Layers of Rocks give us clues to their age
When rocks occur as undisturbed layers, we find that the lowest layers will be the oldest and topmost layers will be the youngest. Frequently rivers cut incisions into the earth's surface and expose such layers. The Grand Canyon in the U.S.A. is the deepest cleft on the earth's surface.
The upper rocks of this canyon are about 200 million years old and contain traces of reptiles, impressions of fern leaves and wings of insects. Halfway down the canyon you find limestone of about 400 million years old which contains the remains of primitive armoured fish. Further down the canyon there are no traces of vertebrates. Three-quarters way down there are no apparent traces of life. Close to the bottom of the canyon the rocks are more than 2 000 million years old.
Rocks as old as those of the bottom of the Grand Canyon have been found to contain a fine-grain flint-like substance called chert. Contained in this chert are simple organisms some of which resemble algae filaments others resemble bacteria.
These were thought to be until recently the earliest known organism (see further down and for a time-line of life go here) and are referred to as cyanobacteria or blue-greens. These organisms are able to extract hydrogen from water and thereby produce oxygen which is essential for other organisms to survive. The chemical agent responsible for this process is called chlorophyll and process is called photosynthesis, and occurs in true algae and higher plants.
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Minik & Frei (2004) wrote a paper that concluded that "Planktonic organisms lived in the Isuan oceans where they produced large amounts of highly 13C-depleted organic matter. The aquatic environment of these organisms comprised relatively oxidized compartments, which allowed solute transport of U. The high biomass productivity of planktonic organisms, the strongly 13C-depleted carbon isotopic signature and the evidence for the presence of oxidized aquatic environments all suggest that oxygenic photosynthesis had developed before 3700 Ma."