Reproductive isolation from a cross between a horse and a donkey or between a mare and a donkey, respectively. These animals are nearly always sterile due to the difference in the number of chromosomes between the two parent species. Both horses and donkeys belong to the genus "Equus", but "Equus caballus" has 64 chromosomes, while "Equus asinus" only has 62. A cross will produce offspring (mule or hinny) with 63 chromosomes, that will not form pairs, which means that they do not divide in a balanced manner during meiosis. In the wild, the horses and donkeys ignore each other and do not
Australopithecus africanus and STS 71), but also presents primitive features including ape-like, curved fingers adapted to tree climbing. Instead of it being a direct ancestor of later hominins and thereby of humans, some researchers believe that "A. africanus" evolved into "Paranthropus", and specifically that "P. robustus" descended from "A. africanus". Both "P. robustus" and "A. africanus" crania seem very alike despite the more heavily built (robust) features of "P. robustus", which, like those of the modern gorilla, are adaptations for heavy chewing. "A. africanus", on the other hand, had a cranium which quite closely resembled that of a modern chimpanzee; yet both