Should there be a new genus to contain Homo habilis?

In February, I revisited the 1964 definition of Homo habilis by Louis Leakey, Philip Tobias and John Napier: “Leakey, Tobias and Napier on the definition of our genus”. The paper is remarkable because it represents the first major attempt to enlarge the anatomical definition of our genus. In that review, I referred to Bernard Wood’s work on the Koobi Fora fossils. I also pointed to Wood’s later essay, published with Mark Collard, that attempted to shrink the definition of Homo, expelling Homo habilis from our genus entirely.

I still have a post in the queue reviewing that paper closely. But in the meantime, Bernard Wood has published a new essay in Nature commemorating the fifty-year anniversary of Leakey, Tobias, and Napier’s paper: “Human evolution: Fifty years after Homo habilis”.

Wood’s essay also touches on the issue of enlarging our genus. He adds a perspective on the role of OH 5 (“Zinjanthropus”) on this problem of the scope of Homo:

Because Nutcracker Man was found in the same layers as the stone tools, the Leakeys assumed that it was the toolmaker, despite its odd appearance. But when Louis announced the discovery, he was not tempted to expand the definition of Homo. That would have eliminated any meaningful distinction between humans and australopiths. Instead he erected a new genus and species, Zinjanthropus boisei (now called Paranthropus boisei), to accommodate it

That is an important idea. I should note that the reduction of Australopithecus into Homo was not an insuperable barrier. By this time, the surfeit of genus names had begun to embarrass those anthropologists who were trying to adopt the ideas of the modern synthesis. Increasingly, Paranthropus, Telanthropus, Atlanthropus, Cyphanthropus, and the like were rejected by the field’s young vanguard. At the time that Mary Leakey found Zinjanthropus, only a minority had begun to use Homo erectus in the place of Pithecanthropus. Only eight years earlier, Ernst Mayr had staked out the position that all hominins should be lumped into Homo. That was the range of views, with a larger, more expansive Homo slowly gaining ground among theorists. But that argument would be much easier with the extremely humanlike reconstruction of OH 7 in 1964, than with OH 5 in 1959.

Wood recounts his own experiences analyzing the Koobi Fora fossil collection, during which he became convinced that the Homo habilis sample actually includes two species. He was not the scientist who named Homo rudolfensis, but his analysis was the first to give it teeth (literally), as he staked out a collection of additional mandibular and cranial specimens in addition to KNM-ER 1470.

Today, Wood believes that the Homo habilis and Homo rudolfensis samples should be placed into yet a third genus.

Although H. habilis is generally larger than A. africanus, its teeth and jaws have the same proportions. What little evidence there is about its body shape, hands and feet suggest that H. habilis would be a much better climber than undisputed human ancestors. So, if H. habilis is added to Homo, the genus has an incoherent mishmash of features. Others disagree, but I think you have to cherry-pick the data to come to any other conclusion. My sense is that handy man should belong to its own genus, neither australopith nor human.

I don’t necessarily disagree about the mishmash of features. But I don’t know which specimens Wood has in mind when he says that H. habilis is larger than A. africanus, unless he is referring to brain size and not body size.

At any rate, proposing a name for such a third genus is probably fruitless under the rules of taxonomic nomenclature. The species name habilis was erected as a species within Homo, while rudolfensis was initially suggested as belonging to Pithecanthropus (which almost everybody considers to be synonymous with Homo, and has the type specimen of Trinil, now part of Homo erectus for most scientists. Homo habilis and Homo rudolfensis may not even be sister species, so it would be nonsensical to name a new genus just based on the assumption that they are monophyletic. Wood and Collard did not provide a new genus name for them; they preferred to put H. habilis and H. rudolfensis into the existing genus, Australopithecus. And Wood acknowledges that many anthropologists think that these two species should be collapsed into one – and some think they both should be collapsed into Homo erectus.

Wood does not mention in his essay the one significant species with a mosaic of australopithecine-like and Homo-like anatomy: Australopithecus sediba. To me, this is the most interesting comparison with the classic Homo habilis definition. Whatever we include in Homo habilis (or break off into Homo rudolfensis) we have no comprehensive anatomical sample across the skeleton. We don’t have any idea what a Homo habilis pelvis would be like, nor can we exclude that known pelvic remains like KNM-ER 3228 may be Homo habilis (or Homo rudolfensis). The pelvic anatomy of A. sediba is more Homo-like than the pelvis of A. africanusor A. afarensis, the hand has features that are more Homo-like than the type specimen of Homo habilis. We don’t know what a Homo habilis femur looks like. Many have argued that the OH 62 skeleton is Homo habilis – with its fragmentary proximal femur – but that depends on an argument about its similarity with STW 53, a specimen that is only Homo habilis by the most generous stretch of taxonomic liberalism.

Consider the problems posed by the Malapa sample in 2010, compared to the Olduvai Homo habilis sample in 1964. The Homo habilis sample presented a larger brain size than known australopithecines that was nonetheless smaller than any known sample of Homo, a human-like hand and foot with some primitive features, and teeth with a handful of features that distinguished them from the known australopithecine (A. africanus, A. robustus and A. boisei) sample. The next thirty years showed that the hand and foot evidence were at best uncertain, leaving the teeth and brain. Malapa initially produced two skeletons, with an australopithecine like brain size, body size, limb proportions and foot, but a series of Homo-like details in the pelvis, teeth, and skull. We now know that both samples represent anatomical mosaics of primitive and derived characteristics. Leakey, Tobias and Napier used their small set of evidence of shared features to enlarge the definition of Homo. Berger and coworkers declined to further enlarge the definition of Homo on the basis of new shared features, instead emphasizing the overall australopithecine-like adaptive pattern represented by the brain and body size.

What a mess early Homo is! Wood draws from this mess the conclusion of repeated divergence and speciation with widespread parallelism.

The ongoing debate about the origins of our genus is part of H. habilis's legacy. In my view, the species is too unlike H. erectus to be its immediate ancestor, so a simple, linear model explaining this stage of human evolution is looking less and less likely. Our ancestors probably evolved in Africa, but the birthplace of our genus could be far from the Great Rift Valley, where most of the fossil evidence has been found.

Far from the Great Rift Valley. Hmmm…

References

Wood B. 2014. Human evolution: fifty years after Homo habilis. Nature 508:31-33. doi:10.1038/508031a