Prehistoric Life 2

by Carl Strang

This year’s winter series is a review of the prehistoric life and geologic history of northeast Illinois. Each chapter will summarize current understanding, gleaned from the literature, of what was going on with life on Earth in a particular span of time, what we know about the local landscape, and what we can say about local life. The Earth is so old that every imaginable environment was here at some point, from ocean depths to mountaintops, from equatorial tropics to tundra, and from wetlands to desert.

Archean Eon (4-2.5 billion years ago)

This eon began with the earliest Earth rocks. Bacteria and other prokaryotes were the only life by the end of the Archean, and they began influencing the planet’s geochemistry. The eon’s division into four eras appears to have been done mathematically, with a 300 million year length for the Neoarchean Era and 400 million for each of the earlier others (in order, Eoarchean, Paleoarchean and Mesoarchean Eras).

Life on Earth. The origin of life in the Archean eon is somewhat speculative. It has been demonstrated that organic chemicals formed in outer space and carried to Earth by comets and meteorites were capable of forming vesicles in water that resemble living cells in some ways. Alternatively, the compounds may have formed near volcanic plumes (Science 300:745) or in the atmosphere (Science 308:1014), or around hydrothermal vents (which some argue are the most likely energy source for the first living forms). The main thought at present is that the first step was the formation of RNA, followed by the RNA replicating itself, followed by some of it getting contained within fatty acid vesicles that were the first membranes, and the whole reproducing and taking in new material. Ultimately DNA formed from RNA. All of life probably descended from a single original micro-organism. (Science 323:198). Ribosomal RNA, of a self-replicating type, when inside the kind of fatty acid vesicle that can form spontaneously, creates osmotic tensions that can lead to the absorption of additional membrane material. Competition between similar combinations of components would introduce a natural selection process very early (Science 314:1558).

The earliest fossil life forms were stromatolites, mats of microbes (mainly cyanobacteria) that formed rocklike reefs, perhaps as far back as 3800 million (3.8 billion) years ago. They lived in shallow seas, and some can be found living in the Bahamas and in Australia today. They were preceded by organisms like today’s bacteria (some, called archaea or achaebacteria, are a different division of life. One of these, a recently discovered deep sea thermal vent species, can survive hotter temperatures than any other known life form. It uses what may be the oldest form of microbe respiration [Science 301:934]). More recent genetic evidence supports the idea that the first organism, ancestral to both bacteria and archaea, lived in a very hot environment (Science 311:1283). The first organisms obtained energy for life by chemically manipulating substances such as hydrogen, hydrogen sulfide, and methane. A huge step came when an organism evolved photosynthesis, the ability to oxidize (acquire electrons from) water, thus capturing energy and producing oxygen as a byproduct. This made possible the stromatolites mentioned above. The Archean eon has been called by some the Age of Prokaryotes. There was at best a trace of oxygen in the Archean atmosphere. The main organic matter consumers may have been methane-producing bacteria. The resulting methane was one major greenhouse gas keeping the planet warm then (Science 298:2341), but it also reacted with nitrogen to produce an atmospheric haze that protected ammonia, another greenhouse gas, from being destroyed by ultraviolet light (Science 328:1266). Recent evidence suggests that the Earth’s magnetic field was active in the Archean, at 50-70% of today’s strength, and this also would have helped shield early life from solar damage (Science 327:1238). Life was limited to aquatic environments.

Local landscape. The farther back in time we look, the less information is available to tell us where Illinois’ part of the Earth’s crust was located, and what it was like. At first, the Earth was still so hot, and plate tectonics so active, that Archean continents were kept small. Only 30-40% of the present volume of continental crust existed by the end of the Archaean. Pieces of today’s continents were scattered all over, in much different relationships than we have today. There is inadequate remaining geological material to tell us whether our part of the Earth’s crust even existed in this eon.

Local life. Any life that may have been here that early was very simple, unicellular though possibly colonial, marine, and limited to prokaryotic organisms.

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