The historical record of Earth is evidenced in its rocks:
2 Biological Processes:
CO2 + H2O+ Sunlight ---> Carbohydrates
Carbohydrates + O2 --->Energy + CO2 + H2O
These are the most familiar forms of metabolism used by
creatures on Earth (plants and animals), but not the only ones. There
are bacteria that use sulfur or methane rather than CO2 and
O2 . There are communities of bacteria and archaea in
sediments under the ocean bottom that have these strange metabolisms, and
that may make up a third of all the biomass on Earth!
Some of these creatures live near hydrothermal vents (which we will discuss
in more detail later) and live completely independently of sunlight
or any other ecosystems. Others rely on debris from dead creatures
drifting down from the waters in the ocean, and thus indirectly rely on
The warmer water helps to dissolve the CO2 better leading to an increase in carbonate production. The increase in carbonate production removes CO2 resulting in the stabilization of temperature (this "feedback" time is ~ 400,000 yr.). Eventually the carbonate rocks are subducted, melted, and the CO2 is again outgassed by volcanos. Without the oceans, or without plate tectonics, this cycle would be much slower or nonexistent.
Stromatolites: Layers of
different types of bacteria that can grow to 30 cm in size. Relatively
rare today, but they can be found in Western Australia.
Modern, living stromatolites can be analyzed to study the different bacteria. The upper bacteria, exposed to the sun, generate energy through photosynthesis (like plants or cyanobacteria). Lower layers live off the waste products of the upper bacteria. As sediments are deposited, the stromatolites grow and the bacteria migrate, creating distinctive layers.
Fossil stromatolite-like structures have been found in Australian rocks dating to 3.5 bya. Only the layered structure is preserved -- individual cells cannot be seen. Therefore, they are not unique evidence for the existence of life because stromatolite-like structures can be made from non biological processes, but they are suggestive.
Individual Fossilized Cells: They are microscopic in size and their individual cells contain organic carbon, and what appears to be DNA molecules. Their appearance is similar to various types of bacteria that are alive today (e.g. cyanobacteria--blue-green algae). Since cyanobacteria contain chlorophyll, giving them their blue-green color, and chlorophyll drives photosynthesis, it could be inferred that the similar microscopic fossilized cells also used photosynthesis. This would indicate fairly advanced life as early as 3.5 b.y.a. Even these cells are disputed by some researchers, however, who argue that the structures are only rock crystals.
Carbon Isotopes: Living organisms prefer carbon-12 to carbon-13 because of the lighter nature of C-12. Consequently, plants and bacteria are slightly enriched in the carbon-12 isotope. All fossil organisms of all ages show this enrichment, even the stromatolite and microscopic fossils from 3.5 b.y.a. It is possible to create this isotopic enrichment through non-biological processes only under very precisely controlled laboratory conditions -- it's very unlikely this could happen in nature. Therefore, the stromatolite and microscopic fossils are biological in origin; they are "life".
Although rocks that date before 3.5 b.y.a. do not reveal fossils (due to reworking and erosion), some show a carbon isotopic enrichment signature, perhaps indicating biological activity as early as 3.85-4.5?? b.y.a. (although, it must be noted that such an enrichment could have been produced by non biological chemical reactions).
If we accept the above evidence, then we must conclude that life gained a foothold on Earth in a relatively short amount of time (100-500 Myr). Life originated quickly once the Earth became habitable. Also, it didn't take long for life to become widespread and relatively sophisticated. The bacteria present 3.5 b.y.a. were quite sophisticated, had chemistry based on DNA molecules. Some were even photosynthetic and produced O2 all through the oceans.
The earliest life on Earth was either photosynthetic or lived off methane or sulfur; there was no oxygen in the atmosphere yet, so respiration had not evolved. Many of the early organisms were anaerobic, meaning they could not survive in the presence of oxygen. These organisms disappear from surface oceans, surviving only under the seafloor, and rocks requiring the presence of oxygen (like the familiar red rocks in Arizona, containing iron oxide) appear about 2.2 bya. This means photosynthesis must have become widespread enough to alter the environment of the entire planet, paving the way for organisms using respiration.
So far all the organisms have been prokaryotes -- single-celled creatures, with no nuclei. Around 1.5 bya the first fossil evidence of eukaryotes -- cells with nuclei -- appears.
Once eukaryotic life appeared -- which took about 2 billion years after the first appearance of life on Earth -- it only took a few hundred million years for multicellular plants and animals to develop, with the Cambrian explosion producing an astonishing variety of types of animals about 540 mya.