The recent announcement of Homo naledi has caused quite a stir both within and beyond the palaeoanthropology community. Hundreds of bones representing at least 15 individuals, all found within a small area deep in the Rising Star Cave system in South Africa, from which they were recovered by an all-woman team. Science journalists really must have thought that Christmas had come early. Amid the waffle and junk, a few factoids have emerged and a couple of pressing questions are exercising the minds of the palaeoliterati. So here is my two-penn’orth.
What do we know? The remains are those of a mix of adult and immature individuals, probably of both sexes, of a hominin population that shows an intriguing blend of derived and primitive traits. From the published photos and descriptions so far, the teeth seem to be consistent with our genus Homo, and the feet are said to be distinctly modern. I find that statement a little worrying, as we have only a sparse collection of early hominin feet. In one of the best-known examples, OH8 from Olduvai Gorge, we are not even certain whose foot it is – Homo habilis or Paranthropus boisei? The naledi hands show long, curved phalanges reminiscent of australopiths though, again, the comparative material is scrappy. The upper arms and shoulders appear to be quite primitive, with none of the torsion seen in the humerus of later Homo species. And the brain is small, with estimates around 460-560 cm3. What this all adds up to is a mosaic of features such as might be expected in a transitional form at a time of rapid evolution under strong selection pressure. Comparison with, for example, the motley specimens that are dumped into Homo habilis or the recently-described Australopithecus sediba are bedeviled by the difficulty of comparing a lot of specimens from one place and (probably) time with sparse bits and pieces from here and there. It would help if we had dates for Homo naledi.Therein lies the first of the pressing questions. How old is this species? The bones are from a patch of sediment deep in a limestone cave system, which is about as difficult a context as one could imagine. There are no useful stratigraphically-related volcanic rocks to give potassium-argon or argon-argon dates or palaeomagnetic evidence. Sediments associated with the bones are not suitable for uranium-series dating. There are flowstones within the cave system that might yield an isotopic date, but their relationship with the find-spot is not clear and direct. The lack of associated animal bones rules out even a biostratigraphical date. What does that leave? Maybe some electron spin resonance measurements on tooth enamel, though that technique is seldom robust and precise when used on its own. The lack of any published dates seems strange until you try to work out how this new find could be dated. Eventually, some absolute numbers will be conjured out of the fossils and their surrounding geology, but that will not be easy. Looking at the fossils themselves, a figure around 1.8 to 2 million years ago is most likely. Anything much more recent would be remarkable.
The other pressing question, one that has got the media really taking notice, is to ask how all those bones got to that place. Quite a few of them are articulating and none shows indications of movement by water or any sediment mass-movement. It looks as if the bones reached that spot as bodies, not as bones washed out of another deposit. The near-absence of remains of any other species is consistent with that conclusion. The find-spot is deep in the cave system, only accessible today through two narrow ‘squeezes’ and certainly in complete darkness. Putting those facts together, the excavators have dared to publicly speculate about deliberate disposal of the dead, early hominins placing their deceased deep into a cave system as some conscious act of ‘burial’. If that were so, it would be quite extraordinary and completely unexpected. Deliberate disposal of the dead is not seen until much later in our evolution, probably in the Sima de los Huesos at Atapuerca, dated to less than half a million years ago, more certainly in Neanderthal burials of the last 60,000 years. However, descriptions and profiles of the Rising Star Cave system all show the present-day morphology, not the cave as it was perhaps 2 million years ago. The excavators’ focus has been on the bones rather than on the formation and geomorphology of the cave. A less complex and difficult alternative access route, now blocked, must surely be more likely than early humans going to such elaborate lengths to ‘bury’ their dead? At any rate, it is an explanation that needs to be thoroughly investigated by experienced cavers before the possibility of deliberate burial gains traction and becomes ‘fact’.Any new fossil human find attracts speculation, and such a large concentration of specimens is bound to cause a lot of excited comment, especially if the press have been emailing every high-profile palaeoanthropolgogist for comment within a few hours of the press conference. The danger is that off-the-cuff preliminary remarks become entrenched positions that then have to be defended. That may apply to the species name Homo naledi. In the published description, the authors do a thorough job of pointing out detailed differences between these remains and those attributed to other early Homo species. The problem is that those other specimens are mostly East African, presenting the challenge of showing that these new fossils are not just the southern form of a rather variable species of which east African Homo ergaster is the more equatorial form. Are there any other specimens from South Africa that might now be recognized as attributable to Homo naledi? The nearby Swartkrans site has yielded a few fragments that are cautiously thought to be from a Homo species contemporaneous with the australopiths for which that site is better known. If those specimens are shown to consistent with the new material, then the proposed name Homo naledi is invalid, either because the Swartkrans specimens were first published as Telanthropus capensis, requiring the name Homo capensis, or because the original finders of the Swartkrans fossils subsequently agreed that T. capensis could be subsumed into Homo erectus. If that is accepted, then the Homo naledi specimens would also be subsumed into H. erectus unless they can be shown to be distinctly different to the original Swartkrans finds. Baffling, isn’t it?
Whatever name is eventually nailed onto these fossils and whenever they are shown to date from, the new finds show just how complex and bushy the hominin clade has been, and just how much completely unexpected new material may remain to be found even in apparently well-explored locations. Some brave soul will have to attempt an overview of all the later australopith and early Homo material to see whether the plethora of species names is really supportable and whether any lines of descent can be inferred. Good luck with that.