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Human evolution is a multidisciplinary scientific inquiry which seeks to understand and describe the origin and development of humanity. Human evolution draws from broad scientific disciplines, most notably physical anthropology and genetics. The term human, in the context of human evolution, refers to the genus Homo, but studies of human evolution usually include other non-human hominids, such as the Australopithecines.
The modern field of paleoanthropology is said to have begun with the discoveries of Neanderthal man in the 19th century. After Charles Darwin published Origin of Species in 1859, and together with the discoveries of Neanderthal and other important specimens, the controversial idea that humans developed from other species arose.
Darwin speculated, based on morphological similarity, that our closest living relatives were chimpanzees and gorillas. Based on the natural range of these creatures, Darwin surmised that human ancestor fossils would ultimately be found in Africa and that humans share a common ancestor with African apes.
It was not until the 1920s that fossils other than neanderthalensis were discovered. In 1925, Raymond Dart described Australopithcecus africanus. The type specimen was the Taung child, an Australopithecine infant. The remains were a remarkably well preserved tiny skull and an endocranial cast of the individual's brain. Although the brain was small (410 cc), its shape was rounded, unlike that of chimpanzees and gorillas, more like a modern human brain. Also, the specimen exhibited short canine teeth and the position of the foramen magnum was evidence of bipedial locomotion. All these traits convinced Dart that the Taung baby was a bipedal human ancestor, a transitional form between "apes" and humans. Another 20 years would go by before Dart's claims were taken seriously, following the discovery of more fossils that resembled Dart's find. The prevailing view of the time was that a large brain evolved before bipedal locomotion. It was thought that intelligence on par with modern humans was a prerequistite to bipedalism.
The Australopithecines are now thought to be the immediate ancestors of the genus Homo, the group to which modern humans belong. Both Australopithecines and Homo are part of the family Hominidae, but recent data has brought into doubt A. Africanus' position as a direct ancestor of modern humans; it may well have been a dead-end cousin. The Australopithecines were originally classified as either gracile or robust. The robust variety of Australopithecus has since been reclassified as Paranthropus. (In the 1930's when the robust specimens were first described, the Paranthropus genus was used. During the 1960s the robust variety was moved into Australopithceus. The recent trend has been back to the original classification as a separate genus.)
The Australopithecus genus
Australopithecus anamensis
Australopithecus africanus
Paranthropus aethiopicus
Paranthropus boisei
Paranthropus robustus
The Homo genus
In modern taxonomy, Homo sapiens is the only extant species of its genus, Homo. Likewise, the ongoing study of the origins of Homo sapiens often demonstrates that there were other Homo species, all of which are now extinct. While some of these other species might have been ancestors of H. sapiens, many were likely our "cousins", having speciated away from our ancestral line. There is not yet consensus as to which of these groups should count as separate species and which as subspecies of another species. In some cases this is due to the paucity of fossils, in others due to the very slight differences used to distinguish species in the Homo genus.
Lived from about 2.4 to 1.5 million years ago (MYA). H. habilis, the first species of the genus Homo, evolved in South and East Africa in the late Pliocene or early Pleistocene, 2.5–2 MYA, when it diverged from the Australopithecines. H. habilis had smaller molars and larger brains than the Australopithecines, and made tools from stone and perhaps animal bones.
Lived from about 1.8 (including ergaster) or from about 1.25 (excluding ergaster) to 0.07 MYA. In the Early Pleistocene, 1.5–1 MYA, in Africa, Asia, and Europe, presumably H. habilis evolved larger brains and made more elaborate stone tools; these differences and others are sufficient for anthropologists to classify them as a new species, Homo erectus. A famous example of Homo erectus is Peking Man; others were found in Indonesia, and sites in Africa and Europe.
Lived from about 1.8 to about 1.25 MYA. Also proposed as Homo erectus ergaster
(Heidelberg Man) lived from about 500 TYA to about 300 TYA. Also proposed as Homo sapiens heidelbergensis and Homo sapiens paleohungaricus.
Lived from about 160 TYA (proposed subspecies). Is the oldest anatomically modern human known.
From about 12 TYA (announced 2004). Nicknamed hobbit for its small size.
Lived from about 250 to 30 TYA. Also proposed as Homo sapiens neanderthalensis. There is ongoing debate over whether the "Neanderthal Man" was a separate species, Homo neanderthalensis, or a subspecies of H. sapiens. While the debate remains unsettled, the preponderance of evidence, collected by examining mitochondrial DNA and Y-chromosomal DNA, currently indicates that there was no gene flow between H. neanderthalensis and H. sapiens, and therefore the two were separate species. In 1997 Dr. Mark Stoneking, then an associate professor of anthropology at Penn State University, stated: "These results [based on mitochondrial DNA extracted from Neanderthal bone] indicate that Neanderthals did not contribute mitochondrial DNA to modern humans… Neanderthals are not our ancestors."² Subsequent investigation of a second source of Neanderthal DNA confirmed these findings.³
Lived from about 200 thousand years ago (TYA) to the present. Between 400,000 years ago and the second interglacial period in the Middle Pleistocene, around 250,000 years ago, the trend in cranial expansion and the elaboration of stone tool technologies developed, providing evidence for a transition from H. erectus to H. sapiens. The direct evidence suggests that there was a migration of H. erectus out of Africa, then a further speciation of H. sapiens from H. erectus in Africa. (There is little evidence that this speciation occurred elsewhere.) Then a subsequent migration within and out of Africa eventually replaced the earlier dispersed H. erectus. However, the current evidence doesn't preclude multiregional speciation, either. This is a hotly debated area in paleoanthropology.
Additional notes
The origins of humanity is often a subject of great political and religious controversy. See: creationism.
The classification of humans and their relatives has changed considerably over time. See the history of hominoid taxonomy.
Speculation about the future evolution of humans is often explored in science fiction. Sometimes evolution to a being of pure spirit is imagined, sometimes continued speciation as humans fill various ecological niches; see adaptive radiation.
Human evolution has possibly reached a peculiar point of development. Their rational understanding of the physical environment and their application of scientific knowledge has given them an unprecedented ability to adapt habitats to their wants and needs (i.e. agricultural development). Many believe this capacity reduces or prevents many theoretical mechanisms believed to be causing evolution. This is an oversimplified view however. It is true surviving well past maturation in industrialized nations is sociologically and technologically protected, thus reducing many of the selective pressures that existed in their former environments, but survival is not the only selective criterion for evolution and factors like reproductive success (i.e. sexual evolution) still vary for a myriad of potential reasons.
Other circumstances, like the scope and connectivity of the human population, will also tend to prevent mechanisms like cladogenesis, thus reducing biodiversity, but other mechanisms like genetic drift and the reduction in selective pressures could possibly cause anagenesis. Most of the natural changes will likely have the character of a negative adaptation (e.g. myopia becomes more and more common due to a lack of selective pressure for superior vision) however, but this is a human characterization of change that is dependent upon human goals and value systems.
As science and technology advances it is possible they will be able to not only consciously adapt their environment to their needs, but adapt their genetic information as well. This new form of evolution by design could more than compensate for the elimination of the natural mechanisms driving evolution. Beyond that it is possible they will abandon their biological machinery in favor of completely artificial systems.
See also
External links
References
- Wolfgang Enard et al. "Molecular evolution of FOXP2, a gene involved in speech and language." Nature, Vol 418 (22 August 2002) p. 870.
- DNA Shows Neandertals Were Not Our Ancestors (http://www.psu.edu/ur/NEWS/news/Neandertal.html)
- Ovchinnikov, et al. "Molecular analysis of Neanderthal DNA from the Northern Caucasus." Nature 404, 490 (2000).
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