Aida Gómez-Robles published a paper in Science Advances three days ago (May 15, 2019 - Dental evolutionary rates and its implications for the Neanderthal–modern human divergence, Vol. 5, no. 5, eaaw1268 DOI: 10.1126/sciadv.aaw1268) in which she finds evidence that "support[s] a pre–800 ka last common ancestor for Neanderthals and modern humans unless hitherto unexplained mechanisms sped up dental evolution in early Neanderthals".
This is really remarkable, as it pushes the conventionally accepted date of divergence back in time from 400 to 800 thousand years ago.
This far older age was worked out by studying ancient Neanderthal teeth from the Sima de los Huesos site in Spain.
Gomez-Robles compared fossilised teeth from archaic hominins such as Australopithecus afarensis, Australopithecus africanus, Paranthropus robustus, Paranthropus boisei, and ancient ones such as Homo habilis, Asian specimens of H. erectus, a Sima de los Huesos H. neanderthalensis and of course, us: Homo sapiens, Other Neanderthal teeth from different sites in Europe were deliberatel left out of the comparative study.
The idea behind the comparison was to see how the shape of teeth in hominins changed over time. Gomez-Robles found that they evolve, and do so at a steady pace. This let estimate when each of these species split from each other.
The conclusion: "The simplest explanation of the results presented in this study is that Neanderthals and modern humans diverged before 0.8 Ma ago", our Last Common Ancestor (LCA) with Neanderthals dates back to 800,000 years ago.
Gomez-Robles goes on to spell out the implications of this more ancient split:
" If the phenotypic LCA of Neanderthals and modern humans was older than 800 ka, this would imply that all fossil hominins younger than this age are no longer valid candidates to occupy this ancestral position. Some fossils younger than this age, however, are frequently considered to be part of the last common ancestral species to Neanderthals and modern humans. These fossils, usually ascribed to Homo heidelbergensis, include European and African specimens, such as Mauer, Arago, Petralona, Bodo, Kabwe, etc., and maybe even some Asian specimens. If Neanderthals and modern humans diverged earlier than 800 ka ago, then all these fossils have to be related either to Neanderthals or to modern humans, or they can be part of a sister lineage to both of them. These fossils, however, cannot be ancestral to Neanderthals and modern humans because they would postdate their evolutionary divergence. An evolutionary relationship between these fossils and both Neanderthals and modern humans would be possible only if they were part of an older ancestral species that persisted in time as a relic species after the actual split of both lineages. Effectively, this scenario would mean that the H. heidelbergensis fossils are part of a sister group to Neanderthals and modern humans but that the evolutionary change from their putative ancestral populations did not involve speciation."
Of course, there are confounding factors that can explain the structural differences in teeth without requiring such an older age, for instance: the teeth evolved quicker in a small population of Neanderthals, isolated from other human groups, making them seem older while they are actually only 400 Ky old.
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Yes, possible, isolation of small groups gives mutations the chance to stick.
ReplyDeleteOr they mixed with someone else and acquired those teeth. Neanderthals kept mixing with us 'moderns' too, until they vanished as a seperate group.
C.S.