Origins: Fourteen Billion Years of Cosmic Evolution (20 page)

CHAPTER 15

The Origin of Life on Earth

T
he search for life in the universe begins with a deep question: What is life? Astrobiologists will tell you honestly that this question has no simple or generally accepted answer. Not much use to say that we’ll know it when we see it. No matter what characteristic we specify to separate living from nonliving matter on Earth, we can always find an example that blurs or erases this distinction. Some or all living creatures grow, move, or decay, but so too do objects that we would never call alive. Does life reproduce itself? So does fire. Does life evolve to produce new forms? So do certain crystals that grow in watery solutions. We can certainly say that you can tell some forms of life when you see them—who could fail to see life in a salmon or an eagle?—but anyone familiar with life in its diverse forms on Earth will admit that many creatures will remain entirely undetected until the luck of time and the skill of an expert reveal their living nature.

Since life is short, we must press onward with a rough-and-ready, generally appropriate criterion for life. Here it is: Life consists of sets of objects that can both reproduce and evolve. We shall not call a group of objects alive simply because they make more of themselves. To qualify as life, they must also evolve into new forms as time passes. This definition therefore eliminates the possibility that any single object can be judged to be alive. Instead, we must examine a range of objects in space and follow them through time. This definition of life may yet prove too restrictive, but for now we shall employ it.

As biologists have examined the different types of life on our planet, they have discovered a general property of Earthlife. The matter within every living Earth creature mainly consists of just four chemical elements: hydrogen, oxygen, carbon, and nitrogen. All the other elements together contribute less than one percent of the mass of any living organism. The elements beyond the big four include small amounts of phosphorus, which ranks as the most important, and is essential to most forms of life, together with still smaller amounts of sulfur, sodium, magnesium, chlorine, potassium, calcium, and iron.

But can we conclude that this elemental property of life on Earth must likewise describe other forms of life in the cosmos? Here we can apply the Copernican principle in full vigor. The four elements that form the bulk of life on Earth all appear on the short list of the universe’s six most abundant elements. Since the other two elements on that list, helium and neon, almost never combine with anything else, life on Earth consists of the most abundant and chemically active ingredients in the cosmos. Of all the predictions that we can make about life on other worlds, the surest seems to be that their life will be made of elements nearly the same as those used by life on Earth. If life on our planet consisted primarily of four extremely rare elements in the cosmos, such as niobium, bismuth, gallium, and plutonium, we would have an excellent reason to suspect that we represent something special in the universe. Instead, the chemical composition of life on our planet inclines us toward an optimistic view of life’s possibilities beyond Earth.

The composition of life on Earth fits the Copernican principle even more than one might initially suspect. If we lived on a planet made primarily of hydrogen, oxygen, carbon, and nitrogen, then the fact that life consists primarily of these four elements would hardly surprise us. But Earth is mainly made of oxygen, iron, silicon, and magnesium, and its outermost layers are mostly oxygen, silicon, aluminum, and iron. Only one of these elements, oxygen, appears on the list of life’s most abundant elements. When we look into Earth’s oceans, which are almost entirely hydrogen and oxygen, it is surprising that life lists carbon and nitrogen among its most abundant elements, rather than chlorine, sodium, sulfur, calcium, or potassium, which are the most common elements dissolved in seawater. The distribution of the elements in life on Earth resembles the composition of the stars far more than that of Earth itself. As a result, life’s elements are more cosmically abundant than Earth’s—a good start for those who hope to find life in a host of situations.

Once we have established that the raw materials for life are abundant throughout the cosmos, we may proceed to ask: How often do these raw materials, along with a site on which these materials can collect and a convenient source of energy such as a nearby star, lead to the existence of life itself? Someday, when we have made a good survey of possible sites for life in our Sun’s neighborhood, we shall have a statistically accurate answer to this question. In the absence of these data, we must take a roundabout path to an answer and ask, How did life begin on Earth?

The origin of
life on Earth remains locked in murky uncertainty. Our ignorance about life’s beginnings stems in large part from the fact that whatever events made inanimate matter come alive occurred billions of years ago and left no definitive traces behind. For times more than 4 billion years in the past, the fossil and geological record of Earth’s history does not exist. Yet the interval in solar system history between 4.6 and 4 billion years ago—the first 600 million years after the Sun and its planets had formed—includes the era when most paleobiologists, specialists in reconstructing life that existed during long-vanished epochs, believe that life first appeared on our planet.

The absence of all geological evidence from epochs more than 4 billion years ago arises from motions of Earth’s crust, familiarly called continental drift but scientifically known as plate tectonics. These motions, driven by heat that wells up from Earth’s interior, continually force pieces of our planet’s crust to slide, collide, and ride by or over one another. Plate tectonic motions have slowly buried everything that once lay on Earth’s surface. As a result, we possess few rocks older than 2 billion years, and none more than 3.8 billion years in age. This fact, together with the reasonable conclusion that the most primitive forms of life had little chance of leaving behind fossil evidence, has left our planet devoid of any reliable record of life during Earth’s first 1 or 2 billion years. The oldest definite evidence we have for life on Earth takes us back “only” 2.7 billion years into the past, with indirect indications that life did exist more than 1 billion years before then.

Most paleobiologists believe that life must have appeared on Earth at least 3 billion years ago, and quite possibly more than 4 billion years ago, within the first 600 million years after Earth formed. Their conclusion relies on a reasonable supposition about primitive organisms. At times a bit less than 3 billion years ago, significant amounts of oxygen began to appear in Earth’s atmosphere. We know this from Earth’s geological record independently of any fossil remains: oxygen promotes the slow rusting of iron-rich rocks, which produces lovely red tones like those of the rocks in Arizona’s Grand Canyon. Rocks from the pre-oxygen era show neither any such colors nor other telltale signs of the element’s presence.

The appearance of atmospheric oxygen was the greatest pollution ever to occur on Earth. Atmospheric oxygen does more than combine with iron; it also takes food from the (metaphorical) mouths of primitive organisms by combining with all the simple molecules that could otherwise have provided nutrients for early forms of life. As a result, oxygen’s appearance in Earth’s atmosphere meant that all forms of life had to adapt or die—and that if life had not appeared by that time, it could never do so thereafter, because the would-be organisms would have nothing to eat, for their potential food would have rusted away. Evolutionary adaptation to this pollution worked well in many cases, as all oxygen-breathing animals can testify. Hiding from the oxygen also did the trick. To this day, every animal’s stomach, including our own, harbor billions of organisms that thrive in the anoxic environment that we provide, but would die if exposed to air.

What made Earth’s atmosphere relatively rich in oxygen? Much of it came from tiny organisms floating in the seas, which released oxygen as part of their photosynthesis. Some oxygen would have appeared even in the absence of life, as UV from sunlight broke apart some of the H
2
O molecules at the ocean surfaces, releasing hydrogen and oxygen atoms into the air. Wherever a planet exposes significant amounts of liquid water to starlight, that planet’s atmosphere should likewise gain oxygen, slowly but surely, over hundreds of millions or billions of years. There too, atmospheric oxygen would prevent life from originating by combining with all possible nutrients that could sustain life. Oxygen kills! Not what we usually say about this eighth element on the periodic table, but for life throughout the cosmos, this verdict appears accurate: Life must begin early in a planet’s history, or else the appearance of oxygen in its atmosphere will put the kibosh on life forever.

By a strange
coincidence, the epoch missing from the geological record that includes the origin of life also includes the so-called era of bombardment, which covers those critical first few hundred million years after Earth had formed. All portions of Earth’s surface must then have endured a continual rain of objects. During those several hundred thousand millennia, infalling objects as large as the one that made the Meteor Crater in Arizona must have struck our planet several times in every century, with much larger objects, each several miles in diameter, colliding with Earth every few thousand years. Each one of the large impacts would have caused a local remodeling of the surface, so a hundred thousand impacts would have produced global changes in our planet’s topography.

How did these impacts affect the origin of life? Biologists tell us that they might have triggered both the appearance and the extinction of life on Earth, not once but many times. Much of the infalling material during the era of bombardment consisted of comets, which are essentially large snowballs laden with tiny rocks and dirt. Their cometary “snow” consists of both frozen water and frozen carbon dioxide, familiarly called dry ice. In addition to their snow, grit, and rocks rich in minerals and metals, the comets that bombarded Earth during its first few hundred million years contained many different types of small molecules, such as methane, ammonia, methyl alcohol, hydrogen cyanide, and formaldehyde. These molecules, along with water, carbon monoxide, and carbon dioxide, provide the raw materials for life. They all consist of hydrogen, carbon, nitrogen, and oxygen, and they all represent the first steps in building complex molecules.

Cometary bombardment therefore appears to have provided Earth with some of the water for its oceans and with material from which life could begin. Life itself might have arrived in these comets, though their low temperatures, typically hundreds of degrees below zero Fahrenheit, argue against the formation of truly complex molecules. But whether or not life arrived with the comets, the largest objects to strike during the era of bombardment might well have destroyed life that had arisen on Earth. Life might have begun, at least in its most primitive forms, in fits and starts many times over, with each new set of organisms surviving for hundreds of thousands or even millions of years, until a collision with a particularly large object wreaked such havoc on Earth that all life perished, only to appear again, and to be destroyed again, after the passage of a similar amount of time.

We can gain some confidence in the fits-and-starts origin of life from two well-established facts. First, life appeared on our planet sooner rather than later, during the first third of Earth’s lifetime. If life could and did arise within a billion years, perhaps it could do so in far less time. The origin of life might require no more than a few million, or a few tens of millions, of years. Second, we know that collisions between large objects and Earth have, at intervals of time measured in tens of millions of years, destroyed most of the species alive on our planet. The most famous of these, the Cretaceous-Tertiary extinction 65 million years ago, killed all the non-avian dinosaurs, along with enormous numbers of other species. Even this mass extinction fell short of the most extensive one, the Permian-Triassic mass extinction, that destroyed nearly 90 percent of all species of marine life and 70 percent of all terrestrial vertebrate species, 252 million years ago, leaving fungi as the dominant forms of life on land.

The Cretaceous-Tertiary and Permian-Triassic mass extinctions arose from the collisions of Earth with objects one or two dozen miles across. Geologists have found an enormous 65-million-year-old impact crater, coincident in time with the Cre-taceous-Tertiary extinction, that stretches across the northern Yucatán Peninsula and the adjoining seabed. A large crater exists with the same age as the Permian-Triassic extinction, discovered off the northwest coast of Australia, but this mass dying might have arisen from something in addition to a collision, perhaps from sustained volcanic eruptions. Even the single example of the Cretaceous-Tertiary dinosaur extinction reminds us of the immense damage to life that the impact of a comet or asteroid can produce. During the era of bombardment, Earth must have reeled not only from this sort of impact, but also from the much more serious effects of collisions with objects 50, 100, or even 250 miles in diameter. Each of these collisions must have cleared the decks of life, either completely or so thoroughly that only a tiny percentage of living organisms managed to survive, and they must have occurred far more often than collisions with ten-mile-wide objects do now. Our present knowledge of astronomy, biology, chemistry, and geology points toward an early Earth ready to produce life, and a cosmic environment ready to eliminate it. And wherever a star and its planets have recently formed, intense bombardment by debris left over from the formation process may even now be eliminating all forms of life on those planets.

More than 4 billion years ago, most of the debris from the solar system’s formation either collided with a planet or moved into orbits where collisions could not occur. As a result, our cosmic neighborhood gradually changed from a region of continual bombardment to the overall calm that we enjoy today, broken only at multi-million-year intervals by collisions with objects large enough to threaten life on Earth. You can compare the ancient and ongoing threat from impacts whenever you look at the full moon. The giant lava plains that create the face of the “man in the Moon” are the result of tremendous impacts some 4 billion years ago, as the era of bombardment ended, whereas the crater named Tycho, fifty-five miles across, arose from a smaller, but still highly signficant, impact that occurred soon after the dinosaurs disappeared from Earth.