Homo sapiens is the scientific name for the human species. Homo is the human genus, which also includes Neanderthals and various other extinct species of hominid. H. sapiens is the only surviving species of the genus Homo. Modern humans are the subspecies Homo sapiens sapiens, distinguished from their direct ancestor, Homo sapiens idaltu (http://en.wikipedia.org/wiki/Homo_sapiens_idaltu).
Subspecies of H. sapiens include Homo sapiens idaltu, roughly translated as “elder wise human” and the only existing subspecies Homo sapiens sapiens. Some sources show Homo sapiens neanderthalensis as a subspecies of H. sapiens. Similarly, the uncovered specimens of the Homo rhodesiensis species have been classified by some as a subspecies, but this classification isn’t commonly accepted.
Scientific examinations of human evolution is concerned, for the most part, with the development of the genus Homo, but normally entails studying other hominids and hominines too, such as Australopithecus. “Modern humans” are defined as the Homo sapiens species, of which the only existing subspecies is known as Homo sapiens sapiens.
Homo sapiens idaltu, the other known subspecies, is extinct. H. neanderthalensis, which became extinct 30,000 years ago, has occasionally been classified as a subspecies, “Homo sapiens neanderthalensis”; genetic research now propose that the functional DNA of modern humans and Neanderthals diverged around 500,000 years ago.
Anatomically modern humans are first seen in the fossil record in Africa about 195,000 years ago, and studies of molecular biology provide proof that the estimated time of divergence from the common ancestor of all modern human populations was 200,000 years ago. The extensive study of African genetic diversity led by Dr. Sarah Tishkoff found the San people to convey the greatest genetic diversity among the 113 distinct populations sampled, making them one of 14 “ancestral population clusters”. The research also traced the origin of modern human migration in south-western Africa, close to the coastal border of Namibia and Angola.
The evolutionary history of primates can be trailed back 65 million years. Primates are one of the oldest of all existing placental mammal groups. The oldest known primate-like mammal species comes from North America, but occupied Eurasia and Africa on a broad scale during the tropical environments of the Paleocene and Eocene. Molecular evidence indicates that the last common ancestor between the humans and the remaining great apes diverged 4 to 8 million years ago.
The gorillas were the first group to separate, then the chimpanzees split off from the line leading up to the humans. The functional portion of human DNA is about 98.4 percent identical to that of chimpanzees when comparing single nucleotide polymorphisms. Therefore, the closest living relatives of humans are chimpanzees and gorillas, as they share a comparatively common ancestor.
Humans are possibly most closely related to two chimpanzee species: the Common Chimpanzee and the Bonobo. Current estimates of proposed consensus between functional human and chimpanzee DNA sequences range between 95 and 99 percent.
Early estimations indicate that the human lineage might have diverged from that of chimpanzees about five million years ago, and from that of gorillas about eight million years ago. However, a hominid skull uncovered in Chad in the year 2001, classified as Sahelanthropus tchadensis, is about seven million years old, and might be proof of an earlier divergence.
Human evolution is defined by a number of essential changes – morphological, developmental, behavioral, and physiological – which have occurred since the split between the last common ancestor of humans and chimpanzees. The first key morphological change was the evolution of a bipedal locomotor adaptation from an arboreal or semi-arboreal one, with all its attendant adaptations knee, low intermembral index, reduced upper body strength.
The human species developed a much larger brain than that of other primates – on average 1,400 cubic centimeters in modern humans, more than twice the size of a chimpanzee or gorilla. The pattern of human postnatal brain growth differs from other apes, and allows for increased periods of social learning and language acquisition in juvenile human individuals. Physical anthropologists argue that the distinctions between the structure of human brains compared to those of other apes are even more substantial than their differences in size.
Other major morphological changes include the evolution or a power and precision grip , a reduced masticatory system, a reduction of the canine tooth, and the drop of the larynx and hyoid bone, making speech achievable. An essential physiological change in humans was the evolution of hidden oestrus, or concealed ovulation, which might have corresponded with the evolution of imperative behavioral changes, such as pair bonding. Another important behavioral change was the development of material culture , with human-made objects becoming more and more frequent and diversified over time. The relationship between all these alterations is the subject of an ongoing debate.
The forces of natural selection have continued to function on human populations, with evidence that particular regions of the genome display directional selection in the past 15,000 years.
Image Caption: Mix of Homo Models. Credit: Wikipedia