Today is Wednesday, which means I answer a question from you lovely readers. If you have a question about human evolution, you can get in touch through email, twitter or facebook. You could even just leave a comment on this post. I’m so easy to contact you have no excuse!
Self-plugging over, let’s get on with it. Today we’re dealing with one from Art, who wonders whether or not Australopithecus (a fossil hominin from 2 – 4 million years ago) is actually an ancestor of gorillas or chimps, rather than humans. He writes
One or two popular writers (Diamond, Dawkins) have suggested the possibility that bipedal Australopithecines may be ancestors of the Chimpanzee and Gorilla. Do you know of any evidence that would rule this out? Do you know of any solid evidence that supports it?
It is certainly true that the Australopiths possessed many ape-like characteristics more similar to chimps and gorillas than humans. These include small brains, prognathic faces, brow ridges, arms and hands adapted for climbing and many more. And yet most palaeoanthropologists still argue that they are more closely related to humans rather than to these other apes they resemble.
This is because evolutionary biology doesn’t identify relationship by comparing the raw number of similarities between a species. Rather, they look for synapomorphies. These are traits which are unique to one particular lineage, having evolved since they diverged from others, so can be used to define it. Thus if you find them in a fossil there’s a very good chance your looking at an ancient member of that lineage.
Many of the similarities between Australopithecus and the non-human apes aren’t synapomorphies. For example, both gorillas and chimps have a small brain, so this can’t be used to define either. As such, the fact Australopithecus has a small brain isn’t evidence it was an ancestor of gorillas or chimps.
Conversely, many of the similarities Australopithecus shares with modern humans are unique to our lineage, indicating that Australopithecus is part of it. Things like habitual bipedalism, use of stone tools and many boring things relating to the shape and size of their various bones are all traits which define the human lineage and are present in Australopiths. These synapomorphies mean that we can say with confidence that Australopithecus is a member of the human family.
Of course, the fact that share so many similarities with the other apes is still interesting. It shows that Australopithecus is an archaic member of the human family, perhaps an ancestor of later species. It sheds light on how the modern human form evolved from a more ape-like one and tells us what our ancestors were like millions of years ago.
But it doesn’t mean they aren’t part of our family.