Fossils indicate that the evolutionary line leading to us had achieved asubstantially upright posture by around 4 million years ago, then began toincrease in body size and in relative brain size around 2.5 million years ago.
Those protohumans are generally known as Australopithecus africanus,
Homo habilis, and Homo erectus, which apparently evolved into each otherin that sequence. Although Homo erectus, the stage reached around 1.7 mil-lion years ago, was close to us modern humans in body size, its brain size was still barely half of ours. - P36