The genus began about 2.3 million years ago. Their ancestors were almost certainly some line of Australopithecine apes.
The physical characteristics of these species are larger brain (above 900ml), and various changes to the skeleton which improve walking and running ability. As the process continues, the forehead becomes more vertical, the skull becomes rounder, the teeth are reduced in size, arms are shorter and legs are longer, and the skeleton becomes more delicate. All species used stone tools, but there is no evidence of when language developed. Human evolution is a much studied topic.