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Fossil Record

The fossil record is the set of remains or traces of past organisms identified in fossiliferous rock formations and sedimentary strata (the stratigraphical record) from previous geologic ages to present. Both strati-graphical and fossil records constitute the geological record. The fossil record, or paleontological record, is a primary source of information for evaluating the history of life, including evolutionary processes and extinction events. Paleontologists have studied the fossils of many thousands of species that lived in the past. They have identified evolutionary successions through time, manifesting the morphological transitions from some species to others.

The fossil record confirms a basic prediction of evolutionary theory: the past organisms were quite different from present ones. Furthermore, the farther back in geological time we look, the simpler and more morphologically similar are the larger taxonomie groups that became fossilized, precisely what to expect if the hypothesis that all life on Earth originated from a universal common ancestor were true. Other features are observed in the fossil record that provide evidence for evolutionary theory: transitional fossils between ancient species and more modern ones, consistent hierarchy in homologies and analogies of past organisms, paleobiogeographic data that show that proposed descendants appeared in the same general area as their predecessors, and so on. Exceptional outcrops containing very well-preserved fossils of past fauna and flora, what is called a Konservat-Lagerstatte, have been found worldwide for the different geological time periods and epochs.

Archean Fossil Record

The oldest fossil traces are the stromatolites of 3,500 million years ago (Paleoarchean era, Archean eon). The stromatolites consist of attached, lith-ified growth structures, accretionary away from a point or limited surface of initiation. The result is a laminated rock built of layer upon layer of sediment and précipitants. The stromatolites are formed by the trapping, binding, and cementation of sedimentary grains by microorganisms, especially cyanobacteria. The first stromatolites were probably built by green sulfur-bacteria and purple bacteria, and they were not very abundant. However, the stromatolites became common during the Paleoproterozoic and Mesoproterozoic eras (early and middle Proterozoic eon, between 2,500 and 1,000 million years ago [mya]), due probably to the great proliferation of colonial cyanobacteria.

The first putative fossil bacteria were found in a stromatolitic formation from the Warrawoona Group (Western Australia) of 3,460 mya. They seem to belong to the domain Eubacteria, but microfossils of extremophilic domain Archaea have also been identified in the Archean eon. Archean life was surely limited to simple nonnucle-ated single-celled microorganisms, or prokaryotes (bacteria). There are no known eukaryotic fossils in Archean rocks, although they might have evolved then and simply not left any fossil. Cyanobacteria and oxygenic photosynthesis first evolved in the Mesoarchean era (3,200–2,800 mya), and were common during the Neoarchean era (2,800–2,500 mya). The combination of oxygen released by photosynthetic cyanobacteria and dissolved iron in oceans formed a distinctive Archean (and Proterozoic) type of rocks, called banded iron formations.

Proterozoic Fossil Record

Pre-Ediacaran Time

While it was involved in oxidizing the oceanic iron and precipitating banded iron oxides (magnetite), biogenic oxygen could not be accumulated in the atmosphere, which continued to be a reducing environment, containing less and less oxygen. This geological process ended in the Proterozoic eon. At that time, the progressive accumulation of oxygen in the atmosphere might have caused the extinction of numerous groups of anaerobic bacteria, what sometimes has been called the oxygen catastrophe of the Paleoproterozoic era (2,500–1,600 mya). It is considered that the atmosphere changed to oxygen-rich during the Orosirian period (2,050–1,800 mya), since the banded iron formations were abundant in the previous periods, mainly during the Siderian period (2,500–2,300 mya) but also during the Rhyacian period (2,300–2,050 mya).

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