Homo naledi, a very primitive looking hominid, once believed to be a very ancient relative of modern humans, has been dated to a very recent period, in this paper (The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa, by Paul HGM Dirks et al., DOI: http://dx.doi.org/10.7554/eLife.24231, published May 9, 2017 Cite as eLife 2017;6:e24231) some 236 to 335 kya. And the abstract concludes:
"...We have constrained the depositional age of Homo naledi to a period between 236 ka and 335 ka. These age results demonstrate that a morphologically primitive hominin, Homo naledi, survived into the later parts of the Pleistocene in Africa, and indicate a much younger age for the Homo naledi fossils than have previously been hypothesized based on their morphology."
Primitive
A second paper (New fossil remains of Homo naledi from the Lesedi Chamber, South Africa, John Hawks, et al., DOI: http://dx.doi.org/10.7554/eLife.24232 Published May 9, 2017 Cite as eLife 2017;6:e24232) points out its really primitive crania, which has a smaller size than that of H. habilis. And this creature lived side by side with our purported African ancestors!
Apparently there are two theories to explain H. naledi:
- It is a ancient relative of H. habilis and retained its primitive morphology in a branch that survived into the times of modern humans.
- It split later on, after H. habilis, maybe from Homo erectus and reverted into a primitive form.
Could these primitive hominids have admixed with Africans and enriched their genome with "diversity"? adding genes and variety that did not appear out of Africa because H. naledi were constrained to their Sub Saharan location?
An out of Africa origin of modern humans Asian (or even, an American) origin for modern humans, who later reached Africa and admixed with H. naledi would surely yield the current genetic variability.
Since Naledi is so young, perhaps viable DNA could be obtained and sequenced. Who can tell what that information will tell us!
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Homo naledi´s coexistence with inmediate ancestors of H. sapiens in Africa seems indeed astonishing, as it puts in evidence some sort of erratic behaviour in the evolution… but it is not the only example of this;
ReplyDelete- H. floresiensis ; presumably a dwarfed H. erectus, with no more than an Oldowan like level of technology, lived in Flores Island until at least 17 Ka…that is to say, in modern H. sapiens times.
- Paranthropus robustus; not precisely homo, but a sagittal crested hominid, persisted until 1 Ma, in South Africa (Swartkrans site, Member 3), and therefore was not only contemporary with real homo like H. habilis/ergaster,...but also with higher brained species.
Even if it is not proved that they shared the same space, these “dead branches” and their coexistence in time with more advanced species, may suggest that the evolution towards modern human was far from being a continuous and monotonously increasing function of time…if it can ever be graphed, I can tentatively imagine it as a highly nonlinear curve which, even being increasing in the long term, would have plenty of singular points and periods of null or negative slope (implying stasis or even evolutive regression during them)…and surely would present considerable variants among the different regions.
Refering to your paragraph…” An Asian (or even an American) origin of modern humans , who later reached Africa and admixed with H. naledi would surely yield the current genetic variability…”, it reflects your previously published idea (even before Yuan et. al. paper) that Africa, necessarily, have had to be back immigrated, in order to get its current high genetics diversity by means of admixture with the incoming groups.
Wheter if H. naledi could have played a role in this process or not…I am not able to give a reasonably valid opinion about the genetic´s point of view, but the global context you mention (Asia/American? origins), …may perfectly leave room for this possibility.
I think that what is known as “Out of Asia” is worth of credit, at least, as a regionally valid model. In fact, H.erectus towards H. sapiens direct evolution in East Asia, is backed up not only by genetic studies, but also by serious anthropological research in a sufficient quantity of human remains…so the hypothesis was also verified by inspecting a lot of skulls.
Aditionally, the resulting local pole of dispersion since 1,5/1,9 Ma, could have triggered evolution in many regions. One of them was America which, by sure, may have played a more relevant role in the whole evolutive process, than merely receiving the final product of it…
And of course,.. why not back into Africa?