One of the key physiological differences between early humans (Homos) and Australopithecines was adult cranial capacity. On average, early humans had brains that were about 35 percent larger than Australopithecus africanus, who is widely considered to be one of two possible immediate ancestors of early humans — the other is Australopithecus garhi.
Early humans such as Homo erectus are from the genus Homo and directly follow evolutionarily from the Australopithecines. As part of the more evolved genus, Homo erectus shows several key physiological differences from his predecessor, Australopithecus.
Another key physiological difference is that early humans developed smaller mouths and teeth, especially molars and premolars. Much of the reason for the smaller mouths was due to the increase in brain size and cranial capacity. However, smaller mouths and teeth also indicated that early humans had softer diets than their evolutionary ancestors. While Australopithecines ate many tough foods such as nuts, early humans ate more soft plants and meat.
A third physiological difference between Homo (all species) and Australopithecines was limb length-Australopithecines had much longer arms, although both genuses were bipedal. Modern human body size and limb proportions first appeared with Homo erectus.
Evolutionary records suggest that many different species of our modern genus, homo, and even some other hominids lived at the same time as one species phased out another. Therefore, it was not necessarily the case that one species was phased out completely before another appeared.