Were Classic SW Asian Neanderthals Admixed?

John Hawks (whom I am fated to miss every time he is in Denver to give a talk), has published a timely new survey of the state of knowledge in multiple disciplines regarding Neanderthals in light of new scientific developments.

The New Genetic Developments

The most profound development has been the sequencing of ancient DNA samples. He notes, in a decidedly glass half empty response to a near miracle of modern technology that "the genetic data from Neandertal skeletal remains are sparse, with fewer than two dozen mtDNA sequences (Dalen et al., 2012), and only six specimens with substantial nuclear DNA information (Green et al., 2010)." We also have a few nuclear genomes from the Denisova Cave in the Altai Mountain area of Russia.

Genetic data collected from modern populations have disclosed that all ancestrally non-African modern populations show a low percentage of Neanderthal admixture that can be confirmed by direct comparison with the ancient genomes. Likewise, Papuans, Australian aborigines, and populations derived from them, have significant Denisovan admixture in addition to their Neanderthal admixture that can be confirmed by direct comparison to ancient Denisovan genomes. A few of the oldest ancient nuclear DNA samples from Europe, most recently in the case of Otzi the Iceman, have likewise revealed distinctive Neanderthal DNA admixture patterns.

We have no ancient genomes to use as comparison samples from Africa. But, the same statistical methods that suggested that there might be ancient hominin ancestry where direct comparison later confirmed Neanderthal and Denisova admixture in modern genomes has also detected traces of ancient hominin admixture with at least one or two ancient hominin species in Africa that survives today in some Central African Pygmies and Southern African Khoi-san peoples, who are sometimes described as "Paleoafrican" because their uniparental genetic lineages are more basal in the phylogeny of modern human genetics than any others on earth.

The New Archaelogical Developments

There has also been a steady stream of new archaeological evidence from new excavations. The most notable of these has been discovery of a new ancient hominin species, Homo Florenesis sometimes called "Hobbits," over the objections of J.R.R. Tolkien estate, on the island of Flores in Indonesia, very close to the place where Denosivan genetic admixture starts to appear in indigenous populations.

Other important finds have been the discovery of the first post-Toba explosion, pre-Australian and Papuan settlement modern human remains in a cave in the Southeast Asia highland of Zomia, a correction to the dating of the youngest Homo Erectus specimen in Indonesia making it a pre-Toba eruption specimen, the discovery of Neanderthal remains not far from the Denisova Cave, clarification of the sequencing and dating of Neanderthal occupations and subsequent modern human occupations of sites in Europe, and the discovery of the use of a few technological techniques by Neanderthals in Europe that had not previously been documented.

Among other things, the improved dating has shown the arrival of modern humans in Europe to have been a more rapid colonization than had previously been assumed. This has also permitted a more nuanced analysis of the relationship between the arrival of modern humans in Europe, the decline of Neanderthals in Europe, and contemporaneous major volcanic eruptions and climate developments.

New archaeological excavations in Arabia that have just started to become available to member of the public have pushed back the earlier orthodoxy about when modern humans emerged from Africa finding distinctively modern human relics in inland Arabia more than 100,000 years ago with a clear affinity to contemporaneous Nuba complex modern human relics in Africa.

Hawks On Neanderthal Types

Much of Hawks paper simply summarizes some of these new developments, but the really powerful addition that his paper makes is in its revisiting of physical anthropology subtyping of Neanderthals skeletal remains that had culminated in a paper by Howell in 1957. The remainder of the paper then begins to fit these subtypes into a narrative in the context of the other evidence.

Howell distinguished Neanderthal remains by era, roughly speaking those before and after 100,000 years ago (the former being "early" and the latter "classical") and by region, with Southwest Asian instances being distinct from those to the North on a North-South axis, and other distinctions emerging in his study and later ones on an East-West axis in Eurasia.

Hawks makes this key observation on page 6 of the paper:

A problematic aspect of the idea of Levantine Neandertals is that the very features that distinguish them from European Neandertals tend to align them with modern humans. For example, the Amud skeleton has stature and limb proportions that set it apart from European Neandertals, but that fall within the range of variability of the Skhul and Qafzeh skeletal remains. Trinkaus (1995) considered the Near East, including Shanidar and the Levantine samples, to include two forms of hominins: “late archaic” and “modern” forms. He argued that the late archaic forms in the Near East have no close connection to European Neandertals, and that similar features reflect mosaicism or generalized archaic morphology in both evolving populations.

Several workers after Howell added the concept of a north-south axis of Neandertal diversification within Europe. Rosas and colleagues (2006) noted that southern Neandertals tend to have increased heights of the lower face and broader faces than the northern sample of Neandertals within Europe. Because the line separating north and south must run along the very long east-west axis of Europe, there are many possible ways to divide the continent into northern and southern samples.

Connecting The Dots

Hawks pulls his punches and refrains from sweeping conclusions, ending his paper with the simple observation that the genetic evidence and archaeological evidence alike indicate that there were several distinct subtypes of Neanderthals in space and time whose categorization has still to be worked out.

But, being the fool who rushes in where angels fear to tread, it isn't hard for me or anyone else familiar with the data to connect the dots.

The emerging genetic evidence suggests that modern human-Neanderthal hybridization probably took place after the Out of Africa event but well before the Upper Paleolithic revolution with Southwest Asia as a prime candidate for the location of this event, as that is where first contact must have taken place and the modern human range did not extend to Europe or further than India for much of that time period.

The obvious conclusion to jump to would be that the similarity of "classic" era Southwest Asian physical skeletal remains to modern humans, relative to Neanderthals elsewhere and from earlier eras, would be that these similarities are due to the impact of modern human-Neanderthal admixture on the Neanderthal populations of the Southwest Asia.

Likewise, an obvious conclusion to jump to that few professional anthropologists have committed to stating forthrightly, is that the post-Mousterian technological cultures of the Neanderthals may have been the product of the increased capabilities of admixed Neanderthals in those communities relative to their pure Neanderthal predecessor.

Hawks has instead advanced the hypothesis that Neanderthals were more fully within the range of modern human variation than they are given credit for being and were more human-like in general than previously assumed. His public comments lean towards the notion that Neanderthals were more of a subspecies of Homo Sapiens than a separate hominin species.

But, personally, I am more inclined to interpret the evidence of behavioral modernity and morphological similarity between Neanderthals and modern humans, not as much as a sign of convergent evolution or close evolutionary relationships, as it is a manifestation of highly influential admixture with modern humans. Indeed, in Southwest Asia and toward the end of the period of Neanderthal and modern human cohabitation of Europe, Neanderthals may have been more admixed with us than we were with them.

Reference

John Hawks, "Dynamics of genetic and morphological variability within Neanderthals" 90 Journal of Anthropological Sciences 1-17 (2012).