Neandertals converging with modern humans, not evolving separately

In my discussion of Di Vincenzo et al. (2012) and Freidline et al. (2012), I noted how Neandertals appeared to be converging towards a modern human form; this seemed to occur both with respect to the ancestral forms usually assigned to the taxon H. heidelbergensis, but also with respect to earlier Neandertals. Some of the most "modern-looking" Neandertals are late specimens such as Vindija (the main source of the Neandertal genome), and these often appear more similar to modern humans than many earlier (pre-100ka) specimens.

A book chapter by Milford Wolpoff and Sang-Hee Lee ("The African origin of recent humanity" in African Genesis: Perspectives on Hominin Evolution (2012), eds. Sally C. Reynolds and AndrewGallagher) explores a similar topic.

The following figure illustrates a model in which recent African Homo, represented by Herto, the hypodigm of H. sapiens idaltu, and Neandertals represent separately evolving lineages whose precursors are the Broken Hill cranium (Kabwe) and Petralona, a pre-Neandertal European hominin from Greece.

Kabwe is much more primitive than Herto. A recent paper lists it simply as greater than 125ka, and Chris Stringer has cast doubt about its great antiquity. Tim White, on the other hand, considered it a precursor of the Herto skull, which he considered the first evidence of anatomically modern humans in Africa, still possessing archaic traits. This contention was, however, upset by the redating of the Omo remains from Ethiopia to ~195ka, making them much older than Herto. So, if Kabwe is near 125ka, it might even appear that the chronological sequence Omo-to-Herto-to-Kabwe is associated with increasing archaicity. So much for a model of H. sapiens gradually emerging in Africa out of more primitive forms; the recent publication of the Iwo Eleru skulls from Nigeria, and Ishango from the Congo are certainly not adding to our confidence that any such process of linear Ascent of Man in Africa ever took place.

But, I'm digressing; if we can't really find a clear signal of a series of human forms in Africa leading to modern humans, perhaps we can find that Neandertals are becoming less and less like modern humans, as time passes since their last "common ancestor".

So, let's get back to the Wolpoff and Lee chapter. The authors contrast the model of separately evolving lineages with one of paleo-demes, defined as:

A palaeo-deme as used here is a ‘designation, reflecting inclusive genealogically related geographic group(s)’ (Howell, 1999 : 203)

The whole matter revolves around the issue of gene flow and selection. If Herto and Neandertals represent separately evolving lineages, then their distance from each other ought to be greater than the distance of either one to the more archaic specimen from the same region. Herto ought to be closer to Petralona than to Neandertals if Neandertals continued to evolve as a separate lineage for 100-200 thousand years after Petralona lived (depending on the age of that specimen).

But, this is not what is observed at all (right). Herto appears closer to Neandertals Shanidar 1, La Ferassie 1, and Amud than to Petralona. Contrary to expectation, Neandertals are apparently converging with African groups of similar age, they are not diverging from them.

One could also observe, that lower differences occur between Herto and some Neandertals (e.g., with Shanidar 1) than between Herto and some anatomically modern humans (e.g., with Skhul 5, Qafzeh 6, and Jebel Irhoud 1).

So, while we will probably continue to hear about estimates of the "time of divergence" between Neandertals and modern humans, we must remember that this may be a mirage. Of course, when geneticists do their calculations, they can fit a tree model to human evolution, in which lineages branch out, become reproductively isolated, and die out, leaving a single twig, African H. sapiens to survive. But, admixture matters:

In fact, the ability of admixture to "converge" populations is the basis of the multi-regional evolution theory, although that is usually posited in terms of gene flow. But, the basic idea is still the same: our relatively uniform human species may not be entirely the result of tree-like divergence of populations from an original African population, but rather of a confluence of streams of ancestry derived from Lower and Middle Paleolithic populations of Homo.

Here are the facts:

1. Geneticists have sequenced the genomes of two archaic hominins (Neandertals and Denisovans): both are differentially affiliated to regional groups modern humans: archaic humans can no longer be considered irrelevant twigs of the human family tree.
2. Geneticists have identified segments of archaic DNA that have introgressed in certain African populations. It seems that archaic admixture is ubiquitous in the human species, even in its supposed cradle.
3. Neandertals and modern humans do not appear to diverge from each other phenotypically as time goes by: rather they seem to converge; they increasingly become more modern-like as time goes by. The analysis on the left beautifully illustrates this (Freidline et al. 2012): Shanidar 5 Sh5 a late Neandertal is within the modern human convex hull and several other Neandertals are close to modern humans; strange behavior indeed, if these guys are supposed to be diverging on a separate path from African humanity)

There is other evidence besides: Mounier et al. (2011) noted that even within the group of modern skulls the most ancient examples were more archaic-looking.  This would agree with the findings of Crevecoeur et al. (2009) that recent mankind possesses a subset of Late Pleistocene variation. It is also, indirectly supported by the findings of Reich et al. (2010), according to which, the genetic divergence of Neandertals and Denisovans (644ky), two contemporaneous populations living in a part of Eurasia exceed the total genetic divergence of all modern human populations (maximum: San-Papuan, 607ky).

An objection to the multiregional model of evolution revolves around the idea that we have "very little" Neandertal or Denisovan ancestry to qualify as truly "multiregional." But, there are two flaws to this objection:

First, it introduces an arbitrary distinction: no one expects different regional human groups to have contributed equally, or even comparably to recent mankind: some populations (say Homo floresiensisis) may have been truly isolated and may have contributed nothing. Others may have inhabited lands with low carrying capacity (say dense, impenetrable tropical rainforest or the fringes of the glaciers), or may have acquired key innovations later than others. So, while the majority of modern human ancestry may stem from a restricted geographical region (in my opinion the Sahara-Arabia belt around 100 thousand years ago), other human groups (such as Neandertals, Denisovans, archaic Africans, etc.) are not irrelevant: they are our ancestors too.

Second, the idea that admixture with archaic hominins was "little" is based on little more than a misunderstanding of the evidence. Mike Hammer explains this succinctly in this talk:

But remember, this is all relative to Africans, it doesn't tell you if Africans have a lot of Neandertal DNA or a little bit of Neandertal DNA: it's just that non-Africans are elevated one little bit up, a step higher than the Africans with respect to Neandertal sharing.

In other words, it's not only x% Neandertal in Eurasians and 0% in Africans; it can also be 10+x% in Eurasians and 10% in Africans: we can only estimate relative levels of influence, not absolute ones. Absolute levels could only be estimated if we had the genomes of old Africans: we would then see modern Eurasians and Africans arrayed on the Old African-Neandertal line, the former a little closer to Neandertals than the latter. But it is far from clear that modern humans in Africa would have escaped Neandertal gene flow altogether.

To conclude, a final quotation from Wolpoff and Lee:

The Europeans are not evolving in a different direction, away from the rest of humanity. The European palaeo-deme , including Neandertals, evolved to be more similar to the descendents of Herto over time, not less similar. The convergence of African and European palaeo-demes demonstrates the presence of gene flow between them, and we agree with the description of these palaeo-demes as ‘varieties in a single metapopulation’ (Hawks and Cochran, 2006 ). Moreover, later than the Neandertals, post-Neandertal anatomical variation in Europe includes Neandertal features, even features that evolved locally to be characteristic of most or all Neandertals and were rare or absent in other regions. 

I am strongly leaning towards acceptance of something quite akin to multiregionalism; it is almost certainly true in a sense: the evidence for widespread archaic admixture it irrefutable. On the other hand, there does seem to have been a Big Bang in human evolution, associated with mtDNA haplogroup L3, Y-haplogroup BT, and the reduced genetic diversity observed in Eurasians relative to Africans. But this appears to have been more akin to the success of a particular local population within a widespread species, rather than the emergence of an altogether new species.