How do we identify new species from fossils?
When we try to identify different species in nature today, we usually think of that question in terms of interbreeding. We look at different populations, whether they look different or the same, and ask, do they ever interbreed with each other or encounter each other in nature?
In paleontology, we’re looking at species from the past, and we can never examine directly whether they can interbreed. So we have to go by what fossils look like, by the creature’s physical form.
Neanderthals are the most famous human ancestors that we know of. We debated for more than a hundred years whether they should be their own species or part of our species. I can say today with some confidence that they are human ancestors, because we have DNA evidence from them. It shows that today’s humans still have a fraction of Neanderthal DNA.
Recently, our team working in South Africa with Lee Berger, found a new species of hominin—the group consisting of modern humans, immediate ancestors and extinct human species. In the remote Rising Star cave, we found a collection of bones that represent the whole anatomy of what we think is a previously unknown species.
We call that species Homo naledi.
When we looked across the whole skeleton, we found some parts that resemble species we already know about. For example, the skull looks like Homo erectus, but it’s much smaller and would house a smaller brain than any other Homo erectus we’ve found. The feet and lower legs resemble modern humans. The hips resemble earlier hominins.
That mixture of features in Homo naledi is something we’ve never seen, and other features, like the thumb and thigh bone, have never been found before in any hominin.
The combination of known and unknown features made us really confident that we’re looking at a new species, something outside the range of anything we’ve seen before.