Human evolution

Human evolution is the part of the theory of evolution by which human beings emerged as a distinct species. It is the subject of a broad scientific inquiry that seeks to understand and describe how this change and development occurred. The study of human evolution encompasses many 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 hominins, such as the australopithecines.


History of paleoanthropology

The modern field of paleoanthropology began with the discovery of 'Neanderthal man'; and evidence of other 'cave men' in the 19th century. The idea that humans are similar to certain great apes had been obvious to people for some time, but the idea of the biological evolution of species in general was not legitimized until after Charles Darwin published On the Origin of Species in 1859. Though Darwin's first book on evolution did not address the specific question of human evolution— "light will be thrown on the origin of man and his history," was all Darwin wrote on the subject— the implications of evolutionary theory were clear to contemporary readers. Debates between Thomas Huxley and Richard Owen focused on the idea of human evolution, and by the time Darwin published his own book on the subject, Descent of Man, it was already a well-known interpretation of his theory— and the interpretation which made the theory highly controversial. Darwin believed that Caucasians evolved from chimpanzees, that Africans evolved from the less intelligent yet stronger apes, and that Asians evolved from orangutangs. Even many of Darwin's original supporters (such as Alfred Russel Wallace and Charles Lyell) balked at the idea that human beings could have evolved their apparently boundless mental capacities and moral sensibilities through natural selection.

Since the time of Carolus Linnaeus, the great apes were considered the closest relatives of human beings, based on morphological similarity. In the 19th century, it was speculated that our closest living relatives were chimpanzees and gorillas, and based on the natural range of these creatures, it was surmised humans share a common ancestor with African apes and that fossils of these ancestors would ultimately be found in Africa.

It was not until the 1920s that fossils other than neanderthalensis were discovered. In 1924, Raymond Dart described Australopithecus africanus. The type specimen was the Taung Child, an australopithecine infant discovered in Taung, South Africa. The remains were a remarkably well-preserved tiny skull and an endocranial cast of the individual's brain. Although the brain was small (410 cm³), its shape was rounded, unlike that of chimpanzees and gorillas, and more like a modern human brain. Also, the specimen exhibited short canine teeth, and the position of the foramen magnum was evidence of bipedal locomotion. All of these traits convinced Dart that the Taung baby was a bipedal human ancestor, a transitional form between apes and humans. Another 20 years would pass before Dart's claims were taken seriously, following the discovery of more fossils that resembled his find. The prevailing view of the time was that a large brain evolved before bipedality. It was thought that intelligence on par with modern humans was a prerequisite 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 sapiens are part of the tribe Hominini, but recent data has brought into doubt the position of A. africanus 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 1930s, when the robust specimens were first described, the Paranthropus genus was used. During the 1960s, the robust variety was moved into Australopithecus. The recent trend has been back to the original classification as a separate genus.

Before Homo

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 a 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 other cases it is due to the slight differences used to classify species in the Homo genus.

The word homo is Latin for 'person', chosen originally by Carolus Linnaeus in his classification system. It is often translated as 'man', although this can lead to confusion, given that the English word 'man' can be generic like homo, but can also specifically refer to males. Latin for 'man' in the gender-specific sense is vir, cognate with "virile" and "werewolf". The word 'human' is from humanus, the adjectival form of homo.

Homo habilis

H. habilis 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. One of the first known hominids, it was nicknamed 'handy man' by its discoverer, Louis Leakey.

Homo erectus

H. erectus (including H. ergaster) lived from about 1.8 MYA (or from about 1.25 MYA excluding ergaster) to 0.07 MYA. In the Early Pleistocene, 1.5–1 MYA, in Africa, Asia, and Europe, presumably, Homo 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, H. erectus. In addition H. erectus was the first human ancestor to walk truly upright. This was made possible by the evolution of locking knees and a different location of the foramen magnum (the hole in the skull where the spine enters). A famous example of Homo erectus is Peking Man; others were found in Asia (notably in Indonesia), Africa, and Europe. Many paleoanthropologists are now using the term Homo ergaster for the non-Asian forms of this group, and reserving H. erectus only for those fossils found in the Asian region and meeting certain skeletal and dental requirements which differ slightly from ergaster. They may have used fire to cook their meat.

Homo ergaster

H. ergaster lived from about 1.8 to about 1.24 MYA. Also proposed as Homo erectus ergaster

Homo heidelbergensis

H. heidelbergensis (Heidelberg Man) lived from about 800 thousand years ago (TYA) to about 300 TYA. Also proposed as Homo sapiens heidelbergensis and Homo sapiens paleohungaricus.

Homo sapiens idaltu

H. sapiens idaltu lived from about 160 TYA (proposed subspecies). It is the oldest known anatomically modern human.

Homo floresiensis

H. floresiensis, which lived to about 12 TYA (announced 28 October 2004 in the science journal Nature), has been nicknamed hobbit for its small size, probably a result of Island dwarfing. H. floresiensis is intriguing both for its size and its age, being by far the most recent species of Homo that does not lie along the direct evolutionary path of modern humans. Found in 2003 it has been dated to approximatly 18,000 years old. The main find was a fossil believed to be a woman of about 30 years of age. Her brain size was only 380cc (which can be considered small even for a chimp). She was only 1 meter in height.

Homo neanderthalensis

H. neanderthalensis 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 prevailing view of evidence, collected by examining mitochondrial DNA and Y-chromosomal DNA, currently indicates that little or no gene flow occurred 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 Pennsylvania 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 However, supporters of the multiregional hypothesis point to recent studies indicating non-African nuclear DNA heritage dating to one MYA, as well as apparent hybrid fossils found in Portugal and elsewhere, in rebuttal to the prevailing view.

Homo sapiens

H. sapiens has lived from about 200 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 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 does not preclude multiregional speciation, either. This is a hotly debated area in paleoanthropology. Sapiens means wise or intelligent. Current research establishes that human beings are highly genetically homogenous, meaning that the DNA of individual Homo Sapiens is more alike than usual for most species, a result of our relatively recent evolution. Distinctive genetic characteristics have arisen however, primarily as the result of small groups of people moving into new environmental circumstances. Such small groups are initially highly inbred, allowing the relatively rapid transmission of traits favorable to the new environment. These adapted traits are a very small component of the Homo Sapiens genome and include such outward "racial" characteristics as skin color and nose form in addition to internal characteristics such as the ability to breathe more efficiently in high altitudes.

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