Horse outward similarity is an unreliable measure of relatedness. Besides the horse, there are six other species of genus "Equus" in the Equidae family. These are the ass or donkey, "Equus asinus"; the mountain zebra, "Equus zebra"; plains zebra, "Equus quagga"; Grévy's zebra, "Equus grevyi"; the kiang, "Equus kiang"; and the onager, "Equus hemionus". Horses can crossbreed with other members of their genus. The most common hybrid is the mule, a cross between a "jack" (male donkey) and a mare. A related hybrid, a hinny, is a cross between a stallion and a jenny (female donkey). Other hybrids include the zorse,
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