3.5 billion to 700 million years ago


The first fossils of prokaryotic cells are known from 3.5 billion years. Prokaryotic cells were the only living organisms which inhabited the earth for a period of about 1.5 billion years.

It is estimated that the earth formed 4.6 billion years ago (billion years ago). The oldest meteorites date to about this time. Since the earth would have been semi-molten for quite some time, there are no modern rocks that date older than about 4 billion years.
In rocks that are 3.8 billion years ago, there are carbon isotope ratios that suggest the presence of living organisms. There are different forms of carbon called isotopes. Living things use a certain carbon isotope (12C) and exclude a second isotope (13C) while neither is discriminated against in the formation of minerals. After more than 10,000 analyses of carbon ratios in rock sediments, there is only one period in which a qualitative difference is observed. At around 3.8 billion years ago, the ratios of carbon isotopes in certain sediments changed from those made in the absence of life to those made in association with living things. This analysis suggests that living things were present on earth 3.85 billion years ago. This change in carbon isotopes occurred shortly after the appearance of liquid water on the planet (Brocks, 1999; Schidlowski, 1988; Holland, 1997). In the rock strata of Isua, Greenland, there are "fossils" of small organic material which are 3.8 billion years old. Although there is organic matter inside them and there are examples of them splitting (budding in the way that yeast do), it is not clear whether they are living cells or nonliving microspheres (Pflug, 1979; Roedder, 1981; Bridgwater, 1981; Dunlop, 1978).

There are two different types of cells on earth today. Prokaryotic cells are represented by modern bacteria and are the simplest type of cell. Prokaryotic cells are small (typically 10,000 times smaller than eukaryotic cells) and lack membrane-bound internal compartments called organelles. There are 2 major divisions of modern bacteria: the eubacteria and the archebacteria (or archaea). The eubacteria make up most of the bacteria humans commonly encounter while the archaea inhabit many of earth's harshest environments. The first fossils of prokaryotic (bacterial) cells are known from 3.5 billion years ago. These bacteria were photosynthetic (although non-oxygen producing) so it is likely that simpler non-photosynthetic bacteria evolved prior to this (Schopf, 1987). Prehistoric photosynthetic bacteria formed large mounds called stromatolites which could reach 30 ft in height. Today, bacterial stromatolites exist in Australia and the fossil record shows that they were once much more common worldwide. Stromatolites are even known from the Archaeozoic Eon from 3 continents, including some which are 3.5 billion years old. Younger stromatolites contain bacterial fossils (Lowe, 1980; Walter, 1980).

Bacteria are the only known inhabitants of earth for at least the first 1.5 billion years in the history of life. Genetic comparisons suggest that the oldest, most primitive bacterial lineages were anaerobic and the ability to survive in the presence of oxygen evolved separately in different lineages. Photosynthesis seems to be an ancient process, present in half of the major eubacterial lineages (including a number in which photosynthetic ancestors seem to have given rise to nonphotosynthetic descendants (Fox, 1980).

Three billion years ago, cyanobacteria (a type of bacteria better known by the somewhat incorrect name "blue green algae") evolved which release oxygen during photosynthesis. The first bacteria probably released hydrogen and hydrogen sulfide in photosynthesis, instead of oxygen. By 2.7 billion years ago, stromatolite-forming microbes seem to have used oxygenic photosynthesis (Buick, 1992). The amount of oxygen in the atmosphere increased from significantly between 2.4 and 2.2 billion years ago (Rye, 1998). By 2.2 billion years ago, chains of cyanobacteria existed with enlarged cells (that were probably specialized for tasks such as nitrogen metabolism as in modern species)-this is the first known example of cell specialization (Nagy, 1974; Schopf).
The earliest atmosphere did not have much oxygen gas. Before photosynthesis, so much oxygen would have been absorbed by reduced geothermal outputs and by weathering rocks that very little would have been present in the atmosphere. The only source of oxygen gas would have been the dissociation of water molecules by sunlight (Des Marais, 2000). This is evident in observing the sediments formed at that time-metals didn't rust and minerals which form in the presence of oxygen are absent. By 2.1 billion years ago there was enough oxygen in the atmosphere to rust iron for the first time and by about 1.8 billion years ago oxygen was oxidizing significant amounts of iron. Other signs of oxygen also appear about this time such as sterane biomarkers from 1.7 billion years ago and oxidized sulfur deposits from about 1.6 billion years ago.