The critical factor that differentiates humans and apes is bipedal locomotion. Comparative studies of fossils of humans and animals show differences in the structure of skulls and pelvis. Bipedalism freed the hands of our ancestors allowing them to carry loads and make tools. Apart from bipedalism particular aspects of tooth number, size, shape and enamel of teeth are of value in tracing human ancestry.
The earliest fossil evidence supporting the evolution of a creature ancestral to humans was found in Kenya. The fossil a single jawbone has been dated to be 5.5 million years old. More substantial evidence was found in mid 1990s in northeastern Ethiopia where the remains of more than 40 individuals who lived about 4.4 million years ago were unearthed.
The earliest hominid fossils of australopithecines from North Kenya have been dated to be 4.2 to 3.9 million years old. Such fossils were traced in Tanzania and Ethiopia also. The australopithecine fossils have been found at Hadar in Ethiopia and named Lucy had 40% of bones preserved. It helped anthropologists to know how australopithecines looked, stood and moved.
On the basis of fossil evidence it is considered that australopithecines were small statured and had relatively smaller brains than modern humans. The fully bipedal australopithecines lived in arid and semi-arid environments in eastern and southern Africa. They were omnivores subsisting on fruits, vegetables, insects and small game. No stone tools associated with australopithecines.
Global weather cooled down about 2.5 million years ago and it possibly resulted in the evolution of several hominid species. Of these one was robust australopithecines also known, as Paranthropus.They were larger than gracile australopithecines with heavier skulls with bony ridges. Robust australopithecines lived in Africa till about a million years ago and they do not seem to be ancestral to modern humans.