 One theory suggests that the human and chimpanzee lineages diverged somewhat at first, then some populations interbred around one million years after diverging.The brains of most species of "Australopithecus" were roughly 35% of the size of a modern human brain with an endocranial volume average of .Although this is more than the average endocranial volume of chimpanzee brains at the earliest australopiths ("A. anamensis") appear to have been within the chimpanzee range, whereas some later australopith specimens have a larger endocranial volume than that of some early Homo fossils.