Relative dating atmossphere
In the late Precambrian, the first multicellular organisms evolved, and sexual division developed.By the end of the Precambrian, conditions were set for the explosion of life that took place at the start of the Phanerozoic Eon.The presence of fossil molecules in the cell walls of 2.5-billion year-old blue-green algae (cyanobacteria) establishes the existence of rare oxygen-producing organisms by that period.Oceans of the Archean Eon (4.0 to 2.5 billion years ago) contained much volcanic-derived ferrous iron (Fe) in BIFs.Several rock types yield information on the range of environments that may have existed during Precambrian time.
A second major peak, which raised atmospheric oxygen levels to 50 percent PAL, was reached by 600 million years ago.
All life-forms were long assumed to have originated in the Cambrian, and therefore all earlier rocks were grouped together into the Precambrian.
Although many varied forms of life evolved and were preserved extensively as fossil remains in Cambrian sedimentary rocks, detailed mapping and examination of Precambrian rocks on most continents have revealed that additional primitive life-forms existed as early as about 3.48 billion years ago.
) are easily destroyed in an oxidizing atmosphere; confirmation of a reducing atmosphere is provided by unoxidized grains of these minerals in 3.0-billion-year-old sediments.
However, the presence of many types of filamentous microfossils dated to 3.45 billion years ago in the cherts of the Pilbara region suggests that photosynthesis had begun to release oxygen into the atmosphere by that time.
If the uniformitarian principle that the present is the key to the past is valid (meaning the same geologic processes occurring today occurred in the past), then sediments laid down during the Precambrian would have likewise been controlled by the movement and geographic position of the continents.