human species, origins of

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Evolution of humans from ancestral primates. The African apes (gorilla and chimpanzee) are shown by anatomical and molecular comparisons to be the closest living relatives of humans. The oldest known hominids (of the human group) had been the australopithecines, found in Africa, dating from 3.5–4.4 million years ago. But in December 2000, scientists unearthed the fossilized remains of a hominid dating back 6 million years. The first hominids to use tools appeared 2 million years ago, and hominids first used fire and moved out of Africa 1.7 million years ago. Modern humans are all believed to descend from one African female of 200,000 years ago, although there is a rival theory that humans evolved in different parts of the world simultaneously.

Miocene apes
Genetic studies indicate that the last common ancestor between chimpanzees and humans lived 5 to 10 million years ago. There are only fragmentary remains of ape and hominid fossils from this period. Dispute continues over the hominid status of Ramapithecus, the jaws and teeth of which have been found in India and Kenya in late Miocene deposits, dating from between 14 and 10 million years ago. The lower jaw of a fossil ape found in the Otavi Mountains, Namibia, comes from deposits dated between 10 and 15 million years ago, and is similar to finds from East Africa and Turkey. It is thought to be close to the initial divergence of the great apes and humans.

Australopithecines
Australopithecus afarensis, found in Ethiopia and Kenya, date from 3.9 to 4.4 million years ago. These hominids walked upright and they were either direct ancestors or an offshoot of the line that led to modern humans. They may have been the ancestors of Homo habilis (considered by some to be a species of Australopithecus), who appeared about 2 million years later, had slightly larger bodies and brains, and were probably the first to use stone tools. Also living in Africa at the same time was A. africanus, a gracile hominid thought to be a meat-eater, and A. robustus, a hominid with robust bones, large teeth, heavy jaws, and thought to be a vegetarian. They are not generally considered to be our ancestors.

A new species of Australopithecus was discovered in Ethiopia in 1999. Named A. garhi, the fossils date from 2.5 million years ago and also share anatomical features with Homo species. The most complete australopithecine skeleton to date was found in South Africa in April 2000. It is about 1.8 million years old and from a female A. robustus.

The skull of an unknown hominid species, Kenyanthropus platyops, was discovered in Kenya in March 2001. Approximately 3.5 million years old, it is contemporary with the australopithecines, previously the oldest known hominids, leading to the suggestion that humans are descended from K. platyops, rather than the australopithecines as has been thought.

Homo erectus
Over 1.7 million years ago, Homo erectus, believed by some to be descended from H. habilis, appeared in Africa. H. erectus had prominent brow ridges, a flattened cranium, with the widest part of the skull low down, and jaws with a rounded tooth row, but the chin, characteristic of modern humans, is lacking. They also had much larger brains (900–1,200 cu cm), and were probably the first to use fire and the first to move out of Africa. Their remains are found as far afield as China, West Asia, Spain, and southern Britain. Modern human H. sapiens sapiens and the Neanderthals H. sapiens neanderthalensis are probably descended from H. erectus.

Australian palaeontologists announced the discovery of stone tools dated at about 800,000 to 900,000 years old and belonging to H. erectus on Flores, an island near Bali, in 1998. The discovery provided strong evidence that H. erectus were seafarers and had the language abilities and social structure to organize the movements of large groups to colonize new islands. In 2000 Japanese archaeologists discovered that H. erectus were probably building hut-like shelters around 500,000 years ago, the oldest known artificial structures.

Neanderthals
Neanderthals were large-brained and heavily built, probably adapted to the cold conditions of the ice ages. They lived in Europe and the Middle East, and disappeared about 40,000 years ago, leaving H. sapiens sapiens as the only remaining species of the hominid group. Possible intermediate forms between Neanderthals and H. sapiens sapiens have been found at Mount Carmel in Israel and at Broken Hill in Zambia, but it seems that H. sapiens sapiens appeared in Europe quite rapidly and either wiped out the Neanderthals or interbred with them.

The remains of a new species of human, Homo floresiensis, were discovered in 2004 in the Liang Bua limestone caves on Flores Island, Indonesia. The 1-m/3.2-ft-tall skeleton of a female was dated to a period as recent as 13,000 years ago. This dating shows that Homo floresiensis survived much later that the Neanderthals.

Modern humans
There are currently two major views of human evolution: the ‘out of Africa’ model, according to which H. sapiens emerged from H. erectus, or a descendant species, in Africa and then spread throughout the world; and the multiregional model, according to which selection pressures led to the emergence of similar advanced types of H. sapiens from H. erectus in different parts of the world at around the same time. Analysis of DNA in recent human populations suggests that H. sapiens originated about 200,000 years ago in Africa from a single female ancestor, ‘Eve’. The oldest known fossils of H. sapiens also come from Africa, dating from around 195,000 years ago (this date was reached in 2005 by the re-dating of remains originally found in Ethiopia in 1967; the previous estimate was 130,000 years). Separation of human populations occurred later, with separation of Asian, European, and Australian populations taking place between 100,000 and 50,000 years ago.

The human genome consists of between 27,000 to 40,000 genes. Of these only about 1.5% differ between humans and the great apes.

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