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).
OXYGEN IN THE ATMOSPHERE
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.