It is an inescapable fact of genetics that all people alive in the world are genetically related, and that at some point an individual existed whom we all claim as an ancestor. The only points of debate are how long ago that ancesor lived, and where. DNA data suggests all modern humans are genetic descendants of one small inbred group of prehistoric Africans. (Patterns in Prehistory)
This "African Eve Hypothesis" is based on the study of DNA taken from the mitocondria, which are features in human cells where energy to keep the cell functioning is produced. (Patterns in Prehistory)
In a typical modern chronology, the line that would lead to us split off from Old World Monkeys about 25 million years (m.y.) ago; from the gibbons, 18 m.y. ago; from orangutans around 14 m.y. ago; from gorillas some 8 m.y. ago; and from the chimps approximately 6 m.y. ago. (Shadows of Forgotten Ancestors)
The story of human origins in Africa takes a twist with the description of a 6–7-million-year-old cranium from Chad. The discovery hints at the likely diversity of early hominids. A fossil cranium, discovered by Michel Brunet and his colleagues and described in this issue, marks a similar turning point in our understanding of human origins. The fossils — the cranium, a jaw fragment and several teeth — belong to a primitive human precursor, or hominid, that is an astonishing 6–7 million years old. Here we have compelling evidence that our own origins are as complex and as difficult to trace as those of any other group of organisms.
What is remarkable about the chimp-sized cranium TM 266-01-060-1 discovered by Brunet et al . is its mosaic nature. Put simply, from the back it looks like a chimpanzee, whereas from the front it could pass for a 1.75-million-year-old advanced australopith. The hominid features involve the structure of the face, and the small, apically worn, canine crowns. Other hominid features are found in the base of the cranium and in the separate jaw fragment. If we accept these as sufficient evidence to classify S. tchadensis as a hominid at the base, or stem, of the modern human clade, then it plays havoc with the tidy model of human origins. Quite simply, a hominid of this age should only just be beginning to show signs of being a hominid. It certainly should not have the face of a hominid less than one-third of its geological age. Also, if it is accepted as a stem hominid, under the tidy model the principle of parsimony dictates that all creatures with more primitive faces (and that is a very long list) would, perforce, have to be excluded from the ancestry of modern humans. (42)