Human evolution

Human evolution is the process of change and development, or evolution, by which human beings emerged as distinct species. It is the subject of a broad scientific inquiry that seeks to understand and describe how change and development took place. The study of human evolution encompasses many scientific disciplines, but 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 hominids, such as the australopithecines.

History of paleoanthropology

The modern field of paleoanthropology is said to have begun with the discoveries of Neanderthal “man” and evidence of other "cave men" in the 19th century. The idea that humans were similar to certain great apes was obvious to people for some time, but the idea of biological evolution of species in general was not legitimized until after Charles Darwin published Origin of Species in 1859. Though Darwin's first book on evolution did not touch on the question of human evolution—"light will be thrown on the origin of man and his history," was all Darwin wrote on the subject—it was clear to contemporary readers what was at stake. 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 aspect of it which made it highly controversial. 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 ranked as being the closest animals to 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 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 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 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 bipedal 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 Australopithecus. The recent trend has been back to the original classification as a separate genus.).

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Before Homo

• The earliest hominids
  • Aegyptopithecus
  • Sahelanthropus tchadensis
  • Orrorin tugenensis
  • Ardipithecus ramidus
• The Australopithecus genus
  • Australopithecus anamensis
  • Australopithecus afarensis
  • Australopithecus africanus
  • Australopithecus garhi
• The Paranthropus genus
  • 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.

The word homo is simply the Latin for "person", chosen originally by Carolus Linnaeus in his classificatory 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.

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.

Homo erectus

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 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. A famous example of Homo erectus is Peking Man; others were found in Asia (notably in Indonesia), Africa, and Europe.

H. ergaster

Lived from about 1.8 to about 1.25 MYA. Also proposed as Homo erectus ergaster

Homo heidelbergensis

(Heidelberg Man) lived from about 500 TYA to about 300 TYA. Also proposed as Homo sapiens heidelbergensis and Homo sapiens paleohungaricus.

Homo sapiens idaltu

Lived from about 160 TYA (proposed subspecies). Is the oldest anatomically modern human known.

Homo floresiensis

From about 12 TYA (announced 2004). Nicknamed hobbit for its small size.

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 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

H. sapiens

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. "Sapiens" means "wise" or "intelligent."

Additional notes

The origins of humanity have often been 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 as continued the speciation of humans as they fill various ecological niches; see adaptive radiation.

References

1. Wolfgang Enard et al. "Molecular evolution of FOXP2, a gene involved in speech and language." Nature, Vol 418 (22 August 2002) p. 870.
2. DNA Shows Neandertals Were Not Our Ancestors
3. Ovchinnikov, et al. "Molecular analysis of Neanderthal DNA from the Northern Caucasus." Nature 404, 490 (2000).

See also

• Archaeogenetics
• Evolutionary medicine
• FOXP2
• Graphical timeline of human evolution
• Homo neanderthalensis
• Jeffrey H. Schwartz
• Mitochondrial Eve (African Eve theory)
• Multi-regional origin
• Physical anthropology
• Single origin hypothesis
• Theories of the origin of humans

External links

• Relations of the Homo sapiens
• Hominid Species at talkorigins.org
• DNA Shows Neandertals Were Not Our Ancestors
• Neanderthals on Trial Nova Online - Provided by PBS.
• Becoming Human - Provided by PBS.
• Tree of evolution of Man's fossil ancestors
• FOXP2 and the Evolution of Language
• Atlas of the Human Journey (National Geographic)

See also: Human evolution, Human evolution/Species chart, 1859, 1997, 19th century, 2002