From A New History of Life: The Radical New Discoveries about the Origins and Evolution of Life on Earth, by Peter Ward and Joe Kirschvink. Copyright © 2015, Peter Ward and Joe Kirschvink. Reprinted by permission of Bloomsbury Press.

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 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 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. 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 (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. 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 lower­density 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.

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-foot­deep ocean, sterilizing the surface of the Earth in the process.

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.

Necessary life support systems and their history
One of the most critical prerequisites for the origin of life on Earth was to have had atmospheric gases “reducing” enough to permit the formation of prebiotic molecules, the building blocks of Earth life. The chemical processes known as oxidation-reduction can be remembered as “oil-rig.” This speaks to whether a compound is giving up electrons (OIL: oxidation is loss) or getting electrons (RIG: reduction is gain). Electrons are like money that can be swapped for energy: in oxidation, an electron loss pays for gain in energy. In reduction, the gain of an electron is money in the bank—and this money is in the form of energy. For example, oil and coal are “reduced.” That is, they have a lot of energy in the bank that can be freed when they are oxidized as we burn these fuels. In other words, we oxidize them, which produces energy.

The composition of the Earth’s atmosphere early in its history is a controversial and heavily researched topic. While the amount of nitrogen may have been similar to that of today, there are abundant and diverse lines of evidence indicating that there was little or no oxygen available. Carbon dioxide, however, would have been present in much higher volumes than today, and this CO2-rich atmosphere would have created hothouse-like conditions through a super greenhouse effect, with CO2 pressures ten thousand times higher than today.

Today our atmosphere is made up of 78 percent nitrogen, 21 percent oxygen, and less than 1 percent carbon dioxide and methane—and this composition seems to be relatively new. As is becoming all too apparent, our atmosphere can change its composition relatively rapidly, especially in that deceptively small 1 percent that includes carbon dioxide and methane, two of the so-called greenhouse gases (along with water vapor) that are of importance far out of proportion to their atmospheric abundance.

Element cycles and global temperatures
Our human body requires an immense number of complicated processes to foster the strange state we call life. Many of these systems involve the movement of the element carbon. In analogous fashion, the movement of carbon, oxygen, and sulfur are key aspects in maintaining environments suitable for life on Earth. Of these, carbon is most important.

Carbon undergoes an active cycling in and out of solid, liquid, and gas phases. The transfer of carbon between the oceans, atmosphere, and life is referred to as the carbon cycle, and it is this movement that has the most critical effect on a changing planetary temperature brought about by varying concentrations of greenhouse gases. What we refer to as the carbon cycle is really composed of two different (but intersecting) cycles—the short­term and long­term carbon cycles. The short-term carbon cycle is dominated by plant life. Carbon dioxide is taken up during photosynthesis, and some of this carbon becomes locked up as living plant tissue—which is a reduced compound, thus rich in energy that can be liberated. When plants die or leaves fall, this carbon is transferred to soil, and can be again transformed into other carbon compounds in the bodies of soil microbes, other plants, or animals—where the reduced carbon compounds are oxidized with a gain of energy to the organism doing the oxidizing.

At the same time, organisms also convert other carbon molecules to a reduced state, where it can be used for energy. As it passes through a food chain of animals, this same carbon, now in reduced state, can be oxidized and then respired out of the animal or microbe as carbon dioxide gas, and thus the cycle can renew. Other times, however, still locked within plant or animal tissue, the energy-rich reduced carbon might be buried without being consumed by other organisms, to become part of a large organic carbon reservoir within the Earth’s crust. In so doing, this carbon is no longer part of the short-term carbon cycle.

The second, or long-term, carbon cycle involved very different kinds of transformations. The most important is that the long-term cycle involves the transfer of carbon from the rock record into the ocean or atmosphere and back again. The time scale of this transfer is generally measured in millions of years. The transfer of carbon to and from rocks can cause changes in the Earth’s atmosphere larger than those that can be attained by the short-term carbon cycle, because there is more carbon locked up in rocks than in the ocean, the biosphere (the sum total of living organisms), and the atmosphere combined. This may seem surprising, because the amount of living matter alone is huge. But Bob Berner of Yale University has calculated that if every plant on our planet were suddenly burned, with all their carbon molecules then entering the atmosphere, this short-term carbon cycling would increase atmospheric carbon dioxide by about 25 percent. In contrast, long-term changes in the past have accounted in swings both up and down of carbon dioxide of more than 1,000 percent.

A crucial aspect of the Earth’s carbon cycle concerns calcium carbonate, or limestone. This common Earth material makes up the skeletons of most skeletonized invertebrates. It is also found in tiny planktonic plants, called coccolithophorids, whose skeletons accumulate to form the sedimentary rock known as chalk. Coccolith skeletons make up a vital part of Earth’s habitability, because they help control long­term temperature at stable levels. Because of the plate tectonic process known as subduction, eventually some of this chalk is carried by the plate tectonic conveyer belt to subduction zones, long depressions in the Earth’s crust where oceanic crust sinks downward into the Earth’s interior at these depressions. Miles down into the Earth, now well below the surface of the sea bottom, sufficient heat and pressure cause the calcareous and siliceous skeletons to change into new minerals, such as silicates, as well as carbon dioxide gas. These minerals and hot carbon dioxide gas then make their way back to the surface of the Earth as upward-rising magma, rich in gas, where the minerals are extruded as lava, and the gas is liberated into the atmosphere.

This, then, is the key process of the carbon cycle. Carbon dioxide is transformed into living tissue, which eventually decays and helps form the skeletons of other kinds of animals and plants, which eventually fuse into lava and gas deep in the Earth, which is then brought back into the surface to renew the cycle. The long-term carbon cycle thus has a huge effect on atmospheric gas compositions, which itself largely controls global temperature. And since processes of sediment burial and erosion as well as chemical weathering are key components determining how much and how fast carbonate and silicate organism skeletons are produced in the sea, ultimately the amount of minerals going down the hungry maw of the subduction zones will dictate how much carbon dioxide and methane is pumped back into the atmosphere through volcanoes. This entire process is therefore both largely controlled by life and ultimately allows life to exist on Earth. More than just dictating atmospheric concentrations, it produced what might be called a planetary thermostat, for there is a feedback aspect to the cycle that regulates the long-term temperature on Earth.

The thermostat works like this. Let’s say the amount of carbon dioxide spewing from earthly volcanoes increases, causing more carbon dioxide and methane to enter the atmosphere. Making their way into the upper atmosphere, many of these molecules cause heat energy rising up from the surface of the Earth (after getting there first as sunlight) to be reflected back toward the Earth. This is the greenhouse effect. With more heat energy trapped in the atmosphere, the temperature of the entire planet rises, in the short term causing more liquid water to evaporate in the atmosphere as water vapor, which itself is also a greenhouse gas. This warming, however, has interesting consequences. With warmer temperatures, the rates of chemical weathering increase. This is most important with regard to weathering of silicate minerals. As we have seen, this weathering process eventually leads to the formation of carbonate or other new kinds of silicate minerals, but the weathering process itself strips carbon dioxide out of the atmosphere.

As weathering rates increase, more and more carbon dioxide is pulled out of the atmosphere to form other chemical compounds that have no first-order effect on global temperature. As atmospheric CO2 levels begin to drop, so too does global temperature by a less effective greenhouse caused by fewer greenhouse gas molecules in the atmosphere. At the same time, weathering rates decrease as it gets colder, and fewer skeletons are precipitated because there are fewer bicarbonate and silica ions to choose from. Eventually, this results in less skeletal material being subducted, and a lower volume of volcanic carbon dioxide. Now the Earth is cooling rapidly. But as it does so, many ecosystems such as coral reefs or surface plankton regions reduce in size, and thus less atmospheric carbon dioxide is called for. In this world, the volcanoes begin to emit more carbon dioxide than can be used by organisms, and the cycle renews.

The crucial weathering rates are not just affected by temperature. The rapid rise of a mountain chain can cause an uptick in silicate mineral erosion, no matter what the temperature. Rising mountains thus cause a more rapid weathering of these minerals and the removal of more atmospheric CO2. The Earth rapidly cools. Many geologists believe that the rapid uplift of the massive and rugged Himalaya mountain chain caused a sudden drop in atmospheric CO2 levels, and thus brought on (or at least contributed to) the cooling that eventually produced the Pleistocene ice age that began some 2.5 million years ago.

A third factor affecting chemical erosion rates is the kind and abundance of plant life. “Higher” (multicellular) plants are highly efficient at causing physical erosion of rock material, thus creating more surface area for chemical weathering to act on. A sudden rise in plant abundance—or the evolution of a new kind of plant with deeper roots, such as found in most trees—has the same effect as the short-term rise of a new mountain chain: weathering rates increase, causing global temperature to decrease. The opposite—the removal of plants either through mass extinction or human-caused deforestation—causes rapid atmospheric heating.

Even the movement of continents can affect worldwide weathering rates, and hence global climate. Since weathering proceeds faster in higher temperatures, even a world in the midst of a very cold interval will get even colder if continental drift moves large continents to equatorial from higher latitudes.

Chemical weathering is quite slow in the Arctic and Antarctic, but high at the equator. Moving continents to equatorial regions will have an effect on global temperature. Another effect of continental position comes from the relative positions of the continents. No amount of chemical weathering can change global temperature if the crucial solutes and mineral species to be used to build skeletons cannot make their way to the sea. Moving water does this, but if all the continents coalesce, as they did in the formation of Pangaea some 300 million years ago, huge areas of the supercontinent interior would have been bereft of rainfall and rivers to the sea. While untold tons of bicarbonate, dissolved calcium, and silica ions would have been produced in the center of this giant continent, much of it never made it to the world ocean.

Eventually, with reduced rainfall, weathering rates would have lowered even in the higher temperatures, and the feedback system may not have worked quite as well as it does with separated continents. The far lower length of continental coastlines produced by the continental amalgamation would have severely affected world climate, as so much of formerly maritime-influenced and wetland areas would have been transformed into regions far from the sea and its water. Deserts and Arctic alike show low rates of weathering, and hence help make the world warmer by lower rate of atmospheric carbon dioxide uptake by mineral by-products of weathering.

The Phanerozoic carbon dioxide and oxygen curves
Perhaps the most influential physical factors other than temperature that most importantly influenced life’s history on Earth were the changing volumes (manifested as atmospheric gas pressures) of life- giving carbon dioxide (for plants) and oxygen (for animals). The relative amounts of both CO2 and oxygen in our planet’s atmosphere over time have been (and continue to be) determined by a wide range of physical and biological processes, and it comes as a surprise to most people that the level of both have fluctuated significantly until relatively recently in geological time. But why do the levels of these two gases change at all? The major determinants are a series of chemical reactions involving many of the abundant elements on and in the Earth’s crust, including carbon, sulfur, and iron. The chemical reactions involve both oxidation and reduction. In each case, free oxygen (O2) combines with molecules containing carbon, sulfur, or iron, to form new chemical compounds, and in so doing oxygen is removed from the atmosphere and stored in the newly formed compounds. Oxygen is liberated back into the atmosphere by other reactions involving reduction of compounds. This is what happens during photosynthesis in plants, as they liberate free oxygen as a by-product of the reduction of carbon dioxide through a complex series of intermediate reactions.

There have been a number of models specifically derived to deduce past O2 and CO2 levels through time, with the set of equations referred to as GEOCARB being the oldest and most elaborate. This model, used for calculating levels of carbon, was devised by Robert Berner of Yale University. In addition to GEOCARB, separate models have been developed by Berner and his students for calculating O2. Together, the models show the major trends in O2 and CO2 through time. This work represents one of the great triumphs of the scientific method. The importance of the rise and fall of oxygen and carbon dioxide over time is really one of the newest and most fundamental of understandings about life’s history on Earth.

Some believe that by 4 billion years ago, conditions and materials on Earth were correct for life to form. But the fact that a planet is habitable does not automatically mean that it will ever be inhabited. The formation of life from nonlife, the subject of the next chapter, appears to have been the most complex chemical experiment of all time. While astrobiologists seem to constantly refer to how “easy” it must have been to start life on Earth, a more nuanced look implies anything but.

Almost more than any other aspect, it has become clear that the interplay and concentrations of the various components of the Earth’s atmosphere have been dominant determinants of not only what kind of life (or there being any life at all) on our Earth, but the history of that life. The increasing acceptance of the dominant roles of oxygen and carbon dioxide levels in understanding not only large-scale patterns but nuances of life’s progression on our planet is in many respects a twenty-first-century innovation in interpreting Earth history. As is the understanding that two other important gases have played dominant roles in the story of life, and in the pages to come: hydrogen sulfide, or H2S, and methane (CH4). Their stories are written in rock, life, and death as well.