One of the most interesting evolutionary finds in recent years is the prokaryotic form, the methanogen. Their special evolutionary significance has been described by the American biophysicist and microbiologist Carl Woese and his colleagues. The methanogens are best classified as chemoautotrophs. They live in a hot, reducing environment. Their cell walls, their enzymes, and certain of their nucleic acids differ markedly from those of other prokaryotes, although their overall cellular organization is clearly prokarvotic. But especially because the nucleic acids of their ribosomes (essential in protein synthesis) are so different from any others known, Carl Woese and his colleagues have suggested that the methanogens might merit a kingdom of their own. That suggestion has not been acted upon, but it is worth mentioning to highlight the special significance of the methanogens. For our purposes, they represent a group of extraordinary chemoautotrophs that depend on a reducing atmosphere. They could wellbe living representatives of early chemoautotrophs--of those forms that first emerged when the ecosystems were evolving and that retain a need for a reducing atmosphere.
The Gaia hypothesis. This brings us to consider somewhat more carefully the changes that occurred in the atmosphere, hydrosphere, and the surface soils of the earth as life originated and evolved. The idea that life and its environment have co-evolved has been called the Gaia hypothesis by the British scientist James E. Lovelock. As the by-products of liv-ing systems accumulated on the surface waters and in the air and the soil of the earth, living things had to adapt to ever new surroundings. We have already said that the appearance of oxygen as a by-product of photosynthesis changed the atmosphere from a reducing one to an oxidizing one. This made possible chemical respiration as we know it today. Most living things are aerobic and animals, in particular, depend on oxygen for fife. Hence the change to an oxidizing atmosphere was an essential precursor to animal life and evolution.