Fossils and the history of life - Summary
• Fossils are formed when the remains of an organism are preserved in the sediment deposited at the bottom of the water column; the sediment may then form a sedimentary rock by compaction over time. If that sedimentary rock is later exposed at the surface of the earth, the fossils can be removed from it.
• The history of the earth is divided into a series of time stages. Most fossils are from organisms that lived in the past 600 million years. The 600 million year period is divided into three Eras (Paleozoic, Mesozoic, Cenozoic); the Eras in turn are divided into successively into Periods and Epochs.
• Rock ages can be measured absolutely using their radio-isotopic composition, and relatively by correlating their fossil content with other rocks elsewhere. Magnetic time zones also provide useful chronological evidence.
• The fossil record provides evidence of the history of life:
- life originated over 3.5 billion years ago; - prokaryotic cells already existed 3.5 billion years ago; - eukaryotic cells originated maybe 1.8 billion years ago;- photosynthesis expanded at a similar time, and caused an increase in the oxygen concentration of the Earth's atmosphere; - multicellular life appear first, in soft-bodied animals, in the Ediacarian faunas of 670 - 550 million years ago; - hard-bodied animals evolved about 550 million years ago and their appearance led to a much richer fossil record, starting with the Cambrian;- terrestrial ecosystems were colonized from the Ordovician onward, and the fossil record provides good evidence concerning the evolution of reptiles, and the origin of birds and of mammals from reptilian ancestors.
• The completeness of the fossil record can be studied, at a gross level, by the geological continuity of the rock sequence; and at a finer level by estimating the percentage of the total time range represented by the rocks during which sediments were actually being deposited.
• The evolution of mammals from reptiles is an example of adaptive evolution and the fossil record reveals that it proceeded in a series of stages, through various groups of mammal-like reptiles.