Tracking the Course of Evolution


by Richard Cowen


LIFE ON Earth is a fact, although we don't know where and how it began. There is no evidence of life, let alone intelligence or civilization, anywhere in the universe except on our planet. The most reasonable hypothesis to explain these two statements is that life evolved here on Earth.
There are complex organic molecules in interstellar space, on interplanetary dust, in comets, and in the meteorites that hit the Earth from time to time. It makes good chemical sense that such compounds form naturally in interplanetary or interstellar space, because gas clouds, dust particles, and meteorite surfaces are bathed in cosmic and stellar radiation. But life as we know it consists of cells, composed mostly of liquid water that is vital to life. It is impossible to imagine the formation of any kind of water-laden cell in outer space; that could only have happened on a planet that had oceans and therefore an atmosphere.
Planets may have organic compounds delivered to them from space, by way of comets or meteorites, but it is unlikely that this process in itself leads to the evolution of life. Organic molecules must have been delivered to Mercury, Mars, Venus, and the Moon as well as to Earth, only to be destroyed by inhospitable conditions on those lifeless planets.
Experiments show that it is fairly simple to form large quantities of organic compounds in planetary atmospheres and on planetary surfaces, given the right conditions. Space-borne molecules thus may add to the supply on a planetary surface, but they would never be the only source of organic molecules that led to the origin of life.
In testing the idea that life evolved here on Earth (from nonliving chemicals), we deal entirely with principles of nature that we can study. Geologists and astronomers can use evidence gathered from the Earth, Moon, and other planets to reconstruct conditions in the early solar system. Chemists can determine how complex organic molecules could have formed in those environments. Geologists can try to determine when life became established on Earth, and biologists can design experiments to test whether these facts fit with the idea of evolution of life from nonliving chemicals.

The Formation of the Solar System

The Universe is about 12 or 13 billion years old, according to current estimates. The Sun, planets, asteroids, and comets that make up our solar system are much younger, relative newcomers at an age of 4.55 billion years (that is, 4550 million years), give or take a mere 50 million years!
As astronomers reconstruct it, a cloud of interstellar dust and gas floated in our inconspicuous part of the Milky Way galaxy for several billion years. Then, a nearby supernova explosion blasted new material and a lot of energy into the cloud; as a result, or by coincidence, the cloud began to collapse on itself. Most of the material condensed in the center of the cloud to form a new star, our Sun, but about 1% of the cloud remained in orbit around the new star as dust and gas.
Dust particles collide softly and tend to stick together by electrostatic and gravitational attraction in a process called accretion. "Dust bunnies" form under the bed in the same way. Around a new star, the dust bunnies can build up and compact into substantial solid masses a kilometer or so in diameter. Computer models show that in only a few million years, several thousand bodies the size of large asteroids will coalesce into larger units that we now see as planets. The new planets continue to be bombarded by asteroid-sized objects for perhaps several hundered million years in an era of huge impacts. Sometimes planets may have been shattered in huge impacts, or had fragments splintered off them into space. For example, a body larger than Mars may have hit the Earth just after it formed, knocking its axis into the present 23° tilt that gives us our seasons, and blasting debris into Earth orbit that quickly accreted to form the Moon. Around this time, our Solar System took on its present form, with three or four major terrestrial planets in stable orbits, giant gas planets orbiting outside them, and meteorites, asteroids, and comets still orbiting in space as celestial debris.
Earth is one of four terrestrial (rocky) planets in the inner part of our solar system. Venus and Earth are about the same size, and Mars and Mercury are significantly smaller. They all formed in the same way, and most likely, they were all bombarded heavily in the era of huge impacts. All the rocky planets would have been very hot for a long time, with many active volcanoes. They melted deeply enough to form planetary cores made of iron and to give off gases to form atmospheres. But there their similarity ended, and each inner planet had its own later history.

Life on Planetary Surfaces

Once a planet survives the era of huge impacts and cooled, its surface conditions are largely controlled by its distance from the Sun and by any volcanic gases that erupt into its atmosphere. The geology of a planet therefore greatly affects the chances that life might evolve on it.
Liquid water is vital for life as we know it, so surface temperature is perhaps the single most important feature of a young planet. Surface temperature is primarily determined by distance from the Sun: too close, and liquid water evaporates to water vapor; too far, and water freezes to ice. But that's not all, otherwise the Moon would have oceans like Earth's. Gas molecules tend to escape into space from the weak gravitational field of a small planet. The smaller the planet, the faster gases are lost and the heavier are the molecules that escape. Gases are also lost from the atmosphere as they react chemically with surface rocks. They can be released again only by eruptions that melt those rocks. A small planet cools quickly, and its volcanic activity stops as its interior freezes. After that, no further volcanic gases will erupt to return or add gases to the atmosphere. Therefore, a small planet quickly evolves to have a very thin atmosphere or no atmosphere at all.
Volcanic gases include large amounts of water vapor and CO2. Both gases trap solar radiation in the atmosphere (the greenhouse effect), and keep the planetary surface warmer than one might expect simply from its distance from the Sun. For example, Earth would have been frozen for most of its history without CO2 and water vapor in its atmosphere. With these principles in mind, let's look at the prospects for life on the planets of our solar system.
Both Mercury and the Moon had active volcanic eruptions early in their history, but they are small. They cooled quickly and are now solid throughout. Their atmospheric gases either escaped quickly to space from their weak gravitational fields or were blown off by major impacts. Today Mercury and the Moon are airless and lifeless.
Venus is larger than the Moon or Mercury, almost the same size as Earth. Volcanic rocks cover most of the planetary surface. Like Earth, Venus has had a long and active geological history, with a continuing supply of volcanic gases for its atmosphere, and it has a strong gravitational field that can hold most gases. But Venus is closer to the Sun than Earth is, and the larger amount of solar radiation hitting the planet was trapped so effectively by water vapor and CO2 that water molecules may never have condensed out as liquid water. Instead, they remained as vapor in its atmosphere until they were dissociated, broken up into hydrogen (H2), which was lost to space, and oxygen (O2), which was taken up through oxidization of the hot surface rocks of the planet.
Today the dense, massive atmosphere of Venus consists largely of CO2. Atmospheric pressure at the surface is 100 times that of Earth's. Sulfur gases react in the atmosphere to make tiny droplets of sulfuric acid (H2SO4), forming the famous clouds that hide the planetary surface. Water vapor has vanished completely: it has either dissociated or has been bound into molecules of sulfuric acid or carbonic acid. Although the sulfuric acid clouds reflect 80% of solar radiation, CO2 traps the rest, so the surface temperature is about 450°C (850°F). We can be sure that there is no life on the grim surface of Venus.
Mars is much more interesting than Venus from a biological point of view. It is smaller than Earth, and farther from the Sun. But it is large enough to retain a thin atmosphere, mainly composed of CO2. Mars today is cold, dry and windswept, with dust storms that can cover half the planet.
No organic material can survive now on the surface of Mars. There is no liquid water, and the soil is highly oxidizing. But while Mars was still young, and was actively erupting volcanic gases from a hot interior, the planet may have had a thicker atmosphere with substantial amounts of water vapor. The crust may still contain ice in cracks and crevices that could be set free as water if large impacts heated the surface rocks deeply enough to melt it, or if climatic changes were to melt it briefly. Mars had surface water in the distant past. Canyons, channels, and plains look as if they were shaped by huge water floods, and other features look like ancient sandbars, islands, and shorelines. Ancient craters on Mars, especially in the lowland plains, have been eroded by gullies, and sheets of sediment lap around and inside them, sometimes reducing them to ghostly rims sticking out of the flat surface. The flood waters may have drained and dried very quickly, but there may have been temporary oceans. Some estimates suggest that there was water in shallow oceans for as long as 500 m.y. during the early history of Mars.
Mars was too small to sustain geological activity for long. As the little planet cooled, its volcanic activity stopped, and its atmosphere was blasted off by impacts, or lost by slow leakage to space and by chemical reactions with the rocks and soil. The surface is now a dry, frozen waste, and even floods generated by meteorite impact cannot last long enough to sustain life. But life may once have existed briefly on Mars.
In 1996 a team of researchers reported they had found organic compounds and tiny fossil bacteria in ALH84001, a meteorite picked up on the Antarctic ice cap. This meteorite most likely originated on Mars, and was splashed into space by an asteroid impact, to arrive on Earth after spending thousands of years in space. The researchers suggested that the organic compounds and the bacteria were Martian. By 1998 the evidence was looking very weak. The organic molecules are real, but most of them are contamination after the rock reached Earth. The "fossil bacteria" are not real. They are about 1/1000 the size of normal bacteria, physically too small to contain enough genes and chemicals to operate a cell. It remains true that the only planet known to have life is ours, the Earth, and that there is no good evidence for past or present life on Mars. [See a mini-essay by RC for more detail on this.]
The asteroid belt lies outside the orbit of Mars. Some asteroids have had a complex geological history and may once have been part of larger bodies. There is no question of life in the asteroid belt now. No planet or moon outside the orbit of Mars could trap enough solar radiation to form liquid water on its surface to provide the basis for life. Complex hydrocarbon compounds can accumulate and survive on the surfaces of meteorites, or in the atmospheres of the outer planets or some of their satellites, but those bodies are frigid and lifeless.
So we return to Earth as the only known site of life. Earth probably had a molten surface originally, because the catastrophic impact that formed the Moon probably melted Earth completely. After this dramatic event, Earth settled down with a core, mantle, and at least a small area of crust. Impacts and volcanic eruptions continued to release gases to form a thick atmosphere that consisted mainly of CO2, with small amounts of nitrogen, water vapor, and sulfur gases. Any hydrogen quickly escaped to space.
Carbon dioxide in the atmosphere trapped solar radiation, so Earth's surface was warm. But Earth was cool enough to form a crust, and water vapor condensed to form oceans. Oceans in turn helped to dissolve CO2 from the atmosphere and deposit it into carbonate rocks on the seafloor. This process absorbed so much CO2 that Earth did not develop runaway greenhouse heating as Venus had, and very little of Earth's water vapor was broken down in the atmosphere, to be lost as hydrogen leaked to space. Instead, large, shallow oceans covered most of Earth, with a few crater rims and volcanoes sticking out as islands.
Huge impacts on the early Earth would have wiped out any life or proto-life on the planet. The life forms that were our ancestors could not have evolved until after the last sterilizing impact. Smaller late impacts may have encouraged the evolution of life, as comets and meteorites fell on to Earth from space. All comets and a few meteorites carry a significant proportion of organic molecules, and comets may have been a major source of the organic molecules that made possible the evolution of life on Earth.
But processes here on Earth also fostered the formation of important organic chemicals. Lightning and intense ultraviolet (UV) radiation from the young Sun acted on the atmosphere to form small traces of very many gases. Most of the gases dissolved easily in water, and as they formed they rained out into the early oceans, making them rich in carbon. The gases included ammonia (NH3), methane (CH4), carbon monoxide (CO), and ethane. As much as three million tons per year of formaldehyde (CH2O) could have formed. Nitrates accumulated in seawater as photochemical smog, and the nitric acid produced in lightning strikes rained out. But perhaps the most important chemical of all was cyanide (HCN). Cyanide forms easily in the upper atmosphere, from solar radiation and from meteorite impact, and then dissolves in raindrops. Today it is broken down almost at once by oxygen, but early in Earth's history it built up at low concentrations in the ocean. Cyanide seems to be a basic building block for more complex organic molecules. Life probably evolved in chemical conditions that would have killed us instantly.
For updates and Web links on this material, see this UC Davis page.

The text was written in mid-1999. RC, March 2000.

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