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Main articles: Common descent, Origin of life
A central assumption of evolutionary theory is that life on Earth had a single point of origin; all subsequent life-forms are descendents of this progenitor organism. This is called the theory of common descent.
Evidence for common descent may be found in shared traits between living organisms. For example, all living things make use of nucleic acids as their genetic material, and use the same twenty amino acids as the building blocks for proteins. Furthermore all organisms use the same genetic code (with some extremely rare minor deviations) to translate nucleic acid sequences into proteins. Because the selection of these traits is somewhat arbitrary, their universality strongly suggests common ancestry.
Phylogeny, the study of the ancestry of species, has revealed that biological structures with radically different internal organizations can bear a superficial resemblance and perform similar functions. These examples of analogous structures show that there are many ways to perform the same actions. For example, the eye was evolved independently in radically different ways in organisms such as humans and octopuses. Likewise, other structures with similar internal organisation may perform divergent functions. Vertebrate limbs are a favorite example of such homologous structures. Other vestigial structures may exist without purpose in one organism, though they have a clear purpose in others. The human wisdom teeth and appendix are common examples.Further evidence of the universal ancestry of life is that abiogenesis has never been observed under controlled conditions, indicating that the origin of life from non-life, is either very rare or only happens under conditions that are not at all like those of modern Earth.
Since abiogenesis is rare and common descent (especially macroevolution) is a slow process, global biological diversity requires that the Earth is very old. This is compatible with geological evidence that the Earth is approximately 4.6 billion years old. (See Timeline of evolution.)
Information about the early development of life includes input from the fields of geology and planetology. These sciences provide information about the history of the Earth and the changes produced by life. Much information about the early Earth has been destroyed by time. Fossils are important for estimating when various lineages developed. Fossil evidence of life's evolution only exists for relatively recent developments. As fossilization is a rather rare occurrence, this only provides sparse information about the evolution of life.
Since metabolic processes do not leave fossils, research into the evolution of the basic cellular processes is done largely by comparison of existing organisms. Many lineages diverged at different stages of development, so it is possible to determine when certain metabolic processes appeared by comparing the traits of the descendants of a common ancestor. However, not even comparative biology can shed much light on the earliest development of life since all existing organisms share certain traits, including the cellular structure, infection by viruses, and the genetic code. Most scientists interpret this to mean that all existing organisms share a common ancestor, which had already developed the most fundamental cellular processes, but there is no scientific consensus on the relationship of the three domains of life ( Archea, Bacteria, Eukaryota), the origins of viruses, or the origin of life. Attempts to shed light on the earliest history of life generally focus on the behavior of macromolecules, particularly RNA, and the behavior of complex systems.
Though the origins of life are murky, other milestones in the evolutionary history of life are well-known. The emergence of oxygenic photosynthesis (c. 3 billion years ago) and the subsequent emergence of an oxygen-rich, non-reducing atmosphere can be traced through the formation of banded iron deposits, and later red beds of iron oxides. This was a necessary prerequisite for the development of aerobic cellular respiration, believed to have emerged c. 2 billion years ago. In the last billion years, simple multicellular plants and animals began to appear in the oceans. Soon after the emergence of the first animals the Cambrian explosion (a period of unrivaled and remarkable, but brief, organismal diversity documented in the fossils found at the Burgess Shale) saw the creation of all the major body plans ( phyla) of modern animals. About 500 million years ago, plants and fungi colonized the land, and were soon followed by arthropods and other animals, leading to the development of land ecosystems with which we are familiar.
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