Thursday, September 17, 2015

Homo naledi

Perhaps you've heard of the recent discovery of a cache of hominin (human-like) bones in a remote, almost inaccessible cave in South Africa. The remains have been designated a new species of hominin named Homo naledi and their discoverer claims them to be an ancestor of modern humans.
Ann Gauger at Evolution News and Views writes a very helpful overview of the discovery and the significance of the unanswered questions about it. She cautions that until we know the age of the fossils, which we don't at this point, nothing can be said about their position, if any, in the human lineage.

Here are the facts according to Gauger:
1. Many bones were found in a nearly inaccessible cave in South Africa. They appear to be fossils with mixed traits. The shoulders and pelvis seem to be australopithicene in character, while the teeth, legs, and feet appear to be more like the genus Homo.

2. Many bones were found in that nearly inaccessible cave. This may be due to non-intentional natural causes, or perhaps the bones were intentionally placed there by someone (by their own species -- who knows?). The latter would argue for some special care of the dead, and perhaps intelligence like ours (or not -- for another point of view see an article by primatologist Frans de Waal in the New York Times). It also depends on whether there was ever another possible route into the cave, or some sort of natural disaster that collected them in one place.

3. Very few bones of small mammals or birds were found in the cave, which argues against both another previous form of access, or a natural disaster that swept them away.

4. The fossils represent many individuals, because multiple bones of the same type have been found.

5. The fossil skulls are small. How this is interpreted depends on the point of view of the interpreter.
Gauger then raises a pair of important questions:
1. How old are the fossils? This matters because if the bones are 3 million years old and from a single species, they would represent the oldest fossils with traits of our genus Homo, and traits of the australopithicenes. Under some interpretations, this might make them the missing link.

If the fossils are younger than 2 million years, the story remains interesting but not nearly so important. Why? Because that would make them younger than the oldest known Homo fossils, Homo erectus, whose morphology was almost exactly like ours. That would make Homo naledi an interesting side branch among hominins, but definitely not the missing link between us and an ape-like ancestor, if such a thing ever existed.

2. Does the find represent a single species with mixed traits, or is it a mixture of two species? The authors of the paper claim that all the fossils are definitely of the same species, with any differences due to sexual dimorphism and age. (Sexual dimorphism means the sexes look different from one another. Think of how gorilla babies, full-grown male silverbacks, and adult females look compared to each other.) Other scientists doubt that claim, and say the differences indicate separate species.
So often in the past hasty judgments about the significance of hominin fossils had to be retracted later after further study made those judgments obsolete. Gauger properly urges restraint in the interpretation of H. nadeli, but it will be interesting to see if this really is older than the oldest known hominins.