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Prebiotic molecules must form in a strongly reducing atmosphere. Periods of intense asteroid bombardment of Earth created a strongly reducing atmosphere for one to two million years after the bombardment. This provided a brief window for the earliest life molecules and protocells to form. This may have happened in the Hadean Eon, but it did happen after the Late Heavy Bombardment at the beginning of the Archaean Eon.
The Late Heavy Bombardment released methane (CH4) from asteroids and formed a strongly reducing environment for a brief period at the transition between the Hadean and Archaean eons. There is much stronger evidence of life (accepted by most scientists) in a delta carbon 13 test conducted on the Isua Black Shale in Greenland, which is radiocarbon dated to within 20 million years of the end of the Late Heavy Bombardment, 3.83 Ga. If this is a valid indication of life, then either biological life formed within 20 million years of the Late Heavy Bombardment or biological life somehow survived the Late Heavy Bombardment and reemerged. If life formed twice, in the mid Hadean Eon and at the end of the Late Heavy Bombardment, then evolution of microbial life might be common in the universe because periods of asteroid bombardment of young planets would seem to be a common event.
The reason that periods of asteroid and comet bombardment lead to life is that they release methane and make the atmosphere strongly reducing in the range of one million years after the bombardment. They also add hydrogen sulfide and cyanide to bodies of water, which is necessary for early life chemistry. After asteroid and comet impacts, pools might have formed in which cyanide and other compounds become concentrated and formed sediments. This would have provided a pool of compounds that could react and form biological compounds. Although life obtains energy from sunlight, the original life also required a source of energy. The energy source might have been hydrothermal pools such as at Yellowstone National Park where chemicals could cycle between hot and cold parts of the pond.
The Archaean Eon began 3.85 Ga (billion years ago) with the end of the Late Heavy Bombardment. The methane atmosphere left over from the Late Heavy Bombardment must have triggered the initial life processes within one or two million years because the methane atmosphere would have been dissipated after that. The first life was probably photosynthetic because algae is the first life in the fossil record.
Figure 5‑10. Stromatolite from Strelley Pool Chert (SPC) (Pilbara Craton) - Western Australia." Credit. Didier Descouens. Used here per CC BY-SA 4.0.
Although there is possible biogenic (low delta carbon 13) carbon just after the Late Heavy Bombardment, the first definitive signs of living organisms are stromatolites, also called bacterial mats, from 3.5 billion years ago (Figure 5‑10). The bacterial mats formed as cyanobacteria or other early photosynthesizing organisms were underlain by bacteria that consumed them. After these organisms died, another set of photosynthesizing organisms formed a layer above; thus, the stromatolites are layered structures. The following video describes life on the early earth prior to the period of plate tectonics.
The Archaean Eon ended 2.5 Ga with the Great Oxidation Event, which initiated the period of oxygen in the atmosphere. This was a disaster for the anaerobic archaebacteria, but it would eventually lead to the evolution of animals and other aerobic organisms.
Figure 5‑11. Banded iron in a 2.1-billion-year-old rock at the Museum of Mineralogy and Geology, Dresden, Germany. Credit: Andre Karwath
One of the indications that an oxidizing atmosphere (and oxidized oceans) formed 2.5 billion years ago is that iron precipitated out of the earth's oceans 2.5 billion years ago. Iron in ocean water reacted with oxygen in the water and formed iron oxide precipitates (solids). The oxygen in the ocean fluctuated, possibly with intensive algae blooms. These fluctuations formed banded iron formations, in which layers of iron were deposited on the sea floor (Figure 5‑11). Sulfur isotopes indicate that the oxidation of the atmosphere during the Great Oxidation event took place within a short span of time (a few million years), which might also indicate that a fundamental process changed, whether a change in the efficiency of photosynthesis or a change in the composition of volcanic gases.
In summary, the sequence in the Archaean and Proterozoic eons that led to plant and animal life on the modern Earth was as follows. First, an asteroid bombardment released methane into the atmosphere. Second, the crust solidified, and water bodies formed. Third, cyanosulfidic chemistry formed the basic molecules of life. Fourth, protocells evolved. Fifth, photosynthetic microorganisms evolved. Sixth, cyanobacteria evolved within the photosynthetic microorganisms. Seventh (next chapter) , eukaryotic green algae evolved. Eighth (next chapter), plants evolved from eukaryotic green algae.
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Cyanobacteria in a microbial mat, such as formed the early stromatolites. Credit: NASA.
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