A New History of Life (3 page)

We no longer believe, as even the most enlightened of Renaissance thinkers did, that the Earth is the center of the universe, center of the solar system, sole place where life exists in the universe, inhabited by intelligent creatures who are the image of a vast, creating god. We know now that the Earth is but one planet of many—and that its life might be similarly quite unremarkable. The most recent example of this is our search for Earthlike planets, or ELPs. More and more are being found each year,
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a set of discoveries that is changing the conversation about the frequency of life in the cosmos. But does being “Earthlike” mean having life? Let us look at what our planet went through in its early evolution to the point that it became habitable and eventually inhabited by life.

Between the 1990s and the present, two very specific and paradigm-changing transformations swept the studies that together provide us with a history of life on our planet. Prior to about this time, very little attention was paid by Earth historians to our Earth as a planet that is just one among many. And in like fashion, little attention was paid to its life as being only one kind of life that should be present in the vastness of the cosmos. Yet the discovery of planets orbiting other stars utterly changed the status quo, both scientifically and societally.
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These findings were a great jolt that transcended the primary fields interested in planets beyond Earth—astronomy and certain specialized branches of geology concerned with what are now called exoplanets—to biological fields, and even into religion. Geoff Marcy, one of the first of the exoplanet discoverers, recounts that one of the first phone calls he received after the momentous discovery of exoplanets came from the Vatican. The Catholic Church, wise in the ways of astronomy, wanted to know if this planet could support life, with all of the religious implications that entailed.

The very first exoplanet was found in 1992 (a planet orbiting a pulsar),
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followed by the 1995 discovery of a planet orbiting a “main sequence” star, the kind that would be far kinder for the evolution of life than pulsars, which have the nasty habit of emitting great bursts of life-sterilizing energy periodically onto any orbiting planet.

Only a year after this second exoplanet find, another and quite different astronomical discovery further electrified the scientific, political, and public worlds—the report of a meteorite from Mars
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that was inferred by NASA scientists to contain possible fingerprints of life (and perhaps even fossil microbes). Together, these findings helped launch the new field called astrobiology.

Huge sums of money were targeted at history of life subjects and problems that prior to this had been poorly funded and little investigated, such as the origin and nature of the first life on Earth. This great change began in the last half of the 1990s, and by the new century it was one of the most stimulating fields of science. It transformed science and continues to transform the topic of this book: a history of
life on Earth, and our understanding of the potential for life on other planets and the histories of “other” life.

While the phrase “Earth-like planet” (or ELP) is now commonly used, we should consider just which Earth we are talking about: the Earth of early in its history—upper left, a complete “waterworld”—or that of the far lower right, some billions of years from now when the oceans have been lost to space.

That our planet is one of many potentially habitable planets, and that our life is but one of many possible chemical recipes, are now givens to many astrobiologists. But the many needs of complex organisms equivalent to our Earth’s current animals and higher plants are not trivial. Our kind of life is probably not unique (at least in terms of complexity). But one of us (Ward) has argued that the word “rare” is appropriate, and hence his Rare Earth Hypothesis
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: that while microbial life may be common in the universe, the systems and especially time of planetary environmental stability allowing evolution to eventually produce animal equivalents might be rare indeed.

Perhaps it is terrestrial chauvinism, or perhaps it is true that only life such as our own is possible in the universe. But the search for exoplanets has, at its core, the central goal of finding other “Earths.” The question becomes to define just what an Earthlike planet really is. We all have a conception of our planet in the present day: dominated by oceans, a green and blue place, and our place. But as we go back in time and forward in time, we find that the Earth was and absolutely will be a place very different from the planet we now call home. Earthlike is really a time as well as a “place” definition, it turns out.

There are various definitions that are current in astronomy and astrobiology, the two fields most concerned with defining just what kind of planet we live on. At its most inclusive, an Earthlike planet has a rocky surface and higher-density core. In its most restricted sense, it should share important necessities of “life as we know it,” including moderate temperatures and an atmosphere that allows liquid water to form on the surface. “Earthlike planet” is often used to indicate a planet resembling modern Earth, but we know that the Earth has changed greatly during the past 4.567 billion years since it formed. During parts of its history, our own Earthlike planet could not have supported life at all, and for over half of its history complex life such as
animals and higher plants was impossible. The Earth was wet for virtually all of its history. Within 100 million years of the moon-forming event, where a Mars-sized protoplanet slammed into a still-accreting Earth-sized body, there was liquid water. Coincidence? Or simply a result of the great rain of water-heavy comets smashing onto the Earth’s surface and creating an extraterrestrial deluge?

The evidence is found in tiny sand grains of the mineral zircon
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radiometrically dated to as old as 4.4 billion years ago. They have the isotopic fingerprint of ocean water being sucked down into the mantle via a plate-tectonic-style subduction process. Even though our sun was far less energetic in earliest Earth history, there were enough greenhouse gases in the atmosphere to keep our planet warm. But even more important than heat from the sun, the volcanic activity on early Earth may have been ten times what it is now—and consequently a great deal of heat was streaming out of the Earth and warming its oceans and land. Some astrobiologists now think that life on Earth could not start until planetary heat cooled far lower than it was in the first billion years of Earth history, which is one of many reasons to think that Earth life could possibly have started on another planet, such as Mars. But there was another Earthlike planet early in our solar system history: Venus.

Early in its history
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Venus should have been in the sun’s habitable zone, although it now has a surface temperature of nearly 900°F (500°C) due to a runaway greenhouse effect that surely sterilized its surface (although some think there may be microbial life in its atmosphere, this seems to us to be a pretty slim chance). In contrast, the geological record of Mars shows clearly that it once had flowing water, even in major rivers and streams that could round pebbles and form alluvial fans.
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Now the water is lost, frozen, or just a faint vapor in the near vacuum of its atmosphere. Presumably its lower mass prohibited the plate tectonic processes essential for crustal recycling, which lowered the thermal gradients in its metallic core that are needed to generate an atmosphere-protecting magnetic field, and the greater distance from the sun allowed it to slip more easily into a permanent “snowball Earth” condition. If life ever existed on Mars, it might still
exist in the subsurface, powered by the slight geochemical energy of radioactive decay.

Prior to about 4.6 billion years ago
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(from this point on, GA refers to billion years ago) the proto-Earth formed from the coalescence of variously sized “planetesimals,” or small bodies of rock and frozen gases that condensed in the plane of the ecliptic, the flat region of space in which all our planets orbit. At 4.567 GA (rather precisely dated, and numerically easy to remember), a Mars-sized object appears to have slammed into this body, causing the nickel-iron cores of the planets to merge and the moon to condense from a silicon-vapor “atmosphere” that existed briefly afterward. For the first several hundred million years of its existence, a heavy bombardment of meteors continuously pelted the new planet with lashing violence.

Both the lava-like temperatures of the Earth’s forming surface and the energy released by the barrage of incoming meteors during this heavy bombardment phase would surely have created conditions inhospitable to life.
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The energy alone produced by this constant rain of gigantic comets and asteroids prior to about 4.4 billion years ago would have kept the Earth’s surface regions at temperatures sufficient to melt all surface rock, and keep it in a molten state. There would have been no chance for water to form as a liquid on the surface.

The new planet began to change rapidly soon after its initial coalescence. About 4.56 billion years ago the Earth began to segregate into different layers. The innermost region, a core composed largely of iron and nickel, became surrounded by a lower-density region called the mantle. A thin, rapidly hardening crust of still lesser-density rock formed over the mantle, while a thick roiling atmosphere of steam and carbon dioxide filled the skies. In spite of being waterless on its surface, great volumes of water would have been locked up in the interior of the Earth and would have been present in the atmosphere as steam. As lighter elements bubbled upward and heavier ones sank, water and other volatile compounds were expelled from within the Earth and added to the atmosphere.
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The early solar system was a place with new planets and a lot of junk that had not been included in planet formation, all orbiting the
sun. But not all those orbits were the stable, low-eccentricity ellipses that the current planets show today. Many of them were highly skewed, and many more crossed between the orbiting planets and the sun. All solar system real estate was thus subjected to a cosmic barrage, and no more so than between 4.2 and 3.8 GA. Some of these objects—the comets in particular—may have contributed to the planetary budget of water, but this is a subject of rather intense debate. We simply don’t know how much water was delivered by cosmic impacts to the early Earth. The recent discovery that the trace amounts of water present in samples returned from the moon match those of the bulk on Earth argues that most of our hydrosphere and atmosphere was dissolved in the global magma ocean formed in the aftermath of the giant impact of the Mars-sized protoplanet, Thaea.

But any life then existing surely would have paid a price. NASA scientists have completed mathematical models of such impact events. The collision of a 500 km diameter body with the Earth results in a cataclysm almost unimaginable. Huge regions of the Earth’s rocky surface would have been vaporized, creating a cloud of superheated “rock-gas,” or vapors several thousand degrees in temperature. It is this vapor, in the atmosphere, which causes the entire ocean to evaporate into steam, boiling away to leave a scum of molten salt on the seafloor. Cooling by radiation into space would take place, but a new ocean would not rain out for at least several thousand years after the event. Such large, Texas-sized asteroids or comets could evaporate a ten-thousand-foot-deep ocean, sterilizing the surface of the Earth in the process.
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About 3.8 billion years ago, even though the worst barrage of meteor impacts would have passed, there still would have been a much higher frequency of these violent collisions than in more recent times. The length of the day was also different, being less than ten hours long, because the Earth’s spin was faster then. The sun would have appeared to be much dimmer, perhaps a red orb of little heat, for it not only was burning with far less energy than today, but it had to shine through a poisonous, riled atmosphere composed of billowing carbon dioxide, hydrogen sulfide, steam, and methane—and no atmospheric or oceanic oxygen was present. The sky itself would probably have
been orange to brick red in color, and the seas, which surely covered virtually all of the Earth’s surface, would have been a muddy brown in color. But it was real estate with gas, liquid water, and a rocky crust with myriad minerals, rocks, and environments—including those now thought to be necessary for the two-part process of evolving life: producing the many “parts” and then bringing them all together on a factory floor.