Homo habilis skull, KNM ER 1813 (1.9 million years).
The Homo habilis Skull KNM-ER 1813 was discovered by K. Kimeu in 1973 at Koobi, Kenya, and described by R. Leakey in Nature in 1973. There is still controversy about this specimen's classification, with some scientists opting to classify it as an australopithecine and others believing it is a species of Homo. Some paleoanthropologists have raised the possibility that KNM-ER 1813 is the female counterpart to the Homo rudolphenis KNM-ER 1470. While dated to the same time period and sharing some characteristics, KNM-ER 1813 has a much smaller face, brain and teeth than 1470. Other paleoanthropologists argue that its brain size of 510cm3 (in contrast to 1470's 750cm3) indicates a size difference too great to be due to sexual dimorphism and represents a separate species. It is also not the case that this specimen is simply an immature version of H. rudolphenis, as the third molar appears to have been worn down. Instead, it has been suggested that it belongs in a category of Homo habilis, with which it shares similarities in tooth size and shape, cranium size, and face shape.