Exemplar: KNM-ER-1813 [Koobi Fora, Kenya] - 1.9 million y.a.HOMO HABILIS lived from about 2.4 to 1.5 million years ago, and is the earliest known species to show novel differences from the chimpanzee and australopithid skulls. The face is still primitive and projecting, but the jaw is pulled under the brain, with smaller molars (though still much larger than in modern humans), and the skull is thinner, with a distinctive rounded shape, vertical sides and a small forehead above the brows. The first humans have arrived on the scene.
A male habilis may have stood at around 1.3 meters and weighed 37 kilos, and females 1.2 meters and 32 kilos. However, some forms of habilis were apparently smaller, and may have stood little more than a meter tall.
The two signature evolutionary trends in hominids are increasing brain size -- in habilis, to an averge of about 650cc -- and a proportionate reduction in the size of the face. In habilis the brain shape is more humanlike: the bulge of Broca's area, implicated in human language, is visible in at least one habilis brain cast. Another resemblance to modern humans is the reduced, less apelike or australopithid sexual dimorphism.
In habilis, increased brain power coincided with the first known use of manufactured stone or quartz tools (hence the choice of name, which means "handy man"). Habilis is associated with the Oldowan tool industry which is characterized by crude stone flakes, rounded hammer stones, and bones used for digging. Though odd stones or pieces of bone may have been used as weapons or scrapers much earlier, tools now begin to show up regularly near dismembered animal remains. One interpretation of the evidence is that habilis scavaged dead prey from carnivore meals, using stones to extract the marrow from bones too large for carnivores to break. Habilis may also have had a more agile gait than the larger australopithids, and for protection against predators may have moved in larger groups.
Despite its distinctively human cranium and its chronological position near the origin of the human line, habilis had a fairly apelike physical form: its arms were almost as long as its legs. It is therefore a controversial species. Similar in physique to the australopithids, without a clear evolutionary descendant, and appearing highly variable in the fossil record, habilis raises more questions than the available fossils are able to answer.