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In this portrait of Planet Earth—at just about the mid point of its probable lifespan—a biologist discusses the evolution of the network of life and the crucial role played by humans in determining the future of our world.
Unlike most books on earth history, which present the story of life on our planet in terms of one chronological period after another, the author discusses Earth’s teeming diversity in terms of pivotal evolutionary developments. Among these he stresses the importance of symbiosis, sex, and altruism as key determinants of the Earth’s biodiversity.
Symbiosis—when single cells began working together—sparked the sudden appearance of complex animals. Much later symbiotic relationships led to flowering plants that depended on animals for pollination and seed dispersal.
With the advent of sexual selection, there developed an astonishing world of complex behavior and a dizzying array of life forms. In humans, sexual selection exerted a great influence on the development of our large brains.
Altruism—when species learned to work together—resulted in even greater variety and complexity. In early humans, altruism gave rise to ever-widening social circles and the spread of culture.
The author also discusses the role of photosynthesis in establishing and maintaining life on earth; the evidence for ancient natural catastrophes, which caused widespread extinctions; and the importance of religion and the recent use of scientific reasoning in the development and the future of the human species.
This eloquent, panoramic perspective is well designed to foster an appreciation for the scope of life on Earth and to encourage wise stewardship of the natural world on which our survival depends.
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About the Author
Stanley A. Rice, PhD (Durant, OK) is the author of Green Planet: How Plants Keep the Earth Alive, The Encyclopedia of Evolution, The Encyclopedia of Science and Technology, and The Encyclopedia of Biodiversity. He is a professor in the Department of Biological Sciences at Southeastern Oklahoma State University.
LIFE OF EARTH
By Stanley A. Rice
Prometheus BooksCopyright © 2011 Stanley A. Rice
All right reserved.
Chapter OneMEET MOTHER EARTH
This book is the biography of planet Earth, sometimes loosely called Mother Earth. "Mother Earth" is, of course, a figurative term. Earth is not a person and does not have a mind. The ancient Greeks considered Earth to be a goddess, Gaia, but modern science reveals that everything in the history and current operation of Earth has a physical, chemical, or biological explanation. There is no body or mind or goddess-spirit of Earth.
On the other hand, Earth is not just a stage upon which life acts or a storehouse from which life gets its resources. Earth is not merely the playroom and cafeteria of organisms. Twentieth-century ecologist G. Evelyn Hutchinson's phrase "evolutionary play in an ecological theater" suggests that the physical conditions of the Earth control evolution, but he also indicated that the Earth's ecosystems alter those conditions over evolutionary time. Life forms a network of interaction. Usually the most important part of an organism's environment is other organisms. Moreover, this network of life has completely transformed Earth's temperature, its atmosphere, and even its geology. The activities of organisms have filled the air with oxygen, and even much of its limestone is biological in origin. The actors in the evolutionary play are making changes in the stage as they go along. It is as if the actors were the props.
Not only has the network of life transformed the conditions of Earth, but it appears to, in some ways and imperfectly, regulate those conditions, just as your body imperfectly regulates its temperature. Your body maintains a stable temperature because your brain stem sends out commands to your body to speed up or slow down processes that create or disperse heat. The Earth has no body or brain. It seems, however, to have a disembodied, mindless, but largely successful kind of self-regulation. This is why I have titled the book Life of Earth, not Life on Earth.
Many scientists have conceptualized this network of life as Gaia. These scientists are not offering sacrifices at a Delphic Oracle or hallucinating on the carbon dioxide in the oracular caves. They are not nature worshippers. To them, Gaia is a personal name for a living planet that is not, or not quite, a person. Earth does not care for its organisms as would a mother; in fact, some scientists, such as planetary scientist Peter Ward, characterize Earth as being like Medea, rather than Gaia. Medea, another character from Greek mythology, was the vengeful wife of Jason, whom she tricked into eating his own (and her) children. The network of life has often, like a good mother, enhanced the welfare of its inhabitants, but sometimes it has made conditions worse for them. Nevertheless we, and all other species, depend on this web of life as surely as any animal depends on its mother.
One day in 1950, over lunch with his scientific colleagues, physicist Enrico Fermi heard someone speculate about how many advanced civilizations there must be out in space. Fermi quipped, "So, where are they?" He meant that if there were many advanced civilizations, some of them must be more advanced than we are, they must have invented space travel, and at least some of them should have contacted us by now. This has come to be known as "Fermi's Paradox." One answer to this paradox is that there are so few advanced civilizations in the universe that they have not found us yet and probably never will. According to this view, sometimes called the "Lucky Gaia" hypothesis, Earthlike planets, Gaia planets, might be very rare. Very few planets have been as lucky as Earth.
It cannot be denied that, from the very beginning, Gaia has been very, very, very lucky. The network of life arose and developed to its present astounding complexity as a result of a vanishingly rare conjunction of circumstances presumably enjoyed by very few other planets in the universe.
Planets are not at all rare in the universe. More than four hundred planets have been detected orbiting nearby stars. Astronomers can detect these planets and calculate their masses and orbit distances by the fluctuations in light intensity and by the slight swaying movements of the star itself. In one case, scientists were able to generate an image of the planet. Most of these planets have been what astronomers call "hot Jupiters," that is, large gaseous planets orbiting very close to their stars. They resemble a double-star system in which the smaller star never ignited. But a few of the planets are believed to be rocky like Venus, Earth, and Mars. It is therefore likely that rocky planets are common in the universe.
Earth's luck goes far beyond just being a rocky planet, according to the Rare Earth hypothesis of planetary scientists Peter Ward and Donald Brownlee. We can begin, Ward and Brownlee say, by thanking our lucky star, the sun.
Thank Our Lucky Star
First, the sun is a calm and stable star. Many stars, such as the Cepheid variable stars mentioned earlier, fluctuate wildly in their energy output: they are small and dim; then as quickly as one day to a couple of months later, they become larger and more than twice as luminous. Such pulsations in energy may prevent life from ever getting started on any planets that revolve around variable stars. In contrast, the sun has been stable for billions of years. Not perfectly stable, of course. The sun has had occasional "coronal mass ejections," in which it propels energy and particles from its outer layer out into the solar system. One of these mass ejections, on September 1, 1859, was strong enough that it shut down the telegraph systems in the United States and Europe and caused auroras to occur in the skies of even tropical regions. The sun also has an eleven-year sunspot cycle. These variations, however, have not had much effect on Earth. Coronal ejections have not been known to have ever harmed life on Earth (no organisms were harmed when the telegraphs shut down), and solar intensity shifts by only 0.1 percent during the sunspot cycle. The sun has, in fact, changed its energy output over the billions of years of its existence. It has increased the intensity of its radiation by about 30 percent during that time—but it has done so very gradually.
Second, the sun is an isolated star. Many stars have partners, forming multiple-star systems—most commonly, binary systems in which two stars swing around each other like dancers. If the sun were part of such a close family of stars, the other stars would prevent planets from having stable orbits, which might prevent the evolution of life. The sun is also far away from stars that emit so much energy that they would disrupt or destroy life. For example, a supernova anywhere within a few dozen light years of Earth would wipe out all life—but there have been no supernovae in the sun's neighborhood for at least several billion years. Moreover, stars in the centers of galaxies may be so close together that they would disrupt the revolution of one another's planets, even if they are not part of multiple-star systems. But the sun is on a swirling arm far from the center of the galaxy. If we were near the center of the galaxy, many stars would be so close to us that night would not be very dark, and those stars would yank and tug us around and disrupt the stability of our planet's conditions.
Thank God for Jupiter
Ward and Brownlee also point out that Earth resides in a very lucky neighborhood of the solar system. The two sources of luck are Jupiter and the moon. First, consider Jupiter.
When the solar system first formed, it was a disc of small asteroids. Many of these asteroids ran into each other and were crushed into planets by their own gravity. These planets continued to mop up asteroids until about 3.9 billion years ago. After that time, few asteroids remained that could crash into the planets. Most of the craters on the moon (which, as large as some planets, also helped to clear away asteroids) are older than 3.9 billion years. One rare and visible exception is Tycho, the large crater in the moon's southern hemisphere with white radiating ejection plumes visible through amateur telescopes. Tycho was formed by an asteroid that hit the moon only 108 million years ago. The moon, which has no wind or weather, has preserved an intact sample of the asteroid impacts that imperiled the early solar system.
Another important component of the solar system is comets. There are billions of these dirty balls of ice that orbit the sun just beyond the outer edge of the solar system. Most of them remain at the edge of the solar system, but some of them have very elliptical orbits, which bring them close to the sun. They whip around the sun like a slingshot and fly back out into the outer edges of the solar system. While comets are near the sun, solar radiation vaporizes some of the water, creating the comet's "tail" that everyone recognizes. Before 3.9 billion years ago, there were a lot of comets, but they are now, like asteroids, comparatively rare.
The principal reason that asteroids and comets now only rarely fall from the sky is the planet Jupiter. Jupiter is so massive, and has such a powerful gravitational field, that it has sucked up most of the asteroids in the inner solar system, except for those in the asteroid belt, whose orbits have been stabilized by that same Jovian gravitation. Any asteroid or comet that happens to come within several million miles of Jupiter is drawn inevitably into its gaseous embrace. This is exactly what happened to the comet Shoemaker-Levy 9 in 1994. After whipping around the sun and heading back into the outer reaches of the solar system, this comet slipped too close to Jupiter, whose gravity fractured it into pieces. Each piece created a huge flare of radiation as it fell into Jupiter's dense atmosphere, and each of the black spots that remained visible for a few weeks was similar in size to the Earth. Therefore Jupiter continues to clear away asteroids and comets from the solar system. Without Jupiter, asteroids and comets might be hitting Earth so frequently that life would not have a chance to exist for very long.
The Moon—It's Not Just Pretty
And then there is Earth's closest neighbor, which poet Percy Bysshe Shelley (in "The Cloud") described as "That orbed maiden, with white fire laden, whom mortals call the moon." Most planets have moons, but Earth is the only planet in the solar system with a moon so large in relation to it. Mars has two tiny moons, Deimos and Phobos, named after the two horses of the war god's chariot. Jupiter and Saturn have moons larger than ours, but they are tiny in relation to the planetary masses. Our moon is large enough and just far enough away to profoundly influence our planet without severely disrupting it. Everyone knows that the tug of the moon causes the tides. Were it not for tides, there would be no intertidal zone, the only home of thousands of species of organisms. But tides may be of relatively little importance to the planet as a whole, even though they are important to barnacles. The major effect of the moon on Earth, crucial to the survival of life as a whole, is to stabilize Earth's movement.
As planets revolve around their suns, they rotate on their axes. These rotational axes wobble, pointing in different directions at different times. Any planet with a large amount of wobbling would have unstable climatic zones, since sometimes the equatorial zone and sometimes the polar zones would directly face the sun. The part of a planet directly facing its sun will receive the most intense radiation and will be warmest. How could tropical, temperate, and polar plants and animals evolve if the climates of those zones are extremely variable? This appears to have happened with Earth's less fortunate little brother, Mars. Earth, however, has not tilted more than about twenty degrees from the plane of its revolution. Even the little bit of wobbling that Earth does experience has been enough to cause about twenty ice ages during the last two million years of Earth's history. We have the moon to thank for the relative stability of Earth's movements.
The Face of the Earth
The surface of Earth consists of restless plates separated by fractures or faults. The plates are always moving, propelled by rivers and plumes of lava beneath them. Two plates might move apart, with lava erupting from the fracture and creating new plate surface. This is what is happening along the Mid-Atlantic Ridge, which looks like a scar down the middle of the Atlantic Ocean. The plates' movement is making the Atlantic Ocean wider at about the same rate that fingernails grow. Two plates might ram into each other, with one of them slipping underneath the other. This is what is happening in the Marianas Trench near Indonesia. Two plates can scrape against each other, as along the San Andreas Fault in California. Earthquakes and volcanoes are common in all regions where two plates move apart, crush together, or scrape. Because of these movements, the rocks of these plates are continually recycled and renewed. There is probably no part of the ocean floor that is more than 120 million years old.
Meanwhile, continents (which consist of lighter minerals than the plates themselves) sit on the tops of the plates and are tossed around by them. As the plates move, lighter minerals rise to the top, and over billions of years, these lighter minerals have formed the continents. Because they sit on top of restless plates, the continents are also pulled apart and crushed back together. About a billion years ago, the supercontinent called Rodinia was pulled apart into separate continents, which were then crushed back together into a supercontinent called Pangaea about a quarter of a billion years ago. Pangaea was then pulled apart into the modern continents. Continents do not get recycled beneath the plates. Unlike ocean floor plates, continents can be very old.
The oldest continental areas are about 3.8 billion years old. There are, therefore, no continental areas that preserve a record of the heavy asteroid bombardment that occurred about 3.9 billion years ago. Asteroids have been rare during the last 3.8 billion years but common enough to leave a few dozen craters on the continents, from the huge two-billion-year-old Vredefort Crater in South Africa to the small fifty-thousand-year-old Barringer Crater near Interstate 40 in Arizona (figure 1-1).
Finally, the Earth itself is a mighty lucky planet. It happened to form in the "habitable zone" of the new solar system, in which temperatures were in the right range to allow water to exist in liquid form. No medium other than water is known in which lifelike processes can occur. Some scientists speculate that the liquid methane on Saturn's moon Titan may be a medium for life. Even if this is the case, molecules move very slowly in liquid methane, and any metabolism of life-forms on Titan would be very slow, and the resulting life-forms would be very simple. And Earth has plenty of water. The water may have been delivered to the young Earth by comets hitting it before 3.9 billion years ago. The ice in the comets melted and vaporized, creating a haze of steam, much of which was lost into space as new comets continued to rain from the sky. When the collisions became less frequent, and Earth cooled down, the steam became oceans, and water vapor saturated a hot, dense atmosphere of carbon dioxide and nitrogen gases. Mars, Earth's little brother, also had oceans when it was a young planet.
The Earth is also just the right size. If the Earth were too large, its gravity would be so great that complex organisms (not to mention mountains or even continents) would not be able to stand up. If the Earth were too small, however, it would be unable to hold onto its atmosphere, partly due to a lack of sufficient gravity, and also because particles streaming from the sun would have stripped the gases away. Mars, which is about half the size of Earth, has an atmosphere—just barely. Its atmosphere is about 1 percent as thick as ours. At first this seems strange, given that its gravity is at least one-quarter as strong as that of the Earth. But Mars is small enough that its core has cooled off and solidified. On Earth, the currents of molten lava produce a magnetic field that deflects much of the dangerous solar particle stream, except for rare events such as the solar flare of September 1, 1859. Mars has no such protection. The solar particles have scoured away most of its atmosphere, as well as its surface water. When Earth and Mars first formed, they were both wet planets with carbon dioxide atmospheres. Earth kept its atmosphere and evolved; Mars lost its atmosphere and (apparently) died. The surface of Mars is without life; and if there is life on Mars, it is deep in the rocks and therefore microbial in size.
Excerpted from LIFE OF EARTH by Stanley A. Rice Copyright © 2011 by Stanley A. Rice. Excerpted by permission of Prometheus Books. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents
ContentsList of Illustrations....................9
Introduction: A Pile of Rocks in the Middle of Kansas....................11
Chapter 1. Meet Mother Earth....................23
Chapter 2. Inevitable Evolution....................45
Chapter 3. Innovation....................75
Chapter 4. Symbiosis....................93
Chapter 5. Sex....................109
Chapter 6. Altruism....................137
Chapter 7. Religion....................163
Chapter 8. Science....................181
Chapter 9. Faith in Photosynthesis....................201
What People are Saying About This
"A far better read than Genesis. It is truly a love story about our Mother Earth." --(John Perlin, author, A Forest Journey )