MKB: You both have had a lot of experience finding new fossil specimens, so I wanted to ask you about a part of paleo work that’s often very difficult for laypeople to understand. How do you go about distinguishing where a new specimen fits in the human family tree, whether it’s part of an already identified species, or something new? That can seem like a really subjective thing from the outside.
RL: I would say that people have generally gone about explaining this backwards. The very earliest things that are our ancestors, quite frankly they don’t look like us at all. I think it’s much more important to look from the present and go back. When you find 10,000-year-old old skeletons they look just like us. In fact, modern looking goes back to 200,000 years. Then, I think we tend to go further and start really seeing the differences. At 1.5 million years ago, it’s not like us at all. If we presented it this other way, from present back, I think we’d have more understanding from the public.
ML: It really is a lot of work to establish that you’ve got something different and that it’s not just variation within the species. The main comparative example you use is to take the gorilla, which has a huge size and shape difference between males and females. Gorillas have the most variation within a species of all modern primates. You look at that very extreme variation and you assume you’re unlikely get a much higher degree of variation within a species than that. Then you compare all the points on your new specimen with known species and you see if it fits within that range of variation. If it exceeds the gorilla level of variation you’ve got a pretty good case for a new species.
And the truth, with this method, is that you’re likely missing species. If you were to take a series of modern monkey skulls and break them apart the way we find them in the fossil record, there’s no way you’d call them different species. But you know in the modern situation that they are different. If anything, we’re conservative on this.