The classification of the genus Homo is controversial among scientists, because so much of what is usually thought to define humans is related to behavior and the use of language, which cannot be deduced from skeletal remains. It is also a point of controversy between evolutionists and creationists.
An evolutionist point of view
The genus which includes modern humans - Homo sapiens, the genus Homo is thought to have been in existence for approximately two million years, while our species, including the archaic H. sapiens, has occupied only the last 200,000 years or so, H. hablis 2.1 to 1.7-1.5 million year, H. erectus 1.5 million years to circa 90,000 years and anatomically modern H. sapiens from 200,000 years to the present. Behaviorally modern humans, that is humans which displayed the same capacity for culture as extant humans, did not appear until around 50,000 years ago.
The genus seems to have begun in Africa with the advent of H. hablis from Olduvai Gorge, Tanzania and as yet an undifferentiated species from Koobi Fora, Kenya. It is believed that this or some other early species of Homo diverged from the Australopithecines more than two million years ago. The divergence is readily seen in the fossil record where significant differences in the cranial and post post-cranial skeleton can be seen.
What is not so easily seen, though of equal significance, is the emergence and further development of social and cultural traits that we commonly view as human. With respect to material culture, only members of the genus Homo can be confidently shown to have made and used stone tools and to have controlled the use of fire. These extra personal aids enabled Homo (H. erectus) to radiate out of East Africa to the more varied environments of the remainder of the Old World to produce a distribution of individuals far greater than that of the earlier Australopithecines. It is thought that a variety of Homo species may have existed at this point, including close relatives such as Homo neanderthalensis (Neanderthal man). DNA techniques seem to indicate that the original H. sapiens community became very small, to the verge of extinction, before prospering.
The principle trends within the genus Homo through time can be summarized as an increase in brain size and complexity, an increase in the complexity of culture and social relationships and an increasing ability to modify the environment.