Tuesday, November 13, 2012

Dynamics of genetic and morphological variability within Neandertals

John Hawks
Journal of Anthropological Sciences Vol. 90 (2012), pp. 1-17

I got around to reading John Hawks' paper which I thought would be a summary paper of current work on Neandertals.  While it is a summary paper, I found it to be the most riveting description of Neandertal behavior I have ever read.  You probably want to read the entire paper.  I started excerpting the paper but ended up copying about a third of it here.  The paper is highly relevant to the model of a refugia and range shift model for human origins.

Neandertal variation and "varieties"

"Southwest Asian Neandertals"

"This group included the entire known fossil record of the Levant to the Zagros, including Skhul, Tabun, Zuttiyeh, and Qafzeh (all in the Levant) and Shanidar, Iraq.  Howell [1957] noted the divergent opinions of anthropologists about the evolutionary scenario that generated this sample.  He offered the opinion that the Tabun had affinities with Early Neandertal people, and that the region had undergone a trend of "sapiensization" explaining the Skhul sample."

"Howell identified these varieties of Neandertals to clarify his position on the Neandertal ancestry of recent humans.  In his view, several previous authors had been too categorical in their insistence that Neandertals could not have been ancestors of modern peoples.  He allowed that the classic Neandertals may have been too specialized to have given rise to later populations within Europe.  But the early Neandertals were less anatomically specialized and may have been ancestral to modern humans in some other, non-European, region.  Moreover, the Southwest Asian Neandertals appeared to provide evidence of an evolutionary trend toward modern humans."

Emerging problems with Neandertal varieties

"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."

"Mitochondrial DNA"

"The pattern of mtDNA evolution within Neandertals suggests that repeated turnover of the population of European Neandertals did happen.  When considering the entire sample of mtDNA, the amount of variation within the Neandertal sample is approximately equal to the variation within living people across the same geographic range, from Spain to Central Asia (Caramelli et al., 2008, Krause et al., 2007).  The common ancestor of all Neandertal mtDNA sequences lived approximately 200,000 years ago, around the same time as the modern human mtDNA last common ancestor (Dalen et al., 2012).   Taken by themselves, these comparisons are consistent with the hypothesis that Neandertals had approximately the same population structure and demographic history as modern Eurasians.  However, when we compare earlier and later Neandertals, the picture is more complex.  The sample of Neandertal mtDNA taken from European specimens after 50,000 years ago is depauperate in variation compared to the full sample (Lalueza-Fox et al. 2008).  The lack of variation in later European Neandertals is not consistent with these being a sample drawn from a small geographic area of a large distribution, without demographic turnover (Dalen et al., 2012.)  Instead, it appears that the western part of the Neandertal range underwent at least one episode of large-scale migration and partial population replacement.  A tightly related clade of sequences includes the specimens from Vindija, El Sidron and Feldhofer, seven specimens in all.  These are among the latest Neandertals in the west.  The Central Asian or eastern European portion of the Neandertal range retained greater mtDNA variation in this later time period, possibly indicating that this area was a source for later Neandertals in Western Europe . . ."

"Fabre and colleagues (2009) also emphasized a biogeographic division of mtDNA into easter and western groups.  They used a different methodology, focused upon whether the geographic range of Neandertals could be divided into replicable subsamples.  In addition to the division into Central Asian and European groups, the study also suggested that the Italian and Croatian specimens might belong to a "southern" group.  This study did not consider the times represented by different sites, and adding the dynamic reflected by time would likely change the groupings.  By testing a priori models, the study avoided some of the problems attendant upon the tree-based approaches described above."

"Nuclear DNA"

"Three Neandertals from Vindija have been represented by substantial sequencing of the nuclear genome, averaging nearly 1x coverage for each of them.  Much smaller fractions of the nuclear genome have been recovered from Neanderthal specimens from Feldhofer Cave, El Sidron, and Mezmaiskaya (Green et al., 2010).  All of these except for Mezmaiskaya are among the group of later Western European Neandertals discussed above, all of which fall intoa single mtDNA clade.  This is therefore a highly constrained set of Neandertals in space and time.   The full set of mtDNA extends includes an eastern range with greater diversity and much earlier specimens in Western Europe."

"The most celebrated result from the nuclear DNA evidence is the finding that non-Africa populations today derive a proportion of their ancestry from Neandertals (Green et al., 2010).  The fraction of ancestry represented by such introgression form Neandertals is between 1 and 4 percent of the genealogical ancestry of individuals with European, Asian or other non-African origins.  Some of the similarity of non-Africans to Neandertals may be attributable to the ancient Middle Pleistocene structure of African populations (Eriksson & Manica, 2012), but this effect alone cannot explain the pattern of similarities, which therefore require substantial introgression (Yang et al., 2012).  It is possible that some similarities of living people and Neandertals resulted from gene flow between Neandertals and African contemporaries before the Late Pleistocene dispersal of modern populations . . ."

Reconciling paleogenomics and morphology

". . . Two avenues of evidence will provide more insights about Neandertal population dynamics.  Obviously, uncovering more nuclear genomes from Neandertals or early Upper Paleolithic humans would advance our knowledge greatly.  Tempering this expectation is that the later western Neandertals, with lower genetic diversity, are the ones most likely to provide more genetic data.  Earlier Neandertals, and the Neandertals from central Asia, would be most useful to uncover new knowledge about the population dynamics of this ancient group. A second source of evidence may come from the introgression of Neandertal genes into later human populations.  As we begin to uncover the genes of living people that came from Neandertals, we face the possibility that these genes may represent different ancient Neandertal groups to greater or lesser degrees.  The initial work on Neandertal genetics suggested that most of the population mixture with Neandertals may have happened in west Asia (Green et al., 2010).  That would suggest that European Neandertals are themselves somewhat genetically distinct from the population that gave rise to most Neandertal genes in recent populations.  Comparing different Neandertals with each other will help us uncover the structure of the population that gave rise to Neandertal ancestry in living people . . ."

Population dynamics

" . . . The attention to "early" Neandertals as a group dating to the last interglacial brought with it the understanding that Neandertals had persisted through at least one entire glacial cycle.  Howell (1952) proposed that glacial cycles provided the isolation that enabled classic Neandertals to evolve their specialized anatomy.  Weckler (1954) argued that isolation was one consequence of glaciations, but that long-distance migrations and recolonizations of formerly periglacial habitat was an important cause of population change in Neandertals and the modern humans who encountered them."

"Today, our knowledge of the geographic range of Neandertals confirms their existence across a broad range of climate regimes.  From the Altai to Spain, the known geographic range of Neandertals covered more than 7000 kilometers east to west.  On the longitudinal range is little doubt, because of the mtDNA evidence European Neandertal specimens to Okladnikov Cave (Krause et al., 2007).  Okladnikov is at present the easternmost site to produce skeletal remains attributable to Neandertals, although other sites with similar archaeology are found in the Altai.  The Neandertals also covered a substantial range in latitude.  The northernmost Neandertal site may be Byzovaya, which does not present skeletal remains but does include a Late Mousterian assemblage with some technical links to central European Neandertal sites (Slimak et al., 2011) . . . Although the European climatic conditions oscillated considerably during the Late Pleistocene, the Neandertals seem likely to have been capable of adapting to change conditions, either by tracking ecotones as climate shifted or by changing their subsistence strategies to meet new requirements.  In other words, the archaeological record by itself is sufficient to show us that Neandertal populations were highly dynamic in areas where habitation was possible only during intermittent climatic periods."

"Archaeological evidence along gives us some indications that Neandertals rapidly colonized new regions when they became suitable for habitation.  The possible excursion of Late Mousterian people north of the Arctic Circle to Byzyvaya and Mamontovaya Kurya are strong indicators of such potential, if these sites truly represent Neandertal activity.  Bar-Yosef (1992) suggested that later Neandertal sites in the Levant, including Amud and Kebara, may represent the recolonization of this area from Europe as cold conditions intensified during the Wurm glaciation.  Shea (2008) considered the record of modern and Neandertal activity in the Levant to represent multiple cases of population turnover, as climate shifts caused successive populations of Paleolithic humans to abandon the area or become locally extinct."

"Across northwestern Europe, from Britain to Poland, an area of more than a million square kilometers was abandoned by Neandertals during the early stages of the last glaciation and not reinhabited until after approximately 60,000 years ago.  The intermittent occupation of these parts of Europe was likely not a function of "habitat tracking" by Neandertals, but instead a record of regional expansions and partial extinctions when climatic conditions deteriorated (Hublin & Roebroeks, 2009).  White & Pettitt (2011) suggested a very small Neandertal population size in northwestern Europe during the late Middle Paleolithic, and considered the possibility that the occupation of Britain was maintained as seasonal hunting camps rather than permanent settlements.  This kind of occupation would put movements of several hundred kilometers into the ordinary behavior pattern of individual Neandertals.  At an extreme, the survival of Neandertals on the northwestern tier of Europe may have been precarious (White, 2006).  From the perspective of population dynamics, this does not suggest a dense, stable population, but instead one of great mobility and repeated ability to colonize and exploit new opportunities . . ."


Maju said...

Something that would help would be genomes from Homo rhodesiensis and such. After all, not only of Neanderthals is the Human genus made, you know.

Marnie said...

Thanks for pointing this out.

I had a look online and it seems that the skull that was originally call Homo rhodesiensis found at Kabwe, Zambia (independence 1964, formerly Rhodesia), is now called the Kabwe skull.

It is now variously classified as:

"homo heidelbergensis" [http://humanorigins.si.edu/evidence/human-fossils/fossils/kabwe-1}


simply as "homo sapiens":

Middle Pleistocene Evolution in Africa and the Origin of Modern Humans

So, yes, Homo sapiens in Africa and Neandertals in Europe evolve in parallel, starting in the Gunz-Mindel interglacial. But, there is likely crossover at each subsequent glacial period [MIS 12, MIS 6, MIS 2-4], so sapiens in Africa and Neanderthals in Europe don't speciate. Instead, we get high inter species diversity.

Maju said...

The category Homo rhodesiensis (regardless that the name is old and sounds horribly colonial) keeps existing and has several representatives. Kabwe is just one of them, even if the original one, just like Neanderthal skull is just one of many Neanderthal specimens known.

In one of my last Leherensuge entries, which may be of your interest because I think gives a quick vision of human evolution possibilities in a few maps, I counted 12 H. rhodesiensis specimens from Eritrea to South Africa plus one (Lake Eyasi, Tanzania), which may be transitional to H. sapiens.

Sources for those maps are in the text. The usage of H. erectus instead of H. ergaster is admittedly arbitrary and was done mostly to follow the main source. Notice the "buffer" of ill-defined H. erectus (senso lato) in North Africa and West Asia between the H. rhodesiensis and H. heidelbergensis specimens in the critical 600-400 Ka BP period, making almost impossible that Heidelbergensis-Neanderthals and Rhodesiensis-Sapiens had much genetic interaction before each species coalesced in their respective areas.

Also... I'm not sure at all but one can imagine that the morphological ambiguity of some Neanderthals (and also arguably some Sapiens) in the "buffer zone" could have been caused by admixture with H. erectus/ergaster native to that area.

Maju said...

Erratum: should read: ... "from Djubouti [not Eritrea] to South Africa".

Marnie said...

I had a look at your post over on the Leherensuge . . . very nice maps over time and geography for these fossil finds. Good discussion as well.

Thanks for adding this here. Much appreciated.