Could Plate Tectonics Actually Have Inhibited the Formation of Animal Life on Earth

In this chapter we have contended that plate tectonics facilitated the rise and then the maintenance of animal life on Earth. But might not the opposite ac tually be true? Could it be that plate tectonics actually retarded the rise of animals? This is the contention of two NASA scientists, H. Hartman and C. McKay, who hypothesized that plate tectonics slowed the rate of oxygenation of the Earth atmosphere. In a 1995 article, Hartman and McKay proposed that plate tectonics slowed the rise of oxygenation on Earth and, by inference, on any planet.

As we have detailed in an earlier chapter, animal life did not arise on Earth until less than a billion years ago, whereas life on this planet antedates the first animals by about 3 billion years. One of the most puzzling aspects of life's history on Earth is this singular gap between the first life and the first animal life. Many factors were surely involved, but there is irrefutable evidence that oxygen is a necessary ingredient for animal life (at least on Earth), and there is much evidence that sufficient concentrations of oxygen were not present in the oceans and atmosphere until less than 2 billion years ago. Many scientists suspect that the long time it took for Earth to acquire an oxygen atmosphere accounts for some, or even all, of the delay between the origin of the first life and the origin of animal life on Earth. Hartman and McKay make the novel suggestion that this delay was partly due to the existence of plate tectonics on Earth.

It is universally agreed that the rise of oxygen on Earth was due to the release of free oxygen as a by-product of photosynthesis. The earliest photo-synthetic organisms used an enzymatic pathway called Photosystem 1; however, this system does not release free oxygen. The later-evolved Photosystem 2 does. This latter system may not have evolved until 2.7 to 2.5 billion years ago. Eventually, photosynthesizing organisms such as photosynthetic bacteria and single-celled plants floating in the early seas would have released vast volumes of oxygen. There was probably some source of inorganically produced free oxygen on the early Earth as well. It may be, for example, that ultraviolet rays hitting water vapor in the upper atmosphere created free oxygen, at least in small volumes. However, a net accumulation could not take place until various reducing compounds (which bind the newly released oxygen and keep it from accumulating as a dissolved gas in the oceans or as a gas in the atmosphere) were used up. For example, the amount of iron in the crust of a planet has a major effect, for all of it on the surface in contact with the atmosphere must be oxidized before free oxygen can accumulate. Such reducing compounds emanate from volcanoes, and it can be argued that planets with a higher rate of volcanicity have more reducing compounds in their oceans and atmospheres. Another important source of reducing compounds is organic compounds, produced either through the death and rotting of organisms or through the inorganic formation of organic compounds, such as amino acids. Great volumes of such material are found in the oceans on Earth, but it is usually buried in sediments. In the absence of plate tectonics, argue Hartman and McKay, such sediments become buried in sedimentary basins and are never brought back into contact with the oceans and atmosphere; thus they are removed from active participation in oceanic and atmospheric chemistry. Because they are taken out of the system, oxygen can accumulate faster than in the case where reducing compounds are constantly being rein-troduced into the atmosphere—a case where the dead don't stay buried.

Hartman and McKay make the intriguing point that Mars may have seen the evolution of complex life within 100 million years of the formation of that planet (assuming, of course, that life originated there at all). Their argument is as follows: The rapid removal of reductants on Mars through burial in deep and undisturbed sediment would have allowed oxygenation to occur much more quickly than on Earth (see Figure 9.4), where plate tectonics constantly recycles sediments via subduction, plate collision, and mountain building. All of these processes can cause previously buried sediments to be brought back up to the surface, where their reductants would once more bind whatever atmospheric oxygen was available. Hartman and McKay also point out that volcanicity on a planet like Mars that does not exhibit plate tectonics is much lower than on Earth. Thus the amount of reducing compounds (such as hydrogen sulfide) entering the atmosphere-ocean systems on Mars from volcanic sources would also have been much lower.

Could it be, then, that Earth hosted the evolution and then the maintenance of animal life in spite of plate tectonics? And that plate tectonics actually discourages the attainment of animal life on a given planet because its presence slows the accumulation of the necessary oxygen-rich atmosphere?

Mars

Complex animal life

First animal life

Eukaryotic life

Prokaryotic life

Earth

Earth

First animal life

Eukaryotic life

Prokaryotic life

Figure 9.4 Comparative evolutionary history of a planet without plate tectonics (Mars) and a world with plate tectonics (Earth) expressed in billions of years before the present.

Figure 9.4 Comparative evolutionary history of a planet without plate tectonics (Mars) and a world with plate tectonics (Earth) expressed in billions of years before the present.

We cannot fault the arguments of Hartman and McKay concerning the role of reductants in retarding oxygenation. However, we can point out that plate tectonics would surely increase the rate of biologically produced oxygen on any world, because it enhances biological productivity by recycling nutrients such as nitrates and phosphates. The net productivity on plate tectonic worlds should thus be expected to be far higher than on non-plate-tectonic worlds, so the rate of oxygenation through photosynthesis should also

be much higher on a plate tectonic world, perhaps offsetting the retardant effect of the recycling of sediment-sequestered reductants.

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