The increase in brain size and decrease in jaw and tooth size are associated with the first appearance of the genus Homo; the archeological evidence of a shift in subsistence patterns is often assumed to be associated with behaviors unique to Homo, although this point remains to be definitely demonstrated. The taxonomic interpretation of early Homo fossils were considered contentious when they were first found, and in many ways it remains so today.
The first discoveries of early Homo fossils were made at Olduvai Gorge, not long after Mary Leakey had found Zinj (now Australopithecus boisei) and Louis Leakey pronounced it to be the maker of gorge’s stone tools. Between 1960 and 1963, a series of fossils was uncovered close to the Zinj site, including hand and foot bones, a lower jaw, and parts of the top of the cranium (Wilford, 2007).
There are very strict rules of convention that must be followed when a scientist names a new species of an existing genus. These involve a careful description of the new specimen to show how the animal fits in with the definition of the genus (in this case Homo) and how it differs from other closely related species. The naming of a new species of hominid has always been likely to touch off a good deal of lively discussion within the profession, so Leakey was certainly guaranteed that much. But the fact that at the same time he had to adjust the definition of the genus Homo in order to accommodate his new species transformed what might have been properly scholarly jousting into near outrage.
Previous diagnoses of Homo had focused on the so-called cerebral rubicon: a hominid had to exceed brain size in order to be judged worthy of full manhood. The trouble was that different authorities put the threshold at different levels.
The diagnosis offered by Leakey and his colleagues was more comprehensive, and included habitual bipedal posture and gait, a precision grip, and a brain capacity much smaller than previously proposed (Line, 2005; Wilford, 2007).
Line, P. (2005). Fossil Evidence for Alleged Apemen – Part 1: The Genus Homo [Electronic Version]. Creation Ministries International. Retrieved October 1, 2007 from http://www.creationontheweb.com/content/view/4450/.
Wilford, J. N. (2007). Lost in a Million – Year Gap, Solid Clues to Human Origins [Electronic Version]. The New York Times. Retrieved October 1, 2007 from http://www.nytimes.com/2007/09/18/science/18evol.html?_r=1&ref=science&oref=slogin.