Evolution

The scientific study of human evolution encompasses the development of the genus Homo, but usually involves studying other hominids and hominines as well, such as Australopithecus. "Modern humans" are defined as the Homo sapiens species, of which the only extantsubspecies is known as Homo sapiens sapiens. Homo sapiens idaltu (roughly translated as "elder wise human"), the other known subspecies, is now extinct. Homo neanderthalensis, which became extinct 30,000 years ago, has sometimes been classified as a subspecies, "Homo sapiens neanderthalensis"; genetic studies now suggest that the functional DNA of modern humans and Neanderthals diverged 500,000 years ago. Similarly, the few specimens of Homo rhodesiensis have also occasionally been classified as a subspecies, but this is not widely accepted. Anatomically modern humans first appear in the fossil record in Africa about 195,000 years ago, and studies of molecular biology give evidence that the approximate time of divergence from the common ancestor of all modern human populations was 200,000 years ago. The broad study of African genetic diversity headed by Dr. Sarah Tishkoff found the San people to express the greatest genetic diversity among the 113 distinct populations sampled, making them one of 14 "ancestral population clusters". The research also located the origin of modern human migration in south-western Africa, near the coastal border of Namibia and Angola.

The evolutionary history of the primates can be traced back 65 million years, as one of the oldest of all surviving placental mammal groups. The oldest known primate-like mammal species (those of the genus Plesiadapis) come from North America, but they were widespread in Eurasia and Africa during the tropical conditions of the Paleocene and Eocene. Molecular evidence suggests that the last common ancestor between humans and the remaining great apes diverged between 8 and 4 million years ago, first the gorillas, and then the chimpanzees (genus Pan) split off from the line leading to the humans; the functional human DNA is approximately 98.4% identical to that of chimpanzees when comparing single nucleotide polymorphisms (see human evolutionary genetics). Therefore, the closest living relatives of humans are gorillas and chimpanzees, as they share a common ancestor.

Humans are probably most closely related to two chimpanzee species: Common Chimpanzee andBonobo. Full genome sequencing has resulted in the conclusion that "after 6.5 [million] years of separate evolution, the differences between chimpanzee and human are ten times greater than those between two unrelated people and ten times less than those between rats and mice".Suggested concurrence between the functional human and chimpanzee DNA sequences range between 95% and 99%; It has been estimated that the human lineage diverged from that of chimpanzees about five million years ago, and from that of gorillas 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.

Human evolution is characterised by a number of important morphological, developmental, physiological and behavioural changes, which have taken place since the split between the last common ancestor of humans and chimpanzees. The first major morphological change was the evolution of a bipedal locomotor adaptation from an arboreal or semi-arboreal one, with all its attendant adaptations, such as a valgus knee, low intermembral index (long legs relative to the arms), and reduced upper-body strength.

Later, ancestral humans developed a much larger brain - typically 1,400 cm³ in modern humans, over twice the size of that of a chimpanzee or gorilla. The pattern of human postnatal brain growth differs from that of other apes (heterochrony), and allows for extended periods of social learning and language acquisition in juvenile humans. Physical anthropologists argue that the differences between the structure of human brains and those of other apes are even more significant than their differences in size.

Other significant morphological changes included the evolution of a power and precision grip, a reduced masticatory system, a reduction of the canine tooth, and the descent of the larynx and hyoid bone, making speech possible. An important physiological change in humans was the evolution of hidden oestrus, or concealed ovulation, which may have coincided with the evolution of important behavioural changes, such as pair bonding. Another significant behavioural change was the development of material culture, with human-made objects becoming increasingly common and diversified over time. The relationship between all these changes is the subject of ongoing debate.

The forces of natural selection have continued to operate on human populations, with evidence that certain regions of the genome display directional selection in the past 15,000 years.