The Real Science Behind the X-Files: Microbes, Meteorites, and Mutants

Chapter Two: Visitors from the Void

Introduction
June 8, 2008, 9:46 a.m.; Fort Detrick, Maryland.

Four scientists garbed in bulky space suits sit clustered around an electron microscope in a sterile, BL4 top containment room. Three of the scientists watch intensely as the project director carefully places a drop of amber liquid from a test tube onto a grid. Inside the test tube are microscopic cells removed days earlier from the surface of a nondescript reddish rock -- a rock like many that lie scattered along desert roadways. But the desert this rock came from lies 50 million miles from the nearest road.

The scientists are visibly nervous. They had accepted the current dogma that samples retrieved from the surface of Mars contain no living matter. The Martian atmosphere, only 1 percent that of sister planet Earth, doesn't protect the rocky surface from the intense ultraviolet radiation of the distant sun. Water, in its liquid form so necessary for life, exists at the frigid Martian surface only as ice, and even the ice is restricted to the polar caps. Life might be found by drilling deep below the Martian surface, to where water is liquid and kilometers of rock afford protection from the harsh surface conditions. This kind of deep drilling necessitates the presence of man, and landing men on Mars is still only a paper fantasy. Life shouldn't be found on surface rocks that can be retrieved by robots, yet here they are in the room, waiting and wondering.

The director finishes preparing the specimen for viewing and inserts the grid into the vacuum chamber of the powerful scanning electron microscope. The eyes of all the scientists focus on the microscope's video monitor. The seconds that pass waiting for the image to come into focus seem like years. And the wait has been years. Four years in preparation for the Mars Surveyor 2001 mission. Months spent waiting for the ship to make the giant leap across interplanetary space. More time for the rover with its Athena instrument payload to obtain core samples from within surface rocks, and for the samples to be launched into orbit. And then years waiting for the Mars Sample Return Ship to dock with the orbiting sample container and ferry the precious rocks back to Earth. Finally, after all this time, more waiting while old arguments were revisited about the need for additional precautions to protect Earth from imaginary hordes of deadly alien microbes -- a scenario expounded by nervous politicians who grew up on too many X-Files episodes. All in preparation for this moment.

As the image on the video monitor comes into focus, gasps of shock and disbelief permeate the small room. Is it some hideous alien creature on the screen, some terrifying microscopic monster? No. The image on the screen is a grapelike cluster of cells, identical to those of Staphylococcus aureus, a common terrestrial anaerobic bacterium. As eyes close and shoulders slump, the worst nightmare of the scientists is realized. A seven-year experiment with a slim but tantalizing possibility of answering the age-old question of life on Mars may be invalidated. All because terrestrial bacteria must have hitched a ride into space on the very instruments used to drill into the Martian rocks. But then again, what if a single genesis produced life on Mars and Earth and the bacteria are actually hardy Martian cousins?

This fictional scenario is a real-life worry for scientists currently working on missions to collect rock samples from Mars and bring them safely back to Earth. Contamination of drill bits with Earthly microbes is more than a remote possibility. It is sufficiently difficult to build and launch ships into orbit without the added problems associated with having to work under completely sterile conditions. Faced with the possibility that sterility cannot be assured, how will it be possible to determine what is alien and what is not?

If alien life is found in our solar system, first contact will likely be with simple cells rather then either little green men or the "grays" so ubiquitous in accounts of alien abduction. Since cells are the centerpiece of life on Earth, it is difficult to imagine an alien life-form that is not based on cells. Living on earth, however, does give us a bad case of biological solipsism. Mix together carbon, nitrogen, oxygen, phosphorous, and plenty of water, wait a few billion years, and what could possibly emerge except cellular life?

All life on Earth is composed of cells. English scientist Robert Hooke used a primitive microscope to observe cells in plant material in the mid-seventeenth century, but his observations remained unappreciated for 170 years until the German naturalist Theodor Schwann correctly theorized that all organisms consisted of cells. The next major advance came in 1858, when German physician Rudolf Virchow suggested that all cells come from other cells. This was a remarkable statement in its day, that life springs from life and not non-life. The problem, of course, was getting people to believe him, for who had not seen mold appear on bread where no mold was before? Soon after Virchow's hypothesis was publicized, French microbiologist Louis Pasteur proved to skeptical scientists that life did indeed spring from preexisting life. Modern biology still considers the cell as the basic unit of life.

Cells existed for at least 3.85 billion years before being discovered by Robert Hooke. Rocks of this age from Western Greenland contain tiny bits of chemicals that are thought to be the remnants of early cellular life. Actual fossilized cells are visible in rocks dating back a mere 3.5 billion years from Australia and South Africa. Since the Earth was formed about 4.5 billion years ago, cellular life took 750 million years to develop from the original simmering primordial soup. These ancient cells needed to be especially sturdy, given the turbulent times on early Earth. It is hard to imagine trying to make a home and getting to the important business of evolving given the exploding volcanoes and constant meteor showers. Some of the meteorite impacts were so violent, they vaporized the upper layer of oceans. These turbulent times ended about 3.85 billion years ago, giving our hardy little ancestors a chance to spread throughout the planet. In short, cellular life pretty much formed almost as soon as the planet settled down enough to permit it.

Modern cells come in so many shapes and sizes that it is impossible to predict what an alien cell might look like. Some cells are very large: a chicken egg is a single cell; some human nerve cells can stretch over three feet, from the base of the spinal cord down to the toes. Some cells are extremely small, such as bacteria, and can be seen only with the help of powerful microscopes. But all cells, no matter what the shape or size, are a liquid compartment surrounded by a barrier called a plasma membrane, which is composed of lipids (the same molecules in fats) and proteins. The membrane separates molecules inside the cell from the environment. It must be strong enough to keep out large predators like viruses yet sufficiently permeable to let in food and water and flush out wastes. Formation of a plasma membrane was the key to the origin of life.

All life is a series of chemical reactions, in which molecules must physically come together so that new molecules can be formed. Molecules trapped in a compartment by the plasma membrane are confined to a small space, making it much more likely that they will interact with one another. Imagine how long it would take to build a house if the building materials are scattered randomly all over the city. Now consider building the same house if all the materials lie within a fence that surrounds the property. Although alien membranes might have different types of lipids and proteins, it is hard to imagine life-forms developing in their absence.

Television aliens are rarely cells unless they are some giant space-faring amoebae sucking up the starship Enterprise. Some TV aliens look like giant copies of animals that already exist on Earth. Other aliens look like humans with bad makeup -- after all, costumes have to fit the actors wearing them. Screen aliens do things that give away the fact that they're not your average Tom, Dick, or Harry, like emerging from spaceships on the Washington, D.C., mall with giant robot companions named "Klaatu." While some of the aliens on The X-Files are just as fanciful, such as the morphing, green-blooded bounty hunter from the episode "Herrenvolk," others are much more subtle, and therefore more plausible. There are alien worms buried near a meteorite in the episode "Ice," and strange microbes in flasks labeled "Purity Control" in the episode "The Erlenmeyer Flask"; the aliens in the episodes "Gethsemane" and "Redux" are not necessarily the ones that resemble refugees from Roswell, New Mexico, but rather unclassifiable cells that begin to divide when placed in a nutrient media; and finally, there is the so-called black cancer organism introduced in the episode "Piper Maru," which changes from dust into sluglike worms that invade helpless human hosts.

On The X-Files, differentiation between what is alien and what is merely strange falls within the auspices of Scully and the numerous scientific experts with whom she consults. These researchers use the latest techniques to ferret out the truth of the organisms entrusted to them. It's a dangerous business. Scientists who analyze alien life-forms on The X-Files have much in common with red-shirted security men from the original Star Trek series -- they rarely survive the episode. You would think that after six seasons and so many deaths, scientists would run screaming from the room at first sight of either Mulder or Scully. Yet without these brave and dedicated individuals, the truth would remain hidden.

Although real scientists have not yet faced the question of living aliens, speculation has fueled commentaries for years on such subjects as whether alien bacteria are sailing the heavens on comets or dust, and whether life originated elsewhere and we are all aliens to this planet. Like many of my scientist friends, I love to fantasize about what alien life-forms might be like. As a biochemist, I think more about the insides of alien organisms than the outsides. What would the genetic material look like? Will the proteins be similar? Will similar plasma membranes enclose the cells? Helping to add realism to the scientific investigations of alien organisms on The X-Files has been wonderful because I can make some of my own personal speculations come to life.

How to Identify an Alien in Three Easy Steps

INT. GEORGETOWN MICROBIOLOGY DEPT. -- LUNCH ROOM -- LATE NIGHT

DR. CARPENTER
What you're looking at is a sequence of genes from the bacteria you brought in. Normally, we'd see no gaps in the sequence. But with these bacteria, we do.

SCULLY
Why is that?

Dr. Carpenter takes a deep breath, staring at Scully for a long beat.

DR. CARPENTER
I don't know why, but I'll tell you that my first call would, under any other circumstances, have been to the government.

SCULLY
What exactly did you find?

DR. CARPENTER
A fifth and sixth DNA nucleotide. A new base pair.
(beat)
What you are looking at, Agent Scully, exists nowhere in nature. It would have to be, by definition, extraterrestrial.

-- "The Erlenmeyer Flask"

The X-Files episode "The Erlenmeyer Flask" begins with a high-speed police chase. The fugitive, however, is anything but routine. When cornered by the police, he moves with lightning speed and strikes with a strength well beyond that of a normal man. These attributes by themselves are sufficient to pique the curiosity of Mulder. Add green blood and the ability to breathe under water and even Scully can't help but be intrigued.

When Mulder and Scully visit the laboratory of Dr. Berubi, Harvard Medical School class of '64 and the owner of the fugitive's car, they find him busy doing experiments with microbes and monkeys. Like any practitioner of the arts of molecular biology, Dr. Berubi's lab is decorated with numerous Erlenmeyer flasks. Whether glass or more modern plastic, these conical-shaped containers designed by German chemist Richard Erlenmeyer have been used for over one hundred years to grow microorganisms.

While Mulder and Scully are questioning Dr. Berubi, they become curious about the microbes being grown in Erlenmeyer flasks labeled "Purity Control." Did these microscopic entities contribute to the transformation of the good doctor's colleague from hardworking scientist into authority-evading, water-breathing, green-blooded strongman? Since Dr. Berubi is decidedly uncooperative, Mulder and Scully leave but return later to discover that the doctor has swan-dived from an upper floor window using laboratory gauze around his neck instead of the more traditional rubber ropes around the ankles. The suspicious death of Dr. Berubi has increased Mulder's curiosity. What is a dead genetics expert like Dr. Berubi growing in those Erlenmeyer flasks? And what experiments require so many angry monkeys? Mulder hands a flask of microbes to Scully and asks her to investigate.

The scientific investigation of the cells growing in the Erlenmeyer flask plays a significant role in this episode. Chris Carter, while writing the script, wanted the experiments to be as accurate as possible. Thus began several long conversations between Chris and myself to map out how a scientist would analyze an unknown microbe. Although my expertise is viruses rather then bacteria, I was well aware of the standard series of experiments that scientists perform when faced with an unidentified microorganism. I told Chris that the first step is to grow more of the sample using Erlenmeyer flasks and a nutritive liquid the color of weak coffee. Chris was apparently delighted by the name of the flask as this was to become the title of the episode. The second step, I told him, is to examine the appearance of the microbes using the proper microscopes. Once you know what the cells look like, the third step is to analyze their DNA so the microbes can be classified with respect to known organisms.

Tempting some bacteria to grow and divide in the laboratory has been an endless nightmare for many microbiologists. The litany of excuses for why so many bacteria resist enticements to reproduce has been repeated by many scientists: they might be dead -- but this can't be true for all recalcitrant bacteria -- or they might not be partial to the cuisine. In general, the richer the food fed to difficult bacteria, the sicker they become; starving seems like a natural state for many microbes. The problem scientists face when trying to grow many microorganisms is that some microbes need almost nothing of everything -- just a few special, nearly impossible to discover ingredients, without which the bacteria refuse to cooperate and multiply.

Since portraying the endless trials and errors of trying to find a proper food source for bacteria would have TV viewers yawning as they frantically reach for the remote control, Dr. Berubi has already solved the mystery of growing the organism. Scully therefore proceeds to the second step, determining what the bacteria look like. She takes the flask of microbes to scientist Dr. Anne Carpenter at Georgetown University for enlightenment.

Invariably, investigations of any unknown microbe require microscopes. But which microscopes? There are so many to choose from. More often than not, the wrong type is used by television scientists. It is the misuse of microscopes, which always makes me wince, that made me so pleased when Chris first called for advice. At long last, a show existed that will use the correct microscopes because the person in charge cares about accuracy.

Take the light microscope, for example. Available as children's toys and routinely used by millions of high school students, these microscopes magnify specimens illuminated by light. The earliest light microscopes were simply upside-down telescopes. The Italian astronomer Galileo, when not examining the heavens, turned his telescope over and was amazed to see tiny bugs expand to the size of locusts. The earliest microscopes contained merely a single lens and were not much better than magnifying glasses. In the late-sixteenth century, the compound microscope was invented. By using more than one lens in combination, it provided dramatically improved resolution and a new world became visible overnight. Intricate details of fleas, gnats, hairs, nettle leaves, and razor edges were marveled over by naturalists using microscopes of cardboard and wood covered with vellum or ray skin.

In 1665, Robert Hooke, perhaps the most accomplished experimental scientist of the seventeenth century, published Micrographia, in which he described his observations of the structure of a piece of cork and saw honeycomb divisions that looked like "a great many little boxes...the first microscopical pores I ever saw, and, perhaps, that were ever seen." Hooke named the tiny compartments "cellulae," the Latin name for "small rooms." The world of cells was born. Later in the seventeenth century, the Dutch amateur scientist (but highly skilled lens grinder) Antonie von Leeuwenhoek made the first observations of single-celled organisms like bacteria, opening up an entire new world of creatures never before imagined.

Light microscopes have undergone a number of improvements over the years. The current resolving power of a light microscope is about 200 nanometers (about one hundred thousandth of an inch), roughly four hundred times better than the human eye. The light that is used to illuminate specimens provides the resolution limitations of these microscopes. Light waves are subject to diffraction; the waves bend around obstacles in their way, causing the visible image to become less focused. What this means is that conventional light microscopes are powerful enough to see individual bacteria, but can't resolve defining features on their surfaces. Light microscopes can be used only to view the few giant viruses, despite what numerous television shows would like to suggest.

One of the most exciting recent developments in light microscopes is the confocal microscope. The confocal scope produces a greatly amplified image using a laser beam sent through a tiny pinhole. By scanning a specimen in the directions of height, width, and depth, a three-dimensional image is built up by powerful computers. I first told Chris about confocal scopes for the episode "Herrenvolk," in which Scully uses one to get a three-dimensional image of her smallpox vaccination scar.

Early in the twentieth century, German scientists saw an analogy between streams of electrons and waves of light. The radical idea was that a beam of electrons could be controlled and focused within a vacuum by magnets and work like the glass lenses of a microscope, only with much higher resolution. The first "electron microscope" was built in 1931. Over the following decades, improvements in design turned the electron microscope into one of biology's most important tools to examine the microscopic world.

There are two kinds of electron microscopes and both have been used in X-Files episodes. The first is called a transmission electron microscope. This microscope is used to view molecules like DNA or the interior of cells. The second type, a scanning electron microscope, is used only to see the outsides of objects, like the mutant flies in the episode "The Post-Modern Prometheus" and the alien bacteria in "The Erlenmeyer Flask." Scanning electron microscopes have produced some of the most breathtaking three-dimensional portraits of the very small and brought great insight to the ultrastructure of everything from cells to hair.

The main disadvantage of electron microscopy is that the specimen must be dead. It can provide amazing snapshots of life, but it can't be used to see life in action. Recently, refinements have been made to confocal microscopes such that they now can give electron microscope resolution of live specimens. If any living alien microscopic organisms are ever recovered, this would be the microscope of choice, so that the precious live sample need not be wasted.

I told Chris that the proper microscope to use for an up close and personal view of Berubi's microbes would be a scanning electron microscope. Dr. Carpenter and Scully marvel at the image of the strange cells on the scope's video monitor. When talking with Scully, Dr. Carpenter refers to the microbes as bacteria. Bacteria and similar-looking microbes called Archaea are collectively known as prokaryotes. Prokaryotes are considered by most scientists to be the ancestral cells -- the oldest, simplest, and most primitive life-forms on Earth. Prokaryotes are merely a single cell, one compartment surrounded by the plasma membrane barrier. Floating around in the liquid or cytoplasm of the compartment are all the molecules required for life -- the DNA genome or genetic material, along with proteins, sugars, lipids, minerals, and many others. The absence of smaller compartments inside a prokaryotic cell is their defining characteristic and what differentiates prokaryotes from cells of higher organisms, known as eukaryotes. When Dr. Carpenter describes the strange cells as bacteria, she must believe that they do not sequester their DNA genomes in a separate cubicle called a nucleus as do eukaryotic cells. Failure to separate the DNA from the rest of the cell is the hallmark of a prokaryote.

In my conversation with Chris, we discussed what Dr. Berubi's bacteria should look like. Scientists that specialize in the study of bacteria find their tiny research subjects to be fascinating and picturesque. I, on the other hand, can be more objective since I prefer plants and viruses to bacteria. While bacteria are fascinating, their appearance is prosaic at best. Since the bacteria in the episode were supposed to be alien, I suggested to Chris that he use a picture of something much more interesting, like plant pollen grains (my working with plants has nothing to do with finding them to be fascinating and picturesque). Pollen grains make wonderful alien bacteria. They are symmetrical like bacteria yet have intricate, complex surfaces with multiple pits and protrusions. I was delighted with the electron microscope picture of pollen that Chris used in the episode. The grains looked eerily alien as the camera focused in on their ornate pits. After the episode aired, Chris and I chuckled over his reading on a fledgling X-Files web site that several observant fans noticed that the alien "bacteria" looked suspiciously like pollen.

Since the "bacteria" that Scully and Dr. Carpenter see on the video monitor of the scanning electron microscope do not resemble normal bacteria, Dr. Carpenter tells Scully that she wants to do further studies. My suggestion to Chris was that a scientist would next look inside a cell, since scanning electron microscopy only gives a view of a cell's surface. To begin visually dissecting a cell, Dr. Carpenter could perform a technique called freeze fracturing. After placing the bacteria in a preservative solution, she would flash freeze them in liquid nitrogen. By striking a frozen block containing the cells with a knife, cracks are created that pass through embedded cells. After Dr. Carpenter sprays the cracked surfaces with platinum, the metal cast is removed and an impression of cells in the metal is viewed with a scanning electron microscope. Dr. Carpenter would see an exquisitely detailed, three-dimensional view of the beautifully ordered plasma membrane that separates a bacterium's single cellular compartment from the outside environment. If the strange microbes were like all cells on Earth, she would see that the plasma membrane is composed of two rows of lipids that completely cover the cell. The perfect symmetry of the lipid rows would be broken by scattered proteins that transverse the membrane.

Besides freeze fracturing Dr. Berubi's bacteria to examine their membranes, Dr. Carpenter would have sliced the cells into many thin wafers using a diamond knife. After staining these ultrathin cell sections with lead, she would use a transmission electron microscope to see exquisite details of the interior of the cell. Scully phones Mulder to report that she and Dr. Carpenter have found something astonishing inside the bacteria -- tiny cells that resemble chloroplasts. Scully tells Mulder that bacteria such as these haven't existed for countless years. What Scully is describing are bacteria that may have lived more than 1.5 billion years ago -- bacteria in the process of evolving into eukaryotic cells.

Eukaryotes such as protists (amoebas), animals, and plants have within their cells a number of small cubicles in addition to the nucleus that houses the DNA. These cubicles are called organelles and each is surrounded by a membrane. Some of the little cubicles resemble intact prokaryotic cells, complete with their own little DNA genomes. This observation caused biologist Lynn Margulis to propose that billions of years ago, a large, hungry prokaryote gobbled up one of its harmless little prokaryotic neighbors and forgot to digest it. The tiny prokaryote (no doubt breathing a sigh of relief) continued to live inside the larger cell. Eventually, the two cells came to agreeable terms and developed a symbiotic relationship, with both cells contributing to the well-being of the new entity. The ingested prokaryote lost the ability to ever live a separate life and became an integral part of the larger cell. Together, the new entity developed into the earliest eukaryotic cell. Mitochondria, found in all eukaryotic cells, and chloroplasts, which reside exclusively in plant cells, are descendants of such poorly digested prokaryotic snacks. What Scully believes she has discovered is that Dr. Berubi's bacteria are in an early stage of symbiosis with the progenitors of chloroplasts. She is correct when saying that such bacteria have not existed on Earth for over a billion years.

Dr. Carpenter conducts a third experiment while Scully sleeps in a nearby room. She analyzes some of the organism's DNA. One of the inescapable features of cells on our planet is that DNA is the genetic material, also called the genome, of the cell. If Dr. Berubi's strange-looking bacteria have a genome consisting of standard DNA, then the DNA would be composed of the same four constituents, called nucleotides, that are found in the DNA of all earthly organisms. Just as a protein is a chain of amino acids linked one after another, DNA is a chain of nucleotides. The smallest chains of DNA are the genomes of viruses, some of which are only a few thousand nucleotides long. A DNA chain in humans can be hundreds of millions of nucleotides long.

DNA rarely exists as a single chain. Rather, nearly all DNA is composed of two chains of nucleotides that coil around each other to form the famous "double helix." The two chains of the double helix are held together because nucleotides in one chain pair up and form weak chemical bonds, called hydrogen bonds, with nucleotides in the sister chain. The four nucleotides in DNA, abbreviated A, G, C, and T, always choose the same partners to pair with. The nucleotide A in one chain always pairs with T in the sister chain. Likewise, C always pairs with G. So if a short region of DNA in one chain is AATC, the sister chain would have the sequence TTAG. This means that if you know the order of nucleotides in one chain of a DNA molecule, you automatically know the nucleotide order in its sister chain.

Since DNA has only four different constituents, scientists in the first half of the twentieth century thought that it couldn't possibly contain enough information to be a cell's genetic material. After all, the genetic material, the genome of the organism, must contain the entire program for producing a unique living creature. The structure of DNA also seemed too simple -- merely a uniform double helix regardless of whether the DNA came from bacteria or humans. It seemed much more reasonable that proteins with their twenty different amino acid constituents and their infinite variety of sizes and shapes were the cell's genetic program. Yet a computer program at its most basic level is a language of only two constituents, zeros and ones. The complexity behind programs as intricate as Windows 98 is achieved by the precise order of billions of these two numbers. The complexity of DNA was likewise found to be a function of the order of nucleotides and not the simple number of different constituents

Dr. Carpenter explains to Scully that she has sequenced (determined the order of nucleotides in) one of the organism's genes. If the DNA in a cell is analogous to a computer's hard drive, then genes can be thought of as programs on the hard drive. Genes are the basic unit of heredity. They are the genetic programs that determine everything from the color of your hair to how shy you are around other people. Genes are regions of DNA that get copied into DNA's molecular cousin, RNA. RNA is similar to DNA in that it is also a chain of linked nucleotides, but the nucleotides used to construct RNA are slightly different than the nucleotides that make up DNA. Most RNA molecules are the instruction sheets given to cellular machines called ribosomes, which in turn use the information to construct proteins. The order of nucleotides in thousands of different DNA genes is therefore what determines the order of nucleotides in the corresponding RNA instruction sheets, which get translated by the thousands of ribosomes in a cell into proteins with a particular order of amino acids. Although proteins are not the genetic material of the cell, they are what directly determines your appearance and basic behavioral traits.

The particular gene that Dr. Carpenter would have sequenced is the one called ribosomal DNA. This gene specifies the production of an RNA molecule called ribosomal RNA. Ribosomal RNA, unlike most RNAs, is not an instruction sheet for making proteins. Rather, ribosomal RNA is one of the numerous cellular ingredients that together make up a ribosome. Since all cells need to make proteins, all cells contain ribosomes. When a new creature is discovered, whether it be a simple bacterium or complex animal, scientists examine the order of nucleotides in its ribosomal DNA gene. Once determined, the sequence is compared with ribosomal DNA sequences of other known organisms. Organisms that are more closely related have genes that share more similar orders of nucleotides. By comparing the ribosomal DNA sequences for all known organisms, a "tree of life" can be drawn, with the earliest organisms at the base, and the most recently emerging organisms forming the outermost branches. If species are near each other on the tree, this means that their DNA sequences are closely related.

An organism unrelated to life on our planet might have a cellular biochemistry that is beyond our imagination. Such a creature would probably not have DNA as we know it, let alone have ribosomal DNA genes. If Dr. Carpenter determines that her strange organism contains ribosomal DNA genes and that the nucleotide sequences of these genes are similar to those found in other organisms on Earth, she would conclude that it was simply a strange new type of bacterium. But this is The X-Files, so that isn't what she finds.

When Chris originally talked with me about this script, he needed Dr. Carpenter to discover something about the bacteria that would instantly suggest an extraterrestrial origin. My suggestion was for Dr. Carpenter to discover something completely unexpected when she is sequencing some of the organism's DNA. She would find that Dr. Berubi's bacteria contain DNA with six nucleotides instead of the usual four.

So how could Dr. Carpenter make this startling finding? DNA sequencing is now a routine laboratory procedure. The results of such an experiment are displayed as four ladders, one for each of the four DNA nucleotides, on a piece of X-ray film called a sequencing autoradiograph. With a few minutes of simple instruction, even a child can look at the finished film and determine the order of nucleotides in a sequenced DNA fragment. Although The X-Files' audience was unlikely to have a detailed grasp of modern molecular biology, I suggested to Chris that Dr. Carpenter find gaps in the nucleotide ladders on the X-ray film of the organism's DNA sequence. These gaps could be interpreted as evidence of an additional pair of nucleotides -- a fifth and sixth nucleotide not found in nature.

Dr. Carpenter explains to Scully (and the home audience) that DNA is always composed of two different pairs of nucleotides, the A/T pair and the C/G pair. Evidence for a third pair of nucleotides in DNA is so unexpected and so different from DNA in any life-form on Earth that she has to conclude the bacteria are extraterrestrial. Chris asked me what I would do if I came across such a result in my own work. I laughed and said that I would probably call the government (not that I ever envision making such a phone call). Dr. Carpenter tells Scully in the episode that normally, her first call would have been to the government. But government agent Dana Scully is already present to share the startling news of the existence of extraterrestrials.

Since no normal DNA-sequencing autoradiograph would contain gaps, I agonized over how such an autoradiograph could be artificially doctored for the episode. I soon realized that this was taking scientific accuracy to a ridiculous level. I suggested to Chris that he visit a molecular biology lab at the University of British Columbia, near where The X-Files is filmed, and ask for a standard "sequencing autoradiograph," which I dutifully spelled out. I suggested that Chris have Dr. Carpenter point to the nucleotide-sequence ladders on the X-ray film and exclaim to Scully that she has found gaps in the sequence. This no doubt disappointed the 0.00001 percent of the population who could interpret the real autoradiograph shown in the episode and clearly see that there are no gaps in the DNA sequence.

Dr. Carpenter and Scully find one more intriguing property of the alien bacteria: they are filled with virus. Since thousands of viruses can be present in an infected cell, it is usually a simple matter to recognize if cells are experiencing a virus invasion by using a transmission electron microscope. Viruses can infect nearly all types of cells -- bacterial, fungal, plant or animal -- so it is not surprising that alien bacteria also contain virus. Although it is a rare cell that doesn't need to worry about virus infection, a single virus is adapted to infect organisms only from one of life's six taxonomic kingdoms. Viruses that infect bacteria cannot infect members of the Archaea (the other type of prokaryote), protists (such as amoebas), fungi, plants, or animals, and vice versa. However, viruses can infect several different hosts within a kingdom. Influenza virus, which causes most cases of the flu, can infect birds, pigs, and humans. The virus that I study, my much-beloved turnip crinkle virus, can infect turnips, mustard, Chinese cabbage, and a lovely little weed called Arabidopsis thaliana. Despite viruses' varied appearances and hosts, they all have a single-minded goal: invade a cell, hijack its ribosomes, and force the cell to produce more virus as quickly as possible.

Viruses are simply a set of genes on the prowl. The most rudimentary viruses are merely a protein bag stuffed with one or more pieces of DNA or RNA. Viruses have no independent metabolism. They cannot make proteins or copy their genome, so they must find a cell that is able, if not willing, to help. Relating back to our previous analogy, if the DNA genome of a cell is like a computer's hard drive, then a virus is a bag containing a program on a floppy disk. If there is no way to open the bag, or no computer to insert the disk into, the program is without meaning. Viruses are not cells. The average size of a virus is only about one hundredth the size of a cell. Viruses have no nucleus, no ribosomes, no metabolic activity. They can't absorb nutrients or make proteins. If a virus cannot find a cell to infect, it has no existence.

Since I work on viruses, students often ask me if viruses are alive. I like to answer this question with the question, "How do you define life?" Since this generally leads to vacant stares, I usually issue the reassuring statement that entire books have been written trying to explain the scientific meaning of life.

Most scientists will agree that cells are the basic unit of life, but why? At the very least, something that is alive should have a metabolism, the ability to either absorb or create the materials required for its existence. It should be able to reproduce itself, and evolve into forms better suited to its environment. The simplest organisms that fulfill these parameters are cells. But as a fan of science fiction, I would argue that the future will likely see a change in this definition. With computer and robot designs becoming increasingly sophisticated, the future could see the emergence of sentient machines with control centers that mimic the human brain. Although no one will argue that an automobile is alive, who will argue that an intelligent and self-aware machine is not as alive as a bacterium?

To invade an animal cell, a virus must trick the cell into letting it through the membrane. Getting most large substances through the plasma membrane barrier and into a cell requires the help of receptors, the puzzle piece­like proteins that sit on the surfaces of cells and serve as gatekeepers, allowing in only those substances that the cell needs. When a specific molecule passes by that can fit together with a receptor, a doorway into the cell is activated. Viruses have cleverly exploited this doorway by having proteins or sugars on their surfaces that mimic the shape of natural molecules in the body which normally interact with receptors. When a virus binds to a receptor, the receptor doesn't realize that it's being duped and opens a way for the virus to invade the cell. Since different cells have different sets of receptors, viruses are limited to infecting only the cells that have a receptor with which they can interact. This is why viruses like the human immunodeficiency virus (HIV) can only infect certain types of cells like immune-system cells and various brain cells -- cells that have the virus's receptor, called CD4, on their surfaces.

As the virus sneaks into a cell, the protein bag is opened and the genome of the virus is released. If the virus has a DNA genome, the viral DNA enters the nucleus and commandeers the cell's enzymes to make RNA copies of its genes. The virus subtly alters the cell's ribosomes, causing them to ignore the cell's own RNA and translate only the virus's RNA into proteins. If the viral genome is RNA instead of DNA, the task is even simpler. The viral RNA is directly used to make proteins. Some of the new proteins are enzymes that replicate the genome of the virus. Other proteins are made that form the virus's coat, the bag that surrounds the viral genome. When all is ready, newly made protein bags assemble around freshly made virus genomes. Most animal and bacterial viruses then burst through the superfluous and fatally injured cell and hunt for more virgin cells to infect.

So what was Dr. Berubi doing with an Erlenmeyer flask filled with alien bacteria replete with viruses and chloroplasts? Scully and Mulder speculate that Dr. Berubi was using the bacteria to grow virus and then using the virus to transfer genes into monkeys and his assistant. Using viruses to transfer genes between organisms, even between kingdoms, is not science fiction. Some viruses with DNA genomes can insert their DNA into the genome of the infected host. It is not a difficult matter to splice a foreign piece of DNA into the virus's DNA (for professionals, at least; don't try it at home). Then, when the virus naturally inserts its genome into the DNA of the host, the new piece of DNA is inserted as well. Dr. Berubi was using bacteria as many scientists do, as a convenient means of producing additional copies of the genome of his engineered virus. To successfully amplify a virus genome in bacteria, the virus first must be disguised as an ordinary piece of bacterial DNA. To accomplish this deception, scientists adds bits of bacterial DNA to the virus DNA. One of these DNA bits contains a bacterial origin of replication. In this way, the bacteria's enzymes think that they are replicating the bacteria's DNA when they are really replicating the virus's DNA. Mulder and Scully's idea that Berubi is using the bacteria as a living factory to grow virus is thus a distinct possibility.

Another piece of the puzzle comes from Dr. Berubi's file folder. Dr. Berubi was working on the Human Genome Project, which Mulder calls the largest scientific project ever undertaken in the history of science. Mulder's description of the Human Genome Project is accurate. It is an immense, multibillion-dollar, fifteen-year effort to determine the precise order of the 6 billion nucleotides of the human genome. It is the twentieth-century equivalent to ordering the one hundred known elements of the periodic table, except that the ordering involves 100,000 human genes.

Dr. Berubi is not alone in working on the Human Genome Project. This monumental effort involves over 250 laboratories in the United States along with labs in eighteen other countries. When you consider that the largest genome sequenced to date is that of the common baker's yeast, a seven-year effort involving a mere 12 million nucleotides and six thousand genes, the Human Genome Project seems overwhelming. But by starting with smaller genomes, such as those of bacteria and yeast, technology has led to more efficient automation of DNA sequencing. Although less than 3 percent of the human genome has been finished so far, scientists believe that they will comfortably make their 2005 goal.

Once the genome is sequenced, scientists and medical doctors will quickly be able to identify genes involved in genetic diseases like some forms of cancer. Individuals that are predisposed to particular diseases will also be identified and treatments will shift to prevention-based approaches. As is revealed in subsequent X-Files episodes, the goal of Dr. Berubi and secret forces in the government is to use viruses to insert alien DNA into humans and create alien-human hybrids. Dr. Berubi may have found that only certain individuals are able to survive the introduction of alien DNA. Through his work on the genome project, he could identify which genes give individuals the ability to survive and thus be able to predict who will benefit from the alien DNA and who will die.

While Scully and Dr. Carpenter are working on the strange contents of the Erlenmeyer flask, Dr. Berubi's green-blooded assistant and recipient of the alien DNA is found and carted off to a hospital in an ambulance. While en route the paramedics, fearing their patient is dying, attempt to do a needle decompression procedure. When the needle penetrates the skin, toxic fumes come from the body causing the paramedics to faint.

This scene was written not long after a similar incident became a major news story. In 1994, thirty-one-year-old Gloria Ramirez, now known on the Internet as the "Toxic Lady," was rushed to the emergency room of Riverside General Hospital in California suffering from respiratory and cardiac distress related to her cervical cancer. While drawing blood, a nurse noticed a strange, ammonialike odor and fainted. A doctor took a whiff of the syringe with the patient's blood and also passed out. By the time the incident was over, four additional ER workers had fainted and twenty-eight other people were affected. Gloria Ramirez died in the ER of kidney failure a short time later.

The scene at the hospital must have resembled an X-Files episode. The ER was evacuated. Riverside city HAZ MAT (Hazardous Materials) teams dressed in space suit­like apparel took air samples, decontaminated the room, and sealed Gloria's body. If this were a fictitious incident in an X-Files episode, Mulder and Scully would have arrived soon afterward. Scully would have conducted the autopsy, finding some scientific basis for the white flecks in Gloria's blood. Mulder would have questioned family members about whether Ms. Ramirez complained of being an alien abductee or whether she recently visited sites of illegal pesticide sprayings. The case would be solved and the X-File closed.

Life, however, rarely imitates fiction. The autopsy of Ms. Ramirez was inconclusive. White particles in her blood could not be explained. There were no traces of ingested pesticides, which have occasionally produced odors in ERs. The hospital was ruled out as a source of the toxic fumes. Mass hysteria or previously undiagnosed medical conditions of the ER personnel were deemed highly unlikely. Dr. Julie Gorchynski, one of the first affected, had been a champion surfer in glowing health. Muscle spasms and oxygen loss following the incident have led to a rare destructive bone disorder. This real X-File remains open and unsolved.

Scully returns to finish helping Dr. Carpenter analyze the alien microbes, only to learn that Dr. Carpenter and her entire family were killed in a car crash. The death of Dr. Carpenter affected me in a rather unusual way. You see, Dr. Carpenter was named after me. Chris had told me that he would name the scientist in the episode Dr. Anne Simon. It was pure bad luck that another Anne Simon was a scientist at Georgetown University, where the fictitious Dr. Carpenter worked. The name therefore needed to be changed to my first name and my husband's last name. Before reading the script, I had visions of Dr. Anne Carpenter as a recurring character providing insightful scientific analysis season after season. My only hope now is that Dr. Carpenter faked her own death and is hiding somewhere, waiting for the opportunity to rise again and help Mulder and Scully in their quest for the truth.

Alien Hitchhikers

INT. U.S. CUSTOMS PRIVATE SEARCH ROOM -- NIGHT

CUSTOMS OFFICER
Would you mind telling me the kind of diplomatic work you do, sir? And what material you're transporting in these?

He lifts TWO METAL CYLINDRICAL CANISTERS into view.
ANGLE TO INCLUDE MAN
Reacting to the site of the canisters.

MAN
That is -- those are filled with biohazardous material.

CUSTOMS OFFICER
Then where is the paperwork? And why aren't the containers marked?

MAN
Don't open those. Whatever you do. That material cannot be exposed.

-- "Tunguska"

At the end of long overseas trips, it seems foreordained that my luggage is searched by United States customs agents. The drill is so routine. A stony-faced man scatters my personal belongings all over the counter. Then eyes gleam and a smile tickles his lips when hearing that I am a scientist who works on plant viruses. The inevitable questions begin. "How many farms teeming with diseased crops have you visited, Dr. Simon?" "What insidious viruses are you bringing back into the country, Dr. Simon?" As the minutes tick by and my chances of making that connecting flight go from dim to nonexistent, it's easy to forget where customs agents belong on the tree of life. Still, on these agents' shoulders rests an important responsibility -- protecting the United States from invasion of deadly foreign pathogens, illegal drugs, and Cuban cigars.

While these are not items that I generally carry in my luggage, a passenger in the X-Files' episode "Tunguska" is not so innocent. Despite pleas of diplomatic immunity, agents open his suitcase to find several suspicious-looking glass tubes containing a biohazardous substance. While this revelation should have provoked some care in handling the containers, one slips out of an agent's clumsy fingers and shatters on the hard floor. Within seconds, a black, oily powder congeals into tiny, sluglike worms. The agent gasps in horror as the creatures slither under his pants legs and invade his body.

Unaware of this event, Mulder and Scully receive a tip that they should intercept a rock that is carried by another courier. The black, stony nature of the rock indicates that it is a meteorite. At the Department of Exobiology, Goddard Space Center, Dr. Sacks examines the rock and makes a startling discovery. He tells Mulder and Scully that the rock is indeed a meteorite and one that contains polycyclic aromatic hydrocarbons.

Polycyclic aromatic hydrocarbons, or PAHs, are a set of organic molecules composed of carbons and hydrogens arranged in a ring-shaped pattern. Take a breath in a heavily polluted urban area and your body will have an intimate association with PAHs. The major source of PAHs in the atmosphere is poorly combusted fossil fuels provided by autos and industrial plant emissions. Even minute doses of PAHs can cause cancer. Ever heard the advice against eating too much charcoal-broiled meat? That's because PAHs are released from charcoal, especially mesquite, and absorbed into the grilled food. Vegetables can also take up PAHs from the atmosphere while they are growing, especially large and leafy ones like lettuce and tobacco.

The PAHs that Dr. Sacks finds are in the interior of the mysterious rock, so they did not arise from the meteorite slamming into a barbecue alongside a Los Angeles freeway. Dr. Sacks is excited to find PAHs inside the meteorite because he knows that PAHs are produced by dead, decaying organisms. In other words, PAHs can be a sign that living creatures once resided inside the rock. Dr. Sacks determines that the PAHs in Mulder and Scully's rock are similar to those scientists found in the Antarctic meteorite ALH84001 -- the Martian meteorite with tantalizing but highly controversial signs of ancient life.

Of the twelve pieces of Mars that have been found on Earth, ALH84001 is by far the oldest -- a potato-sized, 4.5-billion-year-old rock that weighs a little over four pounds (1.9 kg). Scientists can say with certainty that these meteorites were once part of the Martian landscape by analyzing gases trapped inside minute bubbles of glass within the rocks. The glass found in ALH84001 is evidence that the rock was once near a cataclysmic event, a comet or asteroid smashing into Mars with such violence that small parts of ALH84001 melted and formed bubbles of glass. Trapped within the tiny bubbles are whiffs of the atmosphere of Mars, which the Viking mission of 1976 measured to be 95 percent carbon dioxide, 2.7 percent nitrogen, 1.6 percent argon, and a few trace gases.

If you think about it, the presence of Martian rocks on Earth is pretty remarkable. For an object to be ejected from the surface of Mars into space, a comet or asteroid would have had to strike the planet with enough force to throw that object outward at a speed of nearly 4 miles per second -- five times the velocity of a rifle. The pieces of Mars so launched would then remain in the solar system (leaving would require a velocity of more than 350 miles per second) for millions of years until captured by the Earth's gravitational field. One of those pieces, ALH84001, wandered homeless through the solar system for 16 million years before plummeting into the Antarctic. Thirteen thousand years later it was picked up by someone, only to languish unappreciated for eight more years in a cabinet at the Johnson Space Center in Houston.

ALH84001 is not your garden-variety Mars meteorite. Unlike its eleven more youthful siblings, it contains PAHs, the first evidence that organic molecules exist on Mars. This finding came as quite a shock. Over twenty years ago, two Viking space probes conducted experiments that tested for traces of organic compounds on the surface of Mars. One test came back positive, the other negative. NASA, not wanting to confuse the public, decided to issue a confident report stating that Mars was a lifeless wasteland devoid of organic materials. But similar positive and negative results were obtained when the two tests were conducted on the surface of Antarctica. NASA chose not to release a further statement that no life existed on Earth. The tests conducted on Earth were not sensitive enough to detect the vast amounts of frozen fungi, algae, bacteria, and diatoms now known to lie deep below the ice surrounding the South Pole. One can only speculate on what the same tests may have missed on Mars.

Besides containing organic PAHs, ALH84001 has many tiny globules of carbonate, a substance that can form when water saturated with carbon dioxide permeates through rock. Associated with the carbonate globules in the meteorite are chains of magnetite beads -- an iron-oxygen compound. Finding carbonate, magnetite, or PAHs by themselves would hardly promote much excitement outside the world of geologists and astrophysicists. But carbonates usually indicate the presence of water, the elixir of life; magnetite beads in precisely the same chain patterns are often found with fossils of Earth bacteria, and the PAHs were of the type produced by decaying microbes. Discovery of all three compounds together within a hundred thousandth of an inch of objects identical to putative terrestrial miniature bacteria, and the tantalizing conclusion reached by respected NASA and Stanford scientists is that life existed and may still exist on Mars.

When NASA held a news conference to announce that ALH84001 contained signs of life, the story was featured on television and newspapers around the world. At my university, microfossils and life on Mars became the major topic of hallway conversations. While some residents of Earth probably wondered what all the fuss was about, others pondered the mind-boggling scientific and religious implications. Life on other planets would mean that we are not alone, that conditions on Earth are not uniquely suited to the development of living organisms.

With all the hype that followed the announcement by NASA, would you be surprised to learn that this was not the first report of fossils in meteorites? In the 1960s, George Claus and Bart Nagy used a hot new instrument called an electron microscope to examine meteorites known as the Orgeuil chondrite and the Ivuna rock. They were shocked to find minuscule shapes that looked like fossils of bacteria. Fearing contamination after impact, Claus and Nagy looked deep into the rocks yet still found the same lifelike structures. Realizing the implication of such findings, Claus and Nagy published a cautious paper in the journal Nature suggesting that maybe, just maybe they had found signs of ancient extraterrestrial life.

Claus and Nagy's paper was received with a tidal wave of criticism and indifference. The samples must have been altered after impact; the fossils were artifacts of the examination procedure; or simply, the "microfossils" had to be the result of natural, inorganic processes in the rock, since they were much too small to be real cells. Life on other planets in our solar system was a topic for Star Trek fans, not serious scientists. Conditions were too harsh on other planets. Even the best possibility for life outside of Earth, Mars, was too cold and the only surface water was frozen. The moons of Jupiter were too distant from the sun and the presence of liquid water was also thought to be highly improbable.

So why are reports of fossils in meteors being taken seriously today? It has to do with the attention given to the question, Where does life exist on Earth? Actually, the question isn't where life exists but where doesn't life exist. There are abundant microbes in the freezing cold of the Antarctic ice. Beneath the burning desert sands are not only bacteria but specialized insects. Twenty-five miles up in the atmosphere float bacteria. At the bottom of the deepest ocean, where no sunlight penetrates, where the temperature hovers right around freezing, and organic nutrients should be too dilute to support anything, exotic zoos of life survive.

But what really led scientists to open their minds to the possibility of fossils in Martian rocks was the abundance of life below our own ground. Microbiologists had once thought that microbes only lived in mud close to the surface. Below the surface layer was solid rock, and what could survive within rock? Yes, there were signs of bacteria deeper below the surface, but the bacteria couldn't be grown in the laboratory, and what couldn't be grown had to be dead or not very interesting.

Then the U.S. Department of Energy decided to check the quality of water below one of its nuclear plants. Deep holes were drilled near the Savannah River in South Carolina. What was uncovered was a world teeming with life, as much life as on the surface, maybe more. Miles beneath the ground, in what has been called the deep, dark biosphere, live bacteria and fungi, basking in the heat of the Earth's core and snacking on rocks and leftover organic material from dead neighbors. The bacteria even make their own organic molecules like methane -- not by photosynthesis, which requires sunlight, but from carbon dioxide and hydrogen gas dissolved in the rock. Some scientists even believe that life began under the planet's surface. The abundance of life below our ground has made many scientists reevaluate whether surface searches for extraterrestrial life are giving misleading results.

Dr. Sacks's excitement in finding that the meteorite contains PAHs is therefore understandable -- he believes that this meteorite may also contain signs of ancient extraterrestrial life. But PAHs by themselves are only one piece of the puzzle. Dr. Sacks tells Mulder and Scully that he wants to search for microfossils in the rock's interior. Scully correctly reminds Dr. Sacks that many scientists doubt that ALH84001 contains any fossils of ancient Martian life.

There are several reasons why a growing number of scientists are skeptical about fossils in ALH84001. To examine the meteorite, NASA scientists used the type of electron microscope that requires coating the subject with metal. Some geologists who specialize in the study of meteorites believe that the metal coating itself created the tiny objects that look like fossils. Other scientists believe that the fossils are just irregularities in the surface of minerals embedded in the rock. To counter these arguments, a French group recently used an advanced microscope called an environmental scanning electron microscope to examine a Tunisian meteorite called Tatahouine. Unlike conventional electron microscopes, this new device does not require coating the sample with metal. Looking at the Tatahouine meteorite in its natural form, they found objects that looked identical to the fossils in ALH84001. The Murchison meteorite from Australia also contains mushroom-shaped bodies that look like microfossils.

One of the arguments used to bolster the claims for Martian life is the striking resemblance of the tiny fossils to mysterious dwarf bacteria known as nanobacteria. Robert Folk of the University of Texas believes that nanobacteria are not only a major constituent of rocks and sediments buried deep below the surface of our planet, but are also found in tap water and tooth enamel. The size of nanobacteria, about one thousandth the volume of normal bacteria, is a cause of concern for many scientists. A cell this size is only big enough to contain a few ribosomes, a few hundred proteins, and only eight genes' worth of DNA. This is far, far less than what is necessary to support even the most primitive of bacteria. For nanobacteria to be real cells, and not artifacts of the investigation methods, then they must have a biochemistry completely at odds with the biochemistry of all other organisms on Earth. While this may be a common occurrence in science fiction, it is much less likely to be true in real life.

Other aspects of the report of ancient life in ALH84001 are also being questioned. Not every scientist believes that carbonates + magnetite + PAHs = life. At the heart of the controversy is the origin of the carbonate globules. Carbonates can form from water and carbon dioxide at low, life-compatible temperatures. It is crucial for the theory of life on Mars that there be evidence of liquid water. But carbonates can also form from liquid carbon dioxide as long as there is no water and a temperature of about 800 degrees Fahrenheit. Ralph Harvey of Case Western University and Harry McSween from the University of Tennessee believe that an asteroid crashed into Mars a billion years ago, liquefying the Martian surface and its covering of carbon dioxide frost, resulting in the carbonates found in ALH84001. According to Harvey and McSween, carbonates in the Martian rock do not imply the presence of water and life but just the opposite -- searing heat in the absence of water, conditions where life could not possibly have existed.

Dr. Sacks is anxious to enter the debate on life in meteorites, so he prepares the meteorite for electron microscopy. Slicing through a section of the rock, Dr. Sacks must have instantly realized that an electron microscope would be superfluous. Oily black particles splatter onto the doctor, then congeal into slimy, sluglike worms. Before Dr. Sacks has time to be euphoric about discovering living organisms within the meteorite, the worms enter his body and he is paralyzed.

An organism that starts out as thousands of individual cells (like the black cancer particles) that then join together to form a single, multicellular creature sounds like a nightmare made for science fiction -- but it isn't. Dictyostelium discoideum (known to fans as "dicti") is an organism called a cellular slime mold. Dicti goes through phases of its life where it resembles an ameoba, followed by an animal, then a plant, and finally a fungus. As an amoeba, single dicti cells are content to live a solitary existence, moving short distances and munching on bacteria for food. When an amoeba is hungry and bacteria are scarce, it releases a chemical that acts as a homing beacon to other dicti cells in the vicinity. The cells gather together and arrange themselves to form a single, multicellular slug. Dicti slugs bear an uncanny resemblance to the larger X-Files black cancer worms.

When the environment becomes dry, dicti slugs stop moving and undergo a transformation into plantlike organisms. In place of the original slugs now stand rigid, immobile "plants" that resemble very small caramel apples on tiny sticks. Spores form inside the "apple" part and burst through to the outside when mature. These spores with their tough cell walls resemble fungi and feed on decaying plant material. Eventually, each spore gives rise to an individual amoeba and the dicti life cycle begins again. I vividly remember an inventive college professor giving me and two hundred other students petri plates with dicti ameoba cells. I took mine back to the dorm and watched spellbound as slugs mysteriously sprang up from microscopic cells and then turned into little balls and sticks. For years I thought that if only dicti were one hundred times larger with a taste for human flesh, it would make a horrific science fiction menace.

While dicti slugs are harmless, the same cannot be said of the black cancer organisms in "Tunguska." Scully finds a colony of black vermiform (wormlike) organisms attached to Dr. Sacks's pineal gland, located just north of the hypothalamus in the brain. The pineal gland was historically viewed as the "seat of the soul" for its role in neurological and psychiatric disorders. One might think that the presence of vermiform creatures nesting in the pineal gland and paralysis imply cause-and-effect, but one would be wrong. Lab rats seem to live quite happily with their pineal glands removed. One dead giveaway that the black cancer creatures are cohabiting a human body is a black slimy film that appears over the eyes of the host. Since the pineal gland is directly connected by nerves to the eyes, the black film could be disrupting the perception of light by the host's pineal gland, thereby affecting the behavior of the host. In the X-Files episode "Piper Maru," infection by the same alien organism led to strange behaviors on the part of the human hosts, almost as if they were possessed by an occupying force.

Another major function of the pineal gland is to ensure that mammals without access to desktop calendars give birth during spring and summer, when the weather is nice and food is abundant. To synchronize the best times for romantic flings, mammals can unconsciously sense the time of the year by how long the sun is out during the day. Daylight is perceived by the eye's retina, which is connected by nerve cells to the pineal gland. The gland secretes the chemical melatonin, which controls fertility in females and the sex drive in males. It's not just rats and mice that obey their pineal glands and give birth during nice weather. Statistics for human births also indicate that more babies are born in spring and summer than in fall and winter. Unfortunately, Dr. Sacks is killed before Scully can complete her analysis on the connection between the pineal gland and the strange worms.

EXT. RURAL AREA (TUNGUSKA, RUSSIA) -- DAY

KRYCEK Tell me what we're doing here.

Mulder stops digging now, too. Looks at Krycek. There's a moment where we wonder if he might not rabbit punch him again. Then:

MULDER June 30, 1908. Tungus tribesmen and Russian fur traders looked up into the southeastern Siberian sky and saw a fireball streaking to earth. When it hit the atmosphere it created a series of cataclysmic explosions that are considered the largest cosmic event in the history of civilization. Two thousand times the force of the A-bomb dropped on Hiroshima.

Mulder continues to dig.

KRYCEK What was it?

MULDER It's been speculated that it was a piece of a comet or an asteroid, even a piece of anti-matter. The power of the blast felled trees in a radial pattern over an area of two thousand kilometers. But no real definitive evidence has ever been found to satisfy an explanation. Of what it was.

Mulder has moved enough earth to slip under the fence now. As Krycek watches him crawl to the other side.

MULDER I think someone found that evidence. And I think the explanation might be something no one ever dreamed.

-- "Tunguska"

Meanwhile, Mulder travels to Russia to search for the origin of the meteorite containing the alien organism. After trekking through the remote wilderness of Siberia, he finds workers unearthing a huge meteorite in the Tunguska region. Mulder would most certainly understand the significance of finding a meteorite in this location. Tunguska is the site of the largest natural explosion in the history of civilization.

At 7:17 a.m. on June 30, 1908, local tribesmen in the trading post of Vanivara in Siberia saw a ball of fire blazing across the cloudless sky followed by a deafening explosion. Whatever occurred four to five miles above the rolling hills and forests of the Tunguska river region released a blast of energy straight downward that was two thousand times greater than the atomic bomb that devastated Hiroshima. Seismographs from around the world picked up pressure waves from an event so catastrophic that the waves circled the Earth twice. Trees, more than 60 million, toppled over in an area the size of Rhode Island. Fiery lights flickered for days afterward, illuminating midnight skies all over Europe and Asia. The lights were so bright that citizens of London thought that their city was on fire. Although the event probably devastated reindeer herds in the isolated region, only two people died. Tunguska is so remote that it took nineteen years for the first Russian expedition to reach the devastated site. Even today, visiting the site means a fifty-mile hike if a convenient helicopter is not available.

Roy Gallant, director of the Southworth Planetarium at the University of Maine, was the first American to visit Tunguska. Roy told me that it takes an experienced eye to see any remaining traces of the catastrophe. Directly beneath where the explosion occurred stands a forest of telephone poles -- sixty-foot trees stripped of their branches that are stark reminders of the downward force of the blast. Remains of other trees three to ten miles away lie on the forest floor in a radial pattern pointing outward from the center of the cataclysm. Otherwise, there are no craters, no signs of further disruption to a landscape that must have once looked like ground zero after an atomic blast.

There are two mainstream explanations for what caused the explosion in Tunguska. One group of scientists led by Chris Chyba, Peter Thomas, and Kevin Zahnle are convinced that minute rock fragments buried in trees point to the explosion of a stony meteor measuring fifty to sixty yards in diameter. Others, including Roy Gallant, believe that only a comet weighing in at over 100,000 tons could explain the mysterious lights that preceded the event by several days. Mulder would probably favor one of the more unusual explanations -- a nuclear explosion from a crashed UFO or passage of a black hole through the Earth. However, a little scientific investigation by Scully would fail to find a trace of radioactive material in the region. She also would conclude that a black hole would have exited through the fishing grounds of Newfoundland, Canada. Why no fisherman bothered reporting the deafening explosion of a black hole shooting out of the ocean or the immense tidal wave that followed are questions that somehow haven't deterred the black hole enthusiasts. According to the X-Files episode "Tunguska," if scientists had dug in the right place or at the correct depth they would have found the real culprit -- a huge black meteorite filled with the alien black cancer organism.

In "Tunguska" and the earlier episode "Ice," meteors are the delivery systems for alien hitchhikers that are able to invade a human host. This brings up several very interesting questions. What are the chances that an alien microbe could survive a trip through space on a vehicle that lacks such basic amenities as an atmosphere, temperature control, sunscreen, refreshments, and brakes? And providing that an organism could survive the rather abrupt explosion or crash landing at the end of the flight, what are the chances that it could become a parasite of Earth's creatures?

Let's first consider whether a microbe could survive an interstellar cruise on a rock. You might be surprised to learn that respected scientists have been asking this rather unusual question for nearly 100 years. At the turn of the twentieth century, a Swedish electrolyte chemist named Svante Arrhenius wondered if bacteria could survive in outer space. His curiosity on this subject must have earned him many puzzled looks and raised eyebrows from colleagues. Arrhenius's delving into the realm of science fiction was spurred by a discovery that seems at first glance to be unrelated to space-faring microbes -- he found that light can exert a force on objects, if the objects are very, very small. Arrhenius observed that clouds of tiny particles blasted off the icy core of a comet are swept back away from the sun like sails in a solar breeze. That is, light waves exert enough force to push small ice particles, like wind pushing the leaves of a tree. As a physical chemist, Arrhenius determined mathematically that objects would need to be between the size of a bacterial spore and a tiny dust particle to be moved by light. Therefore, Arrhenius speculated, if bacteria could survive in space, they could sail between the stars powered by gentle solar breezes.

At first glance, bacteria in space seems like a fantasy worthy of Mulder's imagination. The hardships bacteria would need to overcome seem overwhelming. If the microbes don't die of asphyxiation or dehydration then the intense cold of near absolute zero should turn their cellular liquid into jagged ice spears. Microbes would have as much chance surviving floating freely in space as they would have surviving being stranded on the moon.

But bacteria can survive being stranded on the moon. In 1967, the unmanned lunar lander Surveyor 3 set down gently on the surface of the moon and sent back detailed pictures of a future manned landing site. The cameras of Surveyor 3 remained on the moon for two and a half years before being picked up by Apollo 12 astronauts and returned to Earth. NASA scientists were astonished to find that tiny bacteria had stowed away in the camera before Surveyor 3 was launched from Earth. These bacteria survived their odyssey with no water and after repeated cycles of being frozen and thawed, courtesy of the moon's huge monthly temperature swings. Pete Conrad, commander of the Apollo 12 mission, considered the survival of the wayward bacteria to be the most significant finding of the mission. In retrospect, Pete might not have been surprised at the hardiness of bacteria had he known what Arrhenius knew -- bacterial spores can withstand desiccation in a powerful vacuum and still grow and reproduce afterward.

Surviving in a frigid vacuum is only one hurdle to overcome. If bacteria are to survive the rigors of unprotected space, they must live through constant buffeting by deadly cosmic rays and ultraviolet radiation. These high-energy emissions from the sun make mincemeat of DNA and other essential molecules by blasting apart chemical bonds. Bacteria should have as much chance surviving the intense radiation of interstellar space as they would have surviving in extremely radioactive water surrounding the core of a nuclear power plant.

You guessed it. There are bacteria living in radioactive water inside nuclear power plants. The appropriately named Micrococcus radiophilus manages to exist in conditions that would instantly kill a human and even turn glass brown and crumbly. Bacteria such as Micrococcus radiophilus have evolved several ingenious strategies for withstanding exposure to radiation. The first line of defense is a shell made of sugar and protein that surrounds a bacterium's fragile plasma membrane. The protective nature of the shell can be increased with a dark pigment that acts like sunscreen and is used by bacteria that live in the upper atmosphere. Bacteria also have a task force of enzymes that constantly monitor and repair errors and damage in their DNA. Why, some scientists argue, have bacteria evolved such abilities if not to cruise the heavens?

So it is not outside the realm of extreme possibility that cells might survive on meteors such as the one portrayed in "Tunguska." Proponents of this theory go on to speculate that life on Earth may have originated from microbes landing from outer space. This theory of the origin of life was first voiced by Arrhenius, who called the idea panspermia. Believers in panspermia contend that life did not develop on Earth but was seeded by bacteria from outer space. Panspermia, proponents suggest, is the only answer to the question of how life could have originated from nonlife in less than -- perhaps considerably less than -- 750 million years. Like Mulder, panspermia proponents suggest that aliens might be invading all the time as stowaways on comets, meteroids, or space dust. Believers in panspermia are not just science fiction junkies. Francis Crick (Nobel laureate and co-discoverer of the structure of DNA) supports the theory of panspermia as does noted English astronomer Sir Fred Hoyle. Even Dana Scully, in "Biogenesis," the last episode of the sixth season, concedes that panspermia is a plausible hypothesis. Because it is accepted that bacteria can survive under conditions once thought impossible, debates on the possibility of panspermia now appear in major scientific magazines.

How much life might be cruising the galaxy, hitching rides on comets or sailing on dust? Quite a bit, according to Fred Hoyle. Hoyle, with his colleague Chandra Wickramasinghe, made this highly controversial assertion in 1979 while studying a curious cosmic phenomenon -- galactic dust clouds. First a little history: In the 1920s, the spectrum of light reflected by galactic dust clouds indicated that water was a major dust cloud component. By the 1950s, more sensitive instruments revealed a host of other substances. But what? Hoyle and Wickramasinghe suspected that dust clouds contained a mixture of water and some carbon compounds, possibly graphite (like pencil lead). They tried various mixtures of ice and graphite in the laboratory to see if they could re-create the same peaks and valleys of the light spectrum reflecting off the dust clouds, without success. Another baffling mystery was that only nearly hollow particles of about one micrometer in size could explain a discrepancy between the light diffracted by the clouds and their apparent mass. The puzzle seemed impossible to solve.

For years, Hoyle and Wickramasinghe pondered their riddle. What was mostly water and carbon, with a dash of other common elements thrown in, hollow, and about a micrometer in size? In 1979, the answer hit them: bacteria. In the laboratory, bacteria fit the spectrum of light bouncing off the dust clouds perfectly. Needless to say, the declaration of Hoyle and Wickramasinghe that clouds of bacteria are sailing through the galaxy provoked an outpouring of criticism -- but no one has yet to provide another answer for the riddle of the galactic dust clouds.

So there are structures resembling fossils of microbes in meteorites and evidence that bacteria could survive a trip through space. However, other interpretations of the evidence are also possible. Deep down, like Scully, I will only be convinced when a sample is retrieved off a comet or planet and life is found that could not possibly have originated from Earthly contamination.

Missions are now being planned to do precisely that -- to retrieve samples from other planets. One of the most likely places in the solar system where life may exist is Europa, Jupiter's smallest moon. Europa is one of the most brilliant objects in the solar system. Its flat surface reflects light off a shimmering layer of ice. The Galileo space probe recently traveled so close to the surface that cameras could have picked up a vacation cottage. While such obvious signs of life are unlikely given the frigid -260 degree Fahrenheit surface temperature, pictures provided tantalizing evidence that conditions below the surface may be suitable for living creatures. Photographs show striation patterns indicating that the ice sheet had been broken and rebroken many times -- signs that a turbulent liquid ocean once flowed and might still flow beneath the ice. Other experiments on board Galileo indicate that both Europa and its sister moon Callisto have interiors that are able to conduct electricity. The most likely explanation? The electrical conductor is a substance contained in liquid water. The very strong possibility of liquid water means that the internal temperature of the moons are sufficiently warm to create an ocean of water.

The likeliest heat source so far from the sun is the internal friction of the moon itself moving over a molten core, the same tectonic motion on Earth that frees heat vents in the ocean floor. Since on Earth thermophilic (heat-loving) bacteria thrive around such vents at temperatures above the boiling point of water -- more evidence, if such were needed, for the ubiquity of life on, in, and under the earth's surface -- similar life on Europa appears more and more probable. There is even an atmosphere, with a smattering of oxygen molecules.

Missions are already being planned that will answer the question of whether life is present below the surface of Jupiter's glowing moon. Determining whether the ocean still exists will require landing a probe on the surface, a mission that NASA is studying for the early twenty-first century. The search for life, however, requires more creativity. One plan is to send a pencil-shaped robot that will plunge itself into the ice and deliver an aquabot that will search and report back. Another idea is to send a spacecraft that will orbit Europa and drop a boron-nitride bomb on the ice. The explosion will send up a plume of chunks and chips that can be grabbed when the craft flies through it.

If life is found below the surface of Europa or Mars, would it be similar to life on our planet? Would it share the same biochemistry as ours? The answer depends on the validity of theories like panspermia. If there was a single genesis of life in our solar system, then cells on other planets may share common characteristics with life on Earth, such as DNA for the genetic material and being surrounded by plasma membranes. Some divergence from terrestrial life would certainly be found since life on other planets would be shaped by different environmental conditions. However, bacteria that have lived below our own ground for hundreds of millions of years survive in environments completely unlike the habitats of bacteria above ground, yet these deep dwellers are remarkably similar to their aboveground cousins. A few simple experiments such as those performed by Dr. Carpenter on the contents of the Erlenmeyer flask should be sufficient to establish whether a relationship exists between any cells from Europa and cells from Earth.

There is a possibility that if alien bacteria shared a single genesis with our bacteria, they could have a harmful effect on human physiology. For this reason, stringent containment measures are planned for any rocks returned to Earth. However, the odds should not be any greater than for random Earth bacteria to be pathogenic on humans. Of the countless species of bacteria on our planet, very few are harmful.

INT. ARCTIC COMPOUND -- NIGHT
MICROSCOPE MATTE

If a single celled creature could appear mean and vicious, this is it. Protective spikes encircle the undulating membrane. A whip-like flagellum propels the creature in an excited state.

MURPHY (O.S.)
Tell me that's not a foreign object.

WIDER

Mulder pulls away. Scully looks into the tube. What she sees seems to take her breath away. Scully looks to Mulder.

SCULLY
The same thing is in Richter's blood.

Everyone tenses. She gestures to her work area. Mulder moves to the other microscope. Bear watches intently, nervous, as Mulder peers into the tube.

MICROSCOPE MATTE

Indeed, the same kind of creature is moving about.

WIDER

Mulder pulls up from the scope. Scully turns to the others.

SCULLY
That single celled organism could be the larval stage of a larger animal.

HODGE
That's a big leap, Scully.

SCULLY
The evidence is there.

-- "Ice"

If alien life-forms were unrelated to terrestrial life, could they still be harmful? This would depend on the biochemistry of the alien organism (and whether they have ray guns). For example, let's speculate about the biochemistry of the weaponless alien worm in the X-Files episode "Ice." The worm was discovered in an ice-core sample that was retrieved from deep below the ice of northern Alaska near a meteorite. The ice surrounding the worm contains high levels of ammonia. Mulder speculates that the alien worm originated on a planet with an ammonium atmosphere.

There are many qualities of ammonia that make it ripe for extraterrestrial speculation. Such speculation usually begins with an awareness of the importance of liquid water to life on Earth. Water's structural properties (and omnipresence) are so important to every form of earth-based life that most scientists believe that extraterrestrial life is likely to be found only on planets where water exists in its liquid form. As this represents a fairly narrow range of temperatures -- 32 degrees to 212 degrees F -- the number of possible planets is similarly restricted...that is, unless a substance like water can be found. Consider, therefore, ammonia.

Ammonia consists of a nitrogen atom bonded with three hydrogen atoms. Water is an oxygen atom bonded with two hydrogens. Both molecules are polar. This means that the electrons shared between atoms forming a chemical bond are not shared equally. The electrons are more strongly attracted to the nitrogen or the oxygen and therefore spend less time with the bonded hydrogens. Since electrons have a negative charge, by spending more time with the oxygen or nitrogen than the hydrogens, the oxygen and nitrogen have a more negative charge and the hydrogens are more positively charged. Since negative- and positive-charged atoms attract, the more positive hydrogens are attracted to atoms on other molecules that have a more negative charge. The polar property of water -- the "universal solvent" -- is why it can dissolve proteins, salts, and sugars. Since ammonia is also polar, the essential position of water as the liquid of life on Earth would be filled by ammonia on an ammonium world.

On our temperate planet, ammonia evaporates at room temperature, which is what accounts for its strong odor. If it were much colder and the atmosphere was soaked with ammonia, ammonia would form pools of liquid just like water. Imagine then an ammonia planet: freezing cold with oceans of ammonia. Mulder is correct in stating that if the worm's planet had an ammonium atmosphere, it would have to be a frigid place. Based simply on outside temperature, the worm would feel right at home in the ice of northern Alaska.

Could life develop on a planet with an ammonium atmosphere? Where water is scarce and liquid ammonia is abundant? Possibly. Imagine a world where the primordial soup is mainly ammonia with a dash of carbon dioxide, phosphate, and some metals. Substitute water for ammonia, and conditions might be similar to Earth's during the dawn of life. The ammonia world would need an energy source, say UV light radiating from a nearby star. Our virgin ammonia planet would be a beautiful place. Yellowish-orange oceans of ammonia shimmering beneath a thick, gaseous atmosphere of ammonia, nitrogen, and carbon dioxide. Lightning storms would deliver torrents of ammonia rain to the surface.

Within a few billion years, the first life-forms could evolve on the planet: tiny, single-cell microbes capable of using the sun's energy to build the complex molecules of life -- microbes capable of photosynthesis. On our water world, the sun's energy powers the photosynthetic conversion of water and carbon dioxide into oxygen and sugars. On the ammonium world, photosynthetic microbes could use their sun's energy to convert ammonia into nitrate, by replacing the hydrogens bonded to the nitrogen in ammonia with oxygens. At the same time carbon dioxide would be converted into sugars. New microbes could evolve that feed on the photosynthetic microbes, which eventually could lead to a type of eukaryotic cell.

Creatures of an ammonium world would probably be carbon based. Advanced organisms would eat food rich in nitrate and carbohydrates (sugars). They would breathe in nitrogen and exhale carbon dioxide. The alien worms from an ammonium planet would probably drool at the sight of freshly fertilized plants, since plants are filled with carbohydrates and fertilizer contains nitrates. If the alien worms proved unfriendly, guns and bombs would not be required, just a garden hose. Water would be as noxious a poison to them as ammonia is to us.

On our water-based world, ammonia is a household product handy for killing bacteria. Ammonia is toxic because, just like water, it passes through the plasma membranes of cells. When ammonia gets into bacteria and fungal cells, it acts like drain cleaner, causing instant death. Water should have the same effect on ammonia-based creatures. The worms from the episode "Ice," if they originated on an ammonium-soaked world, would be poisoned by water long before they ever reached the brain. No creature from an ammonia-based planet could ever be a parasite of a water-based organism.

Whether or not you believe in alien hitchhikers, three hundred tons of extraterrestrial organic matter fall to Earth every year courtesy of comets, meteors, and dust. Four billion years ago, when life on Earth was first appearing, there were hundreds or thousands of times as many comets and meteors on collision courses with Earth. William Irvine, a University of Massachusetts astronomer, estimates that enough comets, meteorites, and dust have fallen over the years of Earth's existence to provide all the raw organic materials present on the planet. Life may or may not have originated on our planet, but our molecules are nevertheless the molecules of the universe.

Sea Urchins in the Arctic

INT. MULDER'S APARTMENT -- EARLY MORNING

SCULLY I have come here today, four years later, to report to you on the illegitimacy of Agent Mulder's work. That it is my scientific opinion he became, through the course of these years, a victim of his own false hopes, and of his belief in the biggest of lies.

According to Fox Mulder, proof of the existence of extraterrestrial life would be the greatest discovery in the history of science. It is the holy grail of his life, the search for his sister, kidnapped during a terrifying evening of bright lights when they were both children. In "Gethsemane," the final episode of the fourth season, Mulder believes that he has finally found the elusive proof. A Canadian geodetic survey team, which specializes in mapping land while taking into account the curvature of the earth's surface, finds a body frozen in the wall of an ice cave. It looks like the quintessential alien: small stature, large bald head, wide oval eyes, gray wrinkled skin, and indeterminate sex...another of the "grays," an alien body that fits the description given by numerous "abductees" and Roswell, New Mexico, "eyewitnesses." If it is a hoax, it is a deadly serious one. When an excited Mulder finally reaches the remote site, he is greeted by the bullet-riddled bodies of the survey team. Fortunately, the frozen alien was hidden by the one surviving member. After returning to civilization, an autopsy of the body reveals masses of stringy white tissue not found in normal humans.

Mulder believes that the alien corpse is authentic while Scully is skeptical. Besides the body, the only hard evidence is in the form of ice-core samples taken from the cave. If the ice-core samples prove to be at least two hundred years old, the last time that the cave was not covered with ice, then a body near the ice samples should also date from the same time. Since people in the eighteenth century probably weren't squirreling away fake aliens in the Arctic, the date that the body was frozen would be an important clue in establishing the truth.

At Mulder's request, Scully takes the ice samples to Dr. Vitagliano at the Exobiology Laboratory, Goddard Space Center. At first glance, the samples are just what would be expected for ice that has not melted for hundreds of years -- layers of sedimentation with the surface layer containing hydrocarbon pollutants that not even the Arctic can escape. No surprises, until Dr. Vitagliano analyzes the ice sample more closely. Using a light microscope, he sees that within the ice matrix are cells. When Dr. Vitagliano tells Scully that cells are in the ice, she asks if the cells are from a plant or animal. His answer is cryptic. The cells cannot be classified as plant or animal.

Scully is surprised by Dr. Vitagliano's response because she knows that plant and animal cells are easy to differentiate. Only plant cells have a sturdy rectangular wall composed of proteins and sugars that surrounds the plasma membrane. Plant cells also have chloroplasts, the little cell-like organelles that help make sugars from sunlight, water, and carbon dioxide. Taking up much of the interior space in some plant cells is an enormous bubble called a vacuole, the storage bin and waste pit of the cell. Finally, plant cells do not have little cubicles called lysosomes, which are used by animal cells for storing enzymes that would be detrimental if floating around free. Besides these differences, which are easily observable using a light microscope, plant and animal cells are remarkably similar. Both have nuclei containing the DNA genome and a main compartment within which float membranes, ribosomes, mitochondria, and other constituents.

For the cells to be unclassifiable, they must have a mixture of plant and animal features. Since such cells do not exist in nature and are not shown on camera, let's be imaginative and speculate. Dr. Vitagliano may be looking at cells that lack a cell wall and contain lysosomes like animal cells while having organelles that look like chloroplasts and a large vacuole. Alternatively, the cells could have a cell wall but be completely animal-like within the plasma membrane. When Chris Carter was writing this episode, he phoned to ask what he should call cells that are a mixture of two organisms. I told Chris that these are known as hybrid cells, or chimeras. The word chimera dates back to a creature from Greek mythology, a fire-breathing monster with a lion's head, a goat's body, and a serpent's tail. When I received the script to check it for accuracy, I was amused to see that Chris had used both terms, calling the cells "chimerical hybrids." Chris and I had several conversations about hybrid cells over the years, beginning with the episode "The Host," which contained a charming chimeric organism known as flukeman.

Chris was intrigued to learn that any two cells can be fused together providing that they both have plasma membranes at their outermost surfaces. For plant cells, this means removing the protein-sugar wall that encloses the plasma membrane. The reason why any two cells can be fused is that proteins and lipids move freely within a membrane, making the membrane a highly fluid structure. Bring two cells together, and the membranes flow into each other, much like two drops of liquid mercury can coalesce to become a single large drop. Hybrid cells can result from the fusion of different types of cells from the same organism, say muscle and liver cells, or from the fusion of cells from two different organisms, like humans and aliens. On my suggestion, Chris made the fusion of fluke and human cells one of the possibilities for how flukeman was generated.

Science defines hybrid organisms as the progeny of parents that normally would choose not to intermingle, like man and fluke. Distinct species of organisms arise because members either cannot or will not breed with other organisms. While plants can form fertile hybrids between species, even between genera -- some highly decorative orchids are derived from as many as four different genera -- hybrid animals are rare and sterile. Organism development is a complicated process and both genomes of a hybrid organism must work closely together, which is not usually possible. Mules are the best-known animal hybrids, the natural offspring of male donkeys and female horses. Beefalo, a new hybrid animal whose meat is available in my supermarket, is produced from crossing cows and buffalo. Creatures like flukeman and human-animal hybrids in fictional stories like H.G. Wells's The Island of Dr. Moreau are pure fantasy.

While Scully and Dr. Vitagliano are puzzling over the strange hybrid cells, Mulder's belief in the authenticity of the alien body is shaken when he is told by a Department of Defense employee named Kritschgau that he has been fooled by an elaborate hoax. The body, Kritschgau tells him, is really an artificial creation from the genetic manipulation of chimeric cells. What Kritschgau seems to be stating is that the alien is merely a strange-looking hybrid, cleverly concocted in a laboratory from cells of two natural terrestrial organisms. This would make the alien simply a modern science version of Piltdown Man.

Piltdown Man is the most famous fraud in the history of science. The hoax began in 1912 when amateur archaeologist Charles Dawson discovered fragments of a skull in the Piltdown quarry in Sussex, England. The skull was a tremendous boost for English anthropologists, who felt left out after Continental scientists discovered Neanderthal, Cro-Magnon, and Java Man remains. Piltdown Man outshone all other finds. It was the "missing link," a humanlike skull with elephantlike teeth and an apelike jaw. And that's what it was. A human skull, elephant molars, and an orangutan jaw.

The perpetrators of the hoax were very clever. The teeth were filed to fit the jaw. The skull was broken to obscure the nonexistent connection with the jaw. The bones were boiled and treated with an iron solution to make them look old. When scientists voiced doubts that the jawbone and skull seemed to be from different animals that coincidentally died in the same location, a new skull and jaw were miraculously found. One such coincidence, maybe. Two? It had to be from the same creature. For forty years, anthropologists puzzled over Piltdown Man, the only anomaly in the increasingly fleshed-out theory on the evolution of man. The hoax survived until a new fluorine dating test in 1949 established that the skull was merely six hundred years old and the jaw was twentieth-century ape. Still, it took another four years before a paper was published establishing the nature of the hoax.

Just like Mulder can be fooled by an alien hoax because he so desperately wants to believe, British anthropologists accepted the lie because it was precisely what they wanted to find. At the time, the leading theory of man's evolution was that the braincase would develop before the jaw, leading to a humanlike skull and a simian jaw. Piltdown Man fit the theory perfectly. The British were also pleased because finding Piltdown Man in England meant that the first intelligent humans were, of course, British. Another reason why the Piltdown Man fraud was so easily perpetrated was that none of the principals seemed to have anything to gain. No one became rich over the finding, although reputations were certainly enhanced. Amazing scientific discoveries made by reputable people don't usually elicit accusations of fraud. What the anthropologists needed at the time was an objective scientist like Scully with access to the bones. Someone who would have taken better X rays, tested both the skull and the jaw for similar organic matter, and used a microscope on the teeth to discover the very obvious signs of tampering.

The perpetrators of the Piltdown Man hoax remain a mystery. Was it Charles Dawson, the finder of the "fossils" and now known to be a notorious forger and plagiarizer? Or perhaps it was Sir Arthur Woodward, the Keeper of Geology of the British Museum's Natural History Department who had access to bone collections that could be used to fake the remains. Teilhard de Chardin, before he became a prominent theologian, accompanied Dawson and Woodward into the field, and has since been accused -- if that's the correct word -- by Harvard paleontologist Stephen Jay Gould. Even Sir Arthur Conan Doyle, famous writer of the Sherlock Holmes mysteries, has come under suspicion since he lived in the area.

When Mulder asks Kritschgau why people would want to perpetrate an alien hoax, he replies that the lie is there to divert attention from greater lies. Mulder responds that science today is quite capable of testing the body to discover the truth. But before testing can begin, the body is stolen. The question remains unanswered: Is it real, or is it an elaborate hoax?

If the body had not been stolen, Kritschgau's information that the alien was just a chimera of known terrestrial species could easily have been tested. Over the years, Mulder and Scully have frequently relied on DNA analysis to help determine if strange-looking corpses were man, ape, or alien. DNA can be used as a precise fingerprint because every organism on Earth, except for identical twins and clones, has its own unique genome. The precise order of the 6 billion nucleotides in your genome is a mixture of your parents' genomes. If you were to compare your nucleotide order with that of any other person's, they would differ by only about 0.1 percent, which is still a considerable 6 million nucleotides. If your DNA is only 98.5 percent the same as the DNA of humans, you would be swinging through the trees with your fellow chimps.

If someone commits a crime and leaves a trace of themselves behind -- a wisp of hair, a drop of blood, a flake of skin -- then DNA in these samples can be matched to the perpetrator's DNA. The current ability to match a sample of DNA to the person it came from requires the identification of regions of the human genome that tend to vary among people, the so-called variable regions. The chances that two people will have exactly the same DNA sequence in ten different variable regions is as low as one in several billion. Variable regions of DNA, most of which have no known function, tend to be located in between genes. The reason for variability in nonfunctional DNA regions is simple. Nucleotide changes, also called mutations, can accumulate in nonfunctional parts of the genome over the millions of years of human existence without harm. In the same way, bad sectors that accumulate in unused portions of a computer's hard drive are substantially less damaging than bad sectors in the operating system of the computer. Genes have to specify the building of a precise protein. Mutations in the DNA of a gene can seriously affect the composition and function of the specified protein. Acquire a new mutation in an important gene and the individual might never develop normally enough to be born.

The most common test to match DNA samples is called restriction fragment length polymorphism, or RFLP. When Mulder and Scully talk about the need to conduct DNA analysis on a sample of human blood or tissue, this is usually what they mean. The RFLP test was used in the O. J. Simpson murder trial to determine whose blood was on objects of evidence. It was also used to identify the remains of the Romanovs, the Russian imperial family murdered in 1917, whose bones lay hidden for seventy years.

The RFLP test makes use of natural bacterial enzymes called restriction enzymes, which recognize small sequence patterns in DNA. For example, the enzyme called EcoRI recognizes the nucleotide sequence GAATTC (guanine-adenine-adenine-thymine-thymine-cytosine). The enzyme will snip the DNA double helix wherever this order of nucleotides appears. If GAATTC is found once in a piece of DNA, then the enzyme will cut the DNA at its location, resulting in two pieces of DNA. In a DNA chain that is millions of nucleotides long, the sequence GAATTC will appear many times and the DNA will be cut into many pieces after treatment with the enzyme.

In a variable region of the DNA, not all people will have the same order of nucleotides. If the sequence GAATTC appears in a variable region of the DNA, then the DNA of some people might be slightly different. Instead of GAATTC, some people might have TAATTC. The DNA of people with TAATTC will no longer be cut at this location by the EcoRI enzyme. Since the fragments of DNA after enzyme treatment are separated according to their size, people will have different sizes and numbers of fragments depending on whether the enzymes cut or didn't cut the DNA at a particular place. To perfectly match a human DNA sample with a particular person, a variety of different restriction enzymes that each recognize a different short sequence of DNA are used to examine a number of different variable regions of the genome.

RFLP analysis is useful if you need to identify a specific person. To determine if tissue is human, ape, alien, or some weird chimera, it is easier to simply determine the order of nucleotides in a particular region of DNA. The nucleotide order in the ribosomal DNA sequence has already been determined for thousands of organisms. By sequencing this gene from the DNA of an unknown creature, you can immediately identify its origin or determine if it has a known close relative. Just as (the late) Dr. Anne Carpenter sequenced the ribosomal DNA gene to determine the nature of the unknown bacteria in the episode "The Erlenmeyer Flask," sequencing the ribosomal DNA gene from a sample of the "fake" alien's tissue would tell a scientist the organism from which the tissue came.

Mulder tells Scully that despite Kritschgau's tale of alien hoaxes, he is still inclined to believe that the alien body is genuine. Scully informs Mulder that the chimeric cells found in the ice matrix support Kritschgau's story of how the fake alien was engineered in the lab. Mulder argues that the chimeric cells could be extraterrestrial. Scientifically, Mulder has a point. Kritschgau's explanation that chimeric cells were used to construct the alien indicates a technology that is well beyond what science can accomplish on this planet.

In the X-Files episode "Redux," the sequel to "Gethsemane," Dr. Vitagliano continues his tests on the chimeric cells from the ice-core sample to determine if they are capable of cell division. In other words, he wants to determine if the cells are still living. For real hybrid cells to be capable of cell division, their parent cells must be closely related, like mice and humans. Chris Carter, who wrote the episode, wanted to know how Dr. Vitagliano could discover that not only were the chimeric cells alive, but that they were developing into an organism. I told Chris that Dr. Vitagliano should place the cells in a nutrient-rich solution. If these were normal animal cells, he would use a pink-colored liquid broth enriched with fetal bovine serum. Dr. Vitagliano tells Scully that when he placed the cells in the media, the cells began to divide.

Since Scully would know that media containing baby cow blood would not please even a carnivorous plant, she assumes that the cells must be from an animal. Dr. Vitagliano still maintains that the cells cannot be classified. Scully is confused, and tells him that the cells were able to complete mitotic cell division in the media.

Eukaryotic cells can divide either by mitotic cell division (mitosis) or meiotic cell division (meiosis). Whether the cells are chimeras from the Arctic or parts of your lung, nearly all will divide by mitosis. In mitosis, one cell becomes two, and the two daughter cells are perfect copies of the parent cell. Many of the 100 trillion hardworking cells in your body have very short life spans. One cell dividing to give two is the only way to replace the billions of cells in your body that die every day so that you can live. Skin cells last only between one and thirty-four days and the cells lining your stomach give out after only two days. Not happy with your current liver? If the cells are healthy and able to divide, you'll have a new liver every five hundred days. Some cells in your body don't divide and cannot be replaced. Nerve cells, such as your brain cells, and heart muscle cells are designed to last a lifetime, but the manufacturer's warranty doesn't cover self-destructive activities.

Only cells called germ cells, which give rise to egg and sperm sex cells, divide by the process called meiosis. In meiosis, daughter cells end up with exactly half the DNA of the parent germ cell. This way, when egg and sperm cells join together, they each contribute half of the DNA to the composite cell, called a zygote. From a single cell, the zygote starts mitotic cell division and develops into a complete organism.

Since the ice-core sample cells don't appear to be germ cells, they must divide by mitosis, as Scully suggests. Scully expects the cells to divide like average animal cells would if bathed in nutrient media -- the two daughter cells separate completely from each other after the division. Instead, she is shocked to learn from Dr. Vitagliano that when the cells started dividing, they began to go through the stages of morula, blastula, and gastrula.

I explained to Chris how Dr. Vitagliano could tell that the cells were developing into a complex life-form. In many animals, after the sperm cell fertilizes the egg cell to form a zygote, the zygote starts mitosis and the resulting glob of cells goes through defined stages of development that can be easily identified using a light microscope. Morula, blastula, and gastrula are three of the early developmental stages. Only animal zygote cells have the complete genetic program and activated cytoplasm necessary to proceed through the many stages of organism development. In plants, it's different. While normally a plant develops from a zygote, which is produced when pollen fertilizes an ovule, I had talked to Chris about how I had taken ordinary plant cells and tricked them into believing that they were really "zygotes." The cells proceeded to go through the many stages of plant embryo development. This type of development, which does not begin with a real zygote, is called somatic development. The term "somatic" refers to the type of cell undergoing development. All cells of an organism that are not germ cells are somatic cells.

Scully and Dr. Vitagliano agree that the cells have begun somatic development, which will eventually result in a life-form. For this scene, Chris needed a real live alien organism. Since one was not readily available, he asked for my help. What could he use that would look alien and wouldn't be easily recognized by the average X-Files fan? Tough question. After mulling the possibilities with my scientist husband, Cliff, for several hours, we agreed on our alien -- a pluteus, one of the stages in the development of the common sea urchin. And I knew just where Chris could film a pluteus. One of the foremost labs studying sea urchin development was a stone's throw away from 20th Century Fox, the home of Chris's Ten Thirteen Production Company: Eric Davidson's lab at the California Institute of Technology.

When the audience is treated to a view through the microscope of the developing alien, they see the pluteus, a translucent, pulsing creature with several protrusions coming from one end and a pointed other end. Scully and Dr. Vitagliano are mesmerized by the creature, and with luck, so was the audience. I wondered at the time if any viewers would recognize that the alien creature was really a sea urchin. The next day, I received an answer. A faculty member friend told me that she watched the episode with her husband, a novice X-Files viewer and scientist who studies marine organisms. After the scene where the pluteus "alien" is shown, he turned to my friend and with a puzzled look on his face asked, "What are sea urchins doing in the Arctic?"

In "Gethsemane" and "Redux," as well as "The Erlenmeyer Flask" and "Tunguska," aliens are not little green men or costumed monsters, but something far more sophisticated...and plausible. The aliens are cells. If we are fortunate enough to discover life on Mars or Europa, it is cells that will be brought back to Earth for study. As a scientist, I shiver with anticipation when I think about that hypothetical test tube in the containment room at Fort Detrick. Will cells be found? Will they prove that life in our solar system had a single genesis? And if not, what strange metabolism will these cells have? What type of genetic material? Will they have plasma membranes? Will they have the same twenty amino acids in their proteins as on Earth? My questions are endless. Aliens do not have to step out of spaceships saying, "I come in peace," or blow up the White House to be worthy of our attention. I agree with Mulder. Proof of the existence of extraterrestrial life would be the greatest discovery in the history of science.

It would make for pretty compelling television, too.

Copyright © 1999 by Anne Simon