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Although most humans recognize that variances occur within a species, it is often a point of dispute as to what these differences entail, their importance, and whether discrimination based on race ([[racism]]) is acceptable. Some societies have placed a great deal of emphasis on race, while others have not.
=== Evolution ===
{{main | Human evolution}}
[[Image:Pekingthr.jpg|thumb|left|250px|Reconstructed skull of ''[[Peking Man]]'', a representative of the extinct species believed to be the nearest ancestor of ''Homo sapiens'', ''[[Homo erectus]]''.]]
The study of [[human evolution]] encompasses the development of the genus ''[[Homo (genus)|Homo]]'', but usually involves studying other [[hominidae|hominids]] and [[homininae|hominines]] as well, such as the [[australopithecines]]. "Modern humans" are defined as the ''Homo sapiens'' [[species]], of which the only extant [[subspecies]] is ''Homo sapiens sapiens''; ''[[Homo sapiens idaltu]]'' (roughly translated as "elder wise man"), the other known subspecies, is extinct.<ref>[http://www.anth.ucsb.edu/projects/human/# Human evolution: the fossil evidence in 3D], by Philip L. Walker and Edward H. Hagen, Dept of Anthropology, University of California, Santa Barbara, retrieved [[April 5]], [[2005]].</ref>
The closest living relatives of ''Homo sapiens'' are the [[Common Chimpanzee]] and the [[Bonobo]]. Full [[genome]] sequencing resulted in the conclusion that "After 6.5 [million] years of separate evolution, the differences between chimpanzee and human are just 10 times greater than those between two unrelated people and 10 times less than those between rats and mice." In fact, chimpanzee and human DNA is 96% identical.<ref>[http://news.ft.com/cms/s/43445728-1a44-11da-b279-00000e2511c8.html Human and chimp DNA is 96% identical], by Clive Cookson, ''Financial Times'', [[August 13]], [[2005]], retrieved [[March 17]], [[2006]].</ref> It has been estimated that the human [[lineage (evolution)|lineage]] diverged from that of chimpanzees about five million years ago, and from [[gorilla]]s about eight million years ago. However, a hominid skull discovered in [[Chad]] in 2001, classified as ''[[Sahelanthropus tchadensis]]'', is approximately seven million years old, which may indicate an earlier divergence.
There are two prominent scientific theories of the origins of contemporary humans. They concern the relationship between modern humans and other hominids. The [[recent single-origin hypothesis|single-origin]], or "out of Africa", hypothesis proposes that modern humans evolved in Africa and later [[human migration|migrated]] outwards to replace hominids in other parts of the world. The [[multiregional hypothesis]], on the other hand, proposes that modern humans evolved, at least in part, from independent hominid populations.<ref>Eswaran, Vinayak, Harpending, Henry & Rogers, Alan R. ''[http://www.sciencedirect.com/science/article/B6WJS-4G3SC6X-1/2/aae7c2810f0d87628e228363c0e1bd66 Genomics refutes an exclusively African origin of humans]'', Journal of Human Evolution, In Press, Corrected Proof, retrieved [[May 6]], [[2005]].</ref>
Geneticists Lynn Jorde and [[Henry Harpending]] of the [[University of Utah]] proposed that the variation in human DNA is minute compared to that of other species, and that during the [[Late Pleistocene]], the human population was reduced to a small number of breeding pairs—no more than 10,000—resulting in a very small residual gene pool. Various reasons for this hypothetical bottleneck have been postulated, the most popular being the [[Toba catastrophe theory]].
Human evolution is characterized by a number of important physiological trends, including the [[encephalization|expansion]] of the brain cavity and brain itself, which is typically 1,400 cm³ in volume, over twice that of a chimpanzee or gorilla ([[Cranial capacity|compare capacities]]). The pattern of human postnatal brain growth differs from that of other apes ([[heterochrony]]), allowing for an extended period of [[language acquisition|social learning]] in juvenile humans. [[Physical anthropology|Physical anthropologists]] argue that a reorganization of the structure of the brain is more important than cranial expansion itself. Other significant evolutionary changes included a reduction of the [[canine tooth]], development of [[bipedal locomotion]], and the descent of the [[larynx]] and [[hyoid bone]], making speech possible. How these trends are related and what their role is in the evolution of complex social organization and culture are matters of ongoing debate in the field of [[physical anthropology]].<ref>Boyd, Robert & Silk, Joan B. (2003). ''How Humans Evolved''. New York: Norton & Company. ISBN 0393978540.</ref><ref>Dobzhansky, Theodosius (1963). ''Anthropology and the natural sciences-The problem of human evolution'', ''Current Anthropology'' '''4''' (2): 138-148.</ref>
=== Habitat and population ===
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