Quote: Broom on fossils as ancestors

In the last few years, a surprising number of paleoanthropologists have published papers claiming that various fossil species could not be ancestors of others because the fossils that represent them have the wrong geological age. Usually, the claim has been that a fossil is “too recent” to be an ancestor of living people, or Homo more generally.

The last fifteen years of fossil discovery have shown just now much we still do not know about the diversity of fossil hominins. So I think that purely statistical research on the duration of various species is not very illuminating. We’re not in a position to test how long most hominin species existed, or what their geographic extent may have been.

Still, it’s a topic that seems to return again and again.

I think that I’ve noted before that Robert Broom wrote thoughtfully on exactly this topic. Lee Berger and I reference Broom’s thoughts in our book, Almost Human.

When Raymond Dart published his initial work on the Taung specimen, Broom was the one who had the most insight about the possible age of the hominin skull. Finding Pleistocene baboons in the Taung fossil assemblage, Broom initially considered that the Taung skull itself might be fairly recent in geological age. As time went on, both Broom and Dart appreciated that the Taung hominin specimen and much of the fossil deposit were likely earlier, Pliocene in age.

But from the first publication of the Taung specimen, some critics maintained that a recent geological age must mean that it could not be connected to human origins. In their view, the age was prima facie evidence that the Taung specimen was an extinct ape, not a human relative.

As I was reading some background research today, I found a very clear exposition by Broom in a Scientific American article from 1929: “What the World Owes to South Africa.” The article is a retrospective by Broom of important paleontological findings from South Africa, not only the Taung specimen but a broad array of findings from Permian and Mesozoic times also.

In his discussion of human origins specifically, Broom addresses the relevance of the geological age of the Taung specimen.

Certain anthropologists have maintained that Australopithecus could not have been a human ancestor because it lived at a time after mankind had probably appeared on the earth. Manifestly, as the specimen died when five or six years old, it could not have been the ancestor of any one, but there is no objection to considering this little specimen as a representative of a race which may have survived hundreds of thousands of years; and that one member of the same tribe, perhaps in early Pliocene times, may have been the ancestor of mankind.

It is a cardinal mistake to think that a fossil is literally the genealogical ancestor of some living animals. Fossils are dead creatures and their individual reproductive fates are unknown. All fossils are related by a tree to living forms, however, and that relationship allows us to test hypotheses about the evolution of living forms. Whether an ancient population or species is a literal ancestor of some living animals is potentially testable, but only if the duration of the species and its boundaries are very well documented.

We know that Neandertals, as a population, include many literal ancestors of today’s humans. That does not mean that any particular fossil Neandertals were among those genealogical ancestors. Many of them belonged to local populations that became extinct, even if most regional populations of Neandertals in the long run contributed some genes to the modern human population.

Do we know that highly diverse hominins like Paranthropus could not have been ancestors of living people? Over the past ten years, it has become abundantly clear that hybridization and introgression were widespread among Pleistocene hominins. Genealogical ancestors trace every genealogical line, and the idea that Homo was hermetically sealed from co-existing closely related lineages is hard to imagine. I expect that P. boisei was a human ancestor, if genealogical ancestry and introgression are how we characterize ancestry.

Many anthropologists may admit that such minority ancestors might be interesting but still maintain that hybridization is not the central issue. They are interested in the predominant ancestry of living people, and that the ancestor of living humans must be a unique species at any one moment in time. They assume a model in which species disappear immediately at the moment that they give rise to a daughter species. In such a model, if a fossil representative of a species is, say, 2.0 million years old, then the species cannot be the ancestor of another species represented by a fossil that existed, say, 2.4 million years ago.