A New History of Life (4 page)

Carbon dioxide through time (in billions of years ago), with estimates of values into the future. Zero represents the present.

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 CO
₂
-rich atmosphere would have created hothouse-like conditions through a super greenhouse effect, with CO
₂
pressures ten thousand times higher than today.
¹³

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.

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 short-term and long-term carbon cycles.
¹⁴
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 long-term 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 CO
₂
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 CO
₂
. 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 CO
₂
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.
¹⁵

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.

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 CO
₂
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 (O
₂
) 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.