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First Life
Discovering the Connections between Stars, Cells, and How Life Began
By David Deamer UNIVERSITY OF CALIFORNIA PRESS
Copyright © 2011 David Deamer
All rights reserved.
ISBN: 978-0-520-94895-2
CHAPTER 1
A FIREBALL OVER AUSTRALIA
In the summer of 1981, a small black stone wrapped in aluminum foil changed the course of my life. About the size of a marble and indistinguishable from any other rock that might be found on a beach, this stone had traveled from southeastern Australia to NASA Ames Research Center in Mountain View, California, where researcher Sherwood Chang showed me the specimen and gave me a small sample to test. But the stone had traveled even farther. It was a piece of a meteor that had lit up the night sky over the town of Murchison, Australia, in September 1969. The fall began with a bright orange fireball and rolling thunder, followed minutes later by a shower of black stones strewn over five square miles. During the next few weeks, townspeople and scientists collected more than 100 kilograms of meteorites ranging in size from marbles to bricks.
Why would a marble-sized rock be so significant that it can change one's life? The reason is that before traveling 7,000 miles from Australia to California, this tiny bit of rock traveled to Earth from the asteroid belt between Mars and Jupiter, a distance of 250 million miles, or even billions of miles if we take into account all of the rock's orbits around the sun before reaching Earth. The rock was produced when a smaller asteroid happened to collide with a larger one, knocking off fragments of the surface and leaving behind a crater resembling those that we can see in photographs of asteroid surfaces (Figure 1).
I held in my hand a genuine rock from outer space. But there was more to this meteorite than just the mineral content typical of other stony meteorites. When the original boulder-sized object exploded over Murchison, the surfaces of the smaller fragments were heated by atmospheric friction to white-hot temperatures. In a few seconds, the friction slowed the fragments from an initial velocity of 20 kilometers per second, and they finally fell to the ground at the same speed they would reach if they were dropped from an airplane. The first stones to be discovered were still emitting a smoky smell from their hot surfaces, a distinctive aroma that I would later notice while extracting organic compounds from the meteorite. Whenever I give talks about this work, I evaporate a drop of Murchison extract into a wine glass and pass it around for audience members to sniff, joking that 4,570,000,000 BCE was a very good year.
That age, 4.57 billion years, is the reason why certain meteorites have changed scientific lives. The Murchison meteorite belongs to a relatively rare group of meteorites called carbonaceous chondrites. Their aroma is produced by organic compounds older than Earth itself, some of which were present in the vast molecular cloud of interstellar dust and gas that gave rise to our solar system 4.57 billion years ago. Most of the organic material—nearly 2% of the total mass of a typical Murchison sample—is in the form of a tarlike polymer called kerogen, but there are also hundreds of different compounds that sound like a chemist's laboratory: oily hydrocarbons, fluorescent polycyclic aromatic hydrocarbons (PAHs), organic acids, alcohols, ketones, ureas, purines, simple sugars, phosphonates, sulfonates, and the list goes on. Where did all this stuff come from? Did it have anything to do with the origin of life?
THE MURCHISON METEORITE AND SELF-ASSEMBLY
With a sample of a carbonaceous meteorite in hand, I was ready to do an experiment I had been dreaming about. Ten years earlier, shortly after the Murchison event, Keith Kvenvolden and a group of researchers at NASA Ames had analyzed a sample of the meteorite and convincingly demonstrated that amino acids, one of the essential organic compounds composing all life on Earth, were present in the meteorite. And these were not just the kinds of amino acids found on Earth (which might have been contamination), but more than 70 other kinds that were clearly alien to biology as we know it. This study, and many that followed, established that amino acids, the fundamental building blocks of proteins, can be synthesized by a nonbiological process. From this, it seems reasonable to think that amino acids, at least, would have been available on prebiotic Earth.
I had spent much of my earlier research career studying lipids, which, along with proteins, nucleic acids, and carbohydrates, represent the four major kinds of molecules that compose living organisms. "Lipid" is a catch-all word for compounds like fat, cholesterol, and lecithin that are soluble in organic solvents. In earlier research I had extracted triglycerides (fat) from the livers of rats, phospholipids such as lecithin from egg yolks, and chlorophyll from spinach leaves. All of these procedures used an organic solvent mixture of chloroform and methanol to dissolve the lipids, and I wanted to try the same thing with the Murchison material. The surface of the meteorite certainly had surface contamination from being exposed to the laboratory atmosphere, so I broke it into smaller pieces and carefully obtained an interior sample weighing about 1 gram. Then I ground the sample in a clean mortar and pestle with a mixture of chloroform and methanol as the solvent, and decanted the clear solvent from the heavier black mineral powder. The chloroform solvent had a yellow tint, which meant that it had dissolved some of the organic material in the meteorite. I dried a drop of the solution on a microscope slide, added water, and then examined it at 400× magnification. It was an extraordinary sight. Lipidlike molecules had been extracted from the meteorite and were assembling into cell-sized membranous vesicles resembling microscopic soap bubbles (Figure 2). Could it be that similar compartments were present when the first liquid water appeared on Earth more than four billion years ago? Maybe, just maybe, if we studied the Murchison meteorite we might know what kinds of molecules made up the membranous boundaries of the first cellular life.
But a huge question remained: Where did the stuff come from? For that matter, where does anything come from? To set the stage for telling that story, let's begin with stars, where everything begins.
WHERE DOES EVERYTHING COME FROM? THE LIFE AND DEATH OF STARS
Forty years ago, when a boulder-sized meteorite blazed through the skies above Murchison, Australia, we had only a few speculations about when our universe began and how galaxies, stars, and planets come to be. Now, within a single lifetime, we have definite answers to fundamental questions that have been pondered throughout recorded history. The first hint of such an answer was put forward in 1946 by Fred Hoyle, at the time a young, brash British astronomer. Born in 1915 in Yorkshire, England, Hoyle displayed a creative genius that is a wonderful example of how ideas become grist for the mill of science, and how bad ideas disappear into dust, while the rare gems of good ideas survive scientific grinding to become touchstones for future generations of scientists. Fred Hoyle was full of ideas and was bold enough to publish them. Here are some examples:
• The fossil archaeopteryx (a small, feathered, birdlike dinosaur) in the British Museum of Natural History is a fake.
• Vast molecular clouds in outer space are loaded with microorganisms which brought the first forms of life to Earth.
• Viruses that cause fl u epidemics are brought to Earth when it passes through the tails of comets.
• All the carbon required for life to exist is synthesized in the interior of stars.
• The universe has no beginning or end. The idea that the universe has a beginning is nonsense, and it deserves a silly name: the Big Bang.
• The requirements for the synthesis of carbon are so precise that life could not have accidentally arisen. There must have been an intelligence at work to make it happen.
Of these ideas, only one has survived the experimental and theoretical tests that are characteristic of the scientific enterprise. It is now the consensus that all the carbon circulating throughout the universe, including every carbon atom in your body, was synthesized in extremely hot interiors of dying stars (100 million degrees!) and then blasted out into space when the star reached the end of its lifetime in a nova or supernova explosion. To understand this process, we need to recall a little high school chemistry. All matter is composed of atoms, and all atoms have a tiny nucleus composed of elemental particles called protons and neutrons, which are surrounded by orbital clouds of much lighter electrons. (Protons and neutrons are approximately 1,800 times more massive than electrons.) But in stars, the temperature is so high that the electrons cannot be held in place, so basically, stars like our sun are composed of a gas of naked atomic nuclei, mostly in the form of hydrogen and helium. Hydrogen is the lightest element, with a single proton and no neutrons in its nucleus. Helium is the second lightest element, with two protons and two neutrons in its nucleus, and is the product of the initial hydrogen fusion reaction that makes stars shine. If we could somehow grab a 1-gram sample of the universe and use it to fill a balloon on Earth, the balloon would float away because the visible material in the universe is mostly hydrogen and helium.
What Hoyle brilliantly realized is that, at sufficiently high temperatures, two helium nuclei can fuse to form a nucleus of the lightest metallic element, beryllium, which then fuses with a third helium nucleus to produce carbon. Because three helium nuclei combine to make one carbon, this reaction is called the triple alpha process. (Helium nuclei are also called alpha particles when they are emitted from a radioactive element.)
Hoyle had his revelation about the origin of carbon in 1946, but earlier theoretical models had already shown that if carbon is somehow made available, nitrogen and oxygen can be formed in a process called the carbon-nitrogen-oxygen (CNO) cycle, which turns out to be the primary source of fusion energy in large, hot stars on their way to oblivion as novas and supernovas. Descriptions of the CNO cycle were independently published by Carl von Weizsäcker and Hans Bethe in 1938 and 1939. They did not know how the carbon was made, and this is where Hoyle filled in a significant gap in our knowledge a few years later.
Hoyle published his idea in 1946 but did not include a mathematical analysis, although he hinted at it. In 1957, Hoyle joined William Fowler, and Margaret and Geoffrey Burbidge at Cal Tech to publish an article in Reviews of Modern Physics, which became a classic. The article was a brilliant analysis of nucleosynthesis of elements in stellar interiors, and should have won a Nobel Prize for someone. It did, but not for Hoyle. Discovering the treasures hidden in the scientific landscape is a chancy business, but getting credit for your discoveries is even chancier. The Nobel Prize in 1983 went to Fowler, who certainly deserved it for his many contributions, and the prize was shared with Subrahmanyan Chandrasekhar, who studied stellar evolution. Sir Fred had to be satisfied with a knighthood.
To sum up, we can now account for all the major elements of life in terms of nuclear reactions in stars, a process called stellar nucleosynthesis. The atoms of carbon, nitrogen, oxygen, sulfur, and phosphorus that comprise all life on Earth were once in the center of stars more massive than our sun, forged at temperatures hotter than any hydrogen bomb. And what about hydrogen? Even more astonishing, most hydrogen atoms are as old as the universe, which somehow burst into existence 13.7 billion years ago when time began. As living organisms, we are not in any way separate from the rest of the universe. Instead, we simply borrow a tiny fraction of its atoms for a few years and incorporate them into the patterns of life. The hydrogen and oxygen atoms are in the water that flows through our cells, and the carbon, hydrogen, oxygen, sulfur, and phosphorus are linked together in the proteins, lipids, and nucleic acids that are the stuff of life. This is why we call them biogenic elements.
STEADY STATE AND BIG BANG FACE OFF
We have a fair understanding of how the elements of life are made, but what about the universe itself? This story has been told many times, but it is such a good story that I can't resist telling it again. This means that I must first tell you about George Gamow, who has been part of my life for as long as I can remember. As a teenager, all I knew about the person whose name I mispronounced (it's "Gamoff," not "Gamau") was that I could actually understand what he wrote about cosmology. A tattered paperback copy of his book One ... Two ... Three ... Infinity! has a place of honor on my library shelf.
Gamow and Hoyle, both energetic personalities, full of good and bad ideas, came to loggerheads over one of the great questions of all time: Did the universe have a beginning? One answer had already been suggested by Monsignor Georges Lemaître, a remarkable Catholic priest whose lifetime (1894–1966) corresponded closely with Gamow's (1904–1968). In 1931, Lemaître published a paper in Nature in which he proposed that the universe was expanding, and must therefore have begun as a "primitive atom." This was not just an idea but was based on a substantial mathematical foundation. When Albert Einstein met Lemaître a few years later, he told the priest, "Your calculations are correct, but your physics is abominable."
Lemaître was vindicated two years later when Edwin Hubble presented direct evidence that the light from distant galaxies had a longer wavelength than nearby galaxies, a phenomenon called the red shift. Hubble's observation led to a revelation about our universe, so it deserves a bit of explanation to underline its significance. The simplest way to understand the red shift is by analogy to sound produced by a vibrating structure. Sound travels through air as a vibration of a certain frequency, with lower tones having lower frequencies measured as vibrations per second. For instance, the note A in the middle of a piano keyboard vibrates 440 times per second. Light also has wavelike properties, but its frequency is a trillion times that of sound. The thing to keep in mind is that red light has a lower frequency than blue light.
Most of us have stood near a road when a car passes with its horn blaring. We hear a higher horn tone when the car is approaching, then lower in tone after it passes. This is called the Doppler effect, in honor of Christian Doppler, who first proposed an explanation in 1842. Now imagine that the car passes by at nearly the speed of light, and that we are seeing the headlights rather than listening to the horn. As the car approaches, the headlights would look blue, and as it speeds away from us, they would look redder. This is the effect that Hubble observed, now called the red shift. The most plausible explanation is that galaxies are moving away from us and that the farthest objects have the greatest apparent velocities, some in fact approaching the speed of light.
Gamow loved the idea of an expanding universe. In 1948, he published a superb paper with his student Ralph Alpher entitled "The Origin of Chemical Elements." The main point of the paper is that hydrogen and helium compose most of the matter of the universe, and they are present in a certain ratio: 92% hydrogen to 8% helium, in terms of the number of atoms. Gamow also included in the paper an important prediction: When it popped into existence, the universe must have been very hot, hotter than the hottest stars today. But with the passage of time, and the expansion of the universe, the cosmic temperature must decrease, just as a compressed gas cools off when it is allowed to expand. Gamow predicted that if we could somehow listen to the universe, we should still be able to "hear" this energy as a kind of low rumble of radio waves.
When he submitted the paper for publication, Gamow could not resist adding the name of his friend and colleague Hans Bethe, who had nothing to do with the work. Gamow expected his joke to be caught before publication, but no one noticed. The paper was published in Physical Reviews, appropriately on April 1, 1948, and the authors listed were Alpher, Bethe, and Gamow. If you don't get Gamow's joke, don't feel bad, because his editor and peer reviewers—all professional physicists—missed it, too. The first three letters of the Greek alphabet are alpha, beta, and ITLγITL, which also refer to the primary particles released by radioactive decay.
(Continues...)
Excerpted from First Life by David Deamer. Copyright © 2011 David Deamer. Excerpted by permission of UNIVERSITY OF CALIFORNIA PRESS.
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