Did cold seasonal climates select for cognitive ability?

Paleolithic artefacts (Wikicommons). The northern tier of Eurasia saw an explosion of creativity that pre-adapted its inhabitants for later developments.

The new journal Psych will be publishing a special follow-up issue on J. Philippe Rushton and Arthur Jensen's 2005 article: "Thirty Years of Research on Race Differences in Cognitive Ability." The following is the abstract of my contribution. The article will appear later.

The first industrial revolution. Did cold seasonal climates select for cognitive ability?

Peter Frost

Abstract: In their joint article, Rushton and Jensen argued that cognitive ability differs between human populations. But why are such differences expectable? Their answer: as modern humans spread out of Africa and into the northern latitudes of Eurasia, they entered colder and more seasonal climates that selected for the ability to plan ahead, since they had to store food, make clothes, and build shelters for the winter. 

This explanation has a long history going back to Arthur Schopenhauer. More recently, it has been supported by findings from Paleolithic humans and contemporary hunter-gatherers. Tools become more diverse and complex as effective temperature decreases, apparently because food has to be obtained during limited periods of time and over large areas. There is also more storage of food and fuel and greater use of untended traps and snares. Finally, shelters have to be sturdier, and clothing more cold-resistant. The resulting cognitive demands fall on both men and women. Indeed, because women have few opportunities to get food through gathering, they specialize in more cognitively demanding tasks like garment making, needlework, weaving, leatherworking, pottery, and use of kilns. The northern tier of Paleolithic Eurasia thus produced the "first industrial revolution"—an explosion of creativity that pre-adapted its inhabitants for later developments, i.e., agriculture, more complex technology and social organization, and an increasingly future-oriented culture. Over time these humans would spread south, replacing earlier populations that could less easily exploit the possibilities of the new cultural environment. 

As this cultural environment developed further, it selected for further increases in cognitive ability. In fact, mean intelligence seems to have risen during historic times at temperate latitudes in Europe and East Asia. There is thus no unified theory for the evolution of human intelligence. A key stage was adaptation to cold seasonal climates during the Paleolithic, but much happened later.

https://www.mdpi.com/journal/psych/special_issues/race_differences_cognitive_ability

References

Rushton, J.P. and A.R. Jensen. (2005). Thirty years of research on race differences in cognitive ability. Psychology, Public Policy, and Law 11(2): 235-294.

http://philipperushton.net/wp-content/uploads/2015/02/Thirty-Years-of-Research-on-Race-Differences-in-Cognitive-Ability-2005-by-John-Philippe-Rushton-Arthur-Robert-Jensen.pdf

A look at an early European

 

Kostenki Man, reconstructed by Mikhail Gerasimov (1907-1970). An early European who was not yet phenotypically European.

 

Who were the first Europeans? We now have a better idea, thanks to a new paper about DNA from a man who lived some 38,700 to 36,200 years ago. His remains were found at Kostenki, a well-known Upper Paleolithic site in central European Russia (Seguin-Orlando et al., 2014).

Kostenki Man tells us several things about the first Europeans and, more broadly, the first non-African humans:

The Neanderthal encounter

Modern humans received their Neanderthal admixture when they were just spreading out of Africa some 54,000 years ago. At that time, they had not yet encountered the Neanderthals and were entering the territory of the Skhul/Qafzeh hominids, a semi-archaic people of the Middle East. So we may have got our Neanderthal admixture indirectly. The Skhul/Qafzeh hominids had probably interbred with their Neanderthal neighbors to the north, and our ancestors may have then picked up this admixture while in the Middle East. 

When our ancestors spread farther north into Europe, some 45,000 to 42,000 years ago, they could have interbred directly with Neanderthals, but they didn't. Perhaps the two groups were just too different. They seem to have intermixed only via a third party that was neither fully modern nor fully archaic.

A strange detour ... and then another!

There was initially a large continuous population across northern Eurasia, perhaps composed of nomads who pursued wandering herds of reindeer across the European Plain and its eastward extension into central and northern Asia.

Not long before the time of Kostenki Man, these Northern Eurasians began to split into three regional groups: Western Eurasians, Eastern Eurasians, and the ancestors of Middle Eastern farmers. The degree of reproductive isolation is unclear, however, and gene flow may have continued between all three groups until the onset of the last ice age some 25,000 years ago. This may be why Kostenki Man does not fit perfectly into any of the three groups, although he is genetically closest to Western Eurasians.

Yes, Northern Eurasians were ancestral to the early farming peoples of the Middle East. It seems that early modern humans had to head north, learn to hunt reindeer, and then head south again before they could start farming. Sounds like a strange detour. Wouldn't it have been easier to stay put and do it locally? You know, Middle-Eastern hunter-gatherers becoming Middle Eastern farmers? Apparently not.

It gets even more convoluted. After some of those Northern Eurasians had gone south to the Middle East, some of their farming descendants "returned" to Europe and partially replaced its hunter-gatherers, particularly in southern and central Europe. This second detour has been greeted with disbelief. Dienekes (2014), for instance, has written: "I don't think many archaeologists would derive European farmers from Russia (Russia is actually one of the last places in Europe that became agricultural)."

True, but farming requires a mindset that may have come from those northern hunters (Frost, 2014). When Piffer (2013) looked at human variation in alleles at COMT, a gene linked to executive function, working memory, and intelligence, he found that northern hunting peoples had more in common with farming peoples than with other hunter-gatherers, "possibly due to the higher pressure on technological skills and planning abilities posed by the adverse climatic conditions."

That mindset made farming possible, but the first steps toward farming could not be taken in a cold climate. They had to be taken in a place with a long growing season and a wide variety of domesticable plants and animals, such as in the Middle East. Once farming had developed there, it could move back north, while taking along its technologies, its food crops, and its livestock species. 

Farming can develop in the tropics with a "tropical" mindset, but it looks very different. The farming that arose in West Africa is overwhelmingly women's work and seems to have wholly developed out of female plant gathering. The guinea fowl is the only animal that has been domesticated for food consumption in sub-Saharan Africa.

The Ice Age was not so bad 

The Upper Paleolithic humans of northern and eastern Europe did not die out during the last ice age, as was commonly thought. They survived the glacial maximum intact.

The European phenotype came later

Kostenki Man was dark-skinned, dark-eyed, and rather short. These details, curiously enough, appear not in the paper but in a review of the paper, published by the same journal, as well as in an interview with one of the authors (Associated Press, 2014; Gibbons, 2014). 

So we now have an upper bound for the emergence of the European phenotype, i.e., light skin and a diverse palette of hair and eye colors. The lower bound has been set by the remains of a Swedish hunter-gatherer, dated to 8,000 years ago, who had the "European" allele for light skin at the gene SLC24A5 (Skoglund et al., 2014).

Conclusion

My main criticism centers on the dating to 38,700 - 36,200 years ago. At the Kostenki site, the radiocarbon dating used to be some 10,000 years younger. It was then recalibrated to an older range of dates when a layer of volcanic ash at the site was attributed to a volcano that had erupted in southern Italy some 39,000 years ago. This recalibration was initially controversial, but the controversy has since subsided (Sinitsyn and Hoffecker, 2006). I would not rule out a subsequent re-recalibration.

By retrieving ancient DNA from an early modern human, we have made a key advance in human paleogenetics, perhaps more so than by sequencing the Neanderthal genome. We again see that evolution did not slow down with the emergence of anatomically and behaviorally modern humans some 60,000 years ago. It actually began to speed up, as humans began to enter not only new natural environments but also new cultural environments of their own making.

 

References 

Associated Press (2014). DNA study dates Eurasian split from East Asians, The Columbus Dispatch, November 6

http://hosted2.ap.org/OHCOL/07e34bb59e064cedb7e2776e8db4b4f7/Article_2014-11-06-EU--Eurasian%20Split/id-ae36fa368c634c7383d807942bd5fe67 

Dienekes (2014). Genome of Kostenki-14, an Upper Paleolithic European (Seguin-Orlando, Korneliussen, Sikora, et al. 2014), Dienekes' Anthropology Blog, November 7

http://dienekes.blogspot.ca/2014/11/genome-of-kostenki-14-upper-paleolithic.html  

Frost, P. (2014). The first industrial revolution, Evo and Proud, January 18

http://evoandproud.blogspot.ca/2014/01/the-first-industrial-revolution.html 

Gibbons, A. (2014). European genetic identity may stretch back 36,000 years, Science, News, November 6

http://news.sciencemag.org/archaeology/2014/11/european-genetic-identity-may-stretch-back-36000-years 

Piffer, D. (2013). Correlation of the COMT Val158Met polymorphism with latitude and a hunter-gather lifestyle suggests culture-gene coevolution and selective pressure on cognition genes due to climate, Anthropological Science, 121, 161-171.

https://lesacreduprintemps19.files.wordpress.com/2014/01/correlation-of-the-comt-val158met-polymorphism-with-latitude-and-a-hunter-gather-lifestyle-suggests-culturee28093gene-coevolution-and-selective-pressure-on-cognition-genes-due-to-climate.pdf 

Seguin-Orlando, A., T.S. Korneliussen, M. Sikora, A.-S. Malaspinas, A. Manica, I. Moltke, A. Albrechtsen, A. Ko, A. Margaryan, V. Moiseyev, T. Goebel, M. Westaway, D. Lambert, V. Khartanovich, J.D. Wall, P.R. Nigst, R.A. Foley, M.M. Lahr, R. Nielsen, L. Orlando, and E. Willerslev. (2014). Genomic structure in Europeans dating back at least 36,200 years, Science, Published online 6 November 2014

http://www.sciencemag.org/content/early/2014/11/05/science.aaa0114
http://www2.zoo.cam.ac.uk/manica/ms/2014_Seguin_Orlando_et_al_Science.pdf  

Sinitsyn, A.A., and J.F. Hoffecker. (2006). Radiocarbon dating and chronology of the Early Upper Paleolithic at Kostenki, Quaternary International, 152-153, 164-174.

http://www.sciencedirect.com/science/article/pii/S1040618206000206 

Skoglund, P., H. Malmstrom, A. Omrak, M. Raghavan, C. Valdiosera, T. Gunther, P. Hall, K. Tambets, J. Parik, K-G. Sjogren, J. Apel, E. Willersley, J. Stora, A. Gotherstrom, and M. Jakobsson. (2014). Genomic diversity and admixture differs for stone-age Scandinavian foragers and farmers, Science, 344 (6185), 747-750.

http://www.sciencemag.org/content/344/6185/747.short

 

On Neanderthals, Denisovans, and other archaics

Andaman Islanders. Related peoples once inhabited the coastal regions of southern, southeastern, and eastern Asia.

The past year brought two major advances: the long awaited sequencing of the Neanderthal genome and the genetic sequencing of an another archaic human, the Denisovans of East Asia, whose existence had previously been unsuspected.

The bottom line comes down to four points:

Before the East African ‘big bang’ gave rise to modern humans some 80,000 to 60,000 years ago, there had been at least four groups of archaic humans:

a) Skhul-Qafzeh hominins
- were almost modern anatomically
- were derived from a demic expansion that spread over most of Africa and the Middle East about 110,000 years ago
- evolved directly into modern humans through the rapid expansion of an East African subgroup

b) Neanderthals
- differed much more anatomically from modern humans
- were behaviorally similar to Skhul-Qafzeh hominins
- probably had body fur and other cold adaptations
- were derived from an earlier expansion out of Africa 250,000 - 400,000 years ago
- inhabited Europe, the Middle East (during glacial maxima), Central Asia, and Siberia as far east as Lake Baikal

c) Hobbits
- were small and possibly a form of Homo erectus
- inhabited at least a portion of southeast Asia
- may have been a regional variant of the Denisovans

d) Denisovans (Homo altaiensis?)
- were another archaic population distinct from modern humans
- inhabited Asia from Lake Baikal eastward (see earlier post)

These archaic humans have left significant genetic admixture in all modern human populations. The admixture is about 1 to 4% in Eurasians (from Neanderthals), 8% in Melanesians (from Neanderthals and Denisovans), and 13% in sub-Saharan Africans (from Skhul-Qafzeh-like hominins).

This admixture might have accelerated human evolution by providing modern humans with useful alleles. To date, no such alleles have been found. The admixture seems to be confined to genes of low adaptive value.

Although Denisovans inhabited East Asia, they left no admixture in present-day East Asians. Yet their admixture is discernible in present-day Melanesians. It seems that the first modern humans to replace Denisovans were not ancestral East Asians but rather ancestral Melanesians. This is consistent with archeological and ethnographic evidence that the coastal regions of southern, southeastern, and eastern Asia were initially settled by people related to the indigenous populations of Melanesia, Papua-New Guinea, and Australia. After the last ice age, they were gradually replaced by populations originating in northern Eurasia (see earlier post). Today, they survive in relic groups like the Veddas of Sri Lanka, the Andaman Islanders, the Semang of the Malayan peninsula, and the Aeta of the Philippines.

References

Green, R.E., J. Krause, A.W. Briggs, T. Maricic, U. Stenzel, M. Kircher, et al. (2010). A draft sequence of the Neandertal genome, Science, 328, 710-722.
http://www.sciencemag.org/cgi/reprint/328/5979/710.pdf

Reich, D., R.E. Green, M. Kircher, J. Krause, N. Patterson, E.Y. Durand, et al. (2010). Genetic history of an archaic hominin group from Denisova Cave in Siberia." Nature, 468, 1053-1060.

Watson, E., P. Forster, M. Richards, and H-J. Bandelt. (1997). Mitochondrial footprints of human expansions in Africa, American Journal of Human Genetics, 61, 691-704.

The puzzle of Neanderthal admixture

Skhul V – one of the Skhul-Qafzeh hominins. Were they the middleman between Neanderthal genes and the modern human genome?

When I initially published my last post, I pooh-poohed the rumors about the reconstructed Neanderthal genome. How could modern humans have Neanderthal DNA when no such admixture appeared in previous analyses of mtDNA, dentition, and noncoding DNA? Any Neanderthal admixture would have to be less than 1%.

I then logged on to Razib Khan’s site. The news had just broken: modern humans outside Africa are 1 to 4% Neanderthal. I hurriedly rewrote my post and spent the rest of the day eating crow.

In retrospect, I now realize I had put too much faith in the existing data. I had also ignored the warning signs. The HBD community correctly decoded the silence that had fallen over Pääbo’s research team, as well as a remark made by Linda Vigilant.

Despite the crow-phagia, this is good news. It means the end to a long and bitter conflict between proponents of two models of human origins: ‘Out of Africa’ versus multiregionalism. Anthropologists may still disagree, but the room for disagreement has greatly shrunk from what it was only a week ago. We can now move on.

The winning model is a weak version of Out of Africa. Modern humans are largely descended from a series of population expansions that began around 80,000 years ago somewhere in eastern Africa and culminated c. 60,000 BP in a ‘big bang’ that would create 96-99% of the gene pool outside Africa. The remaining 1-4% came from archaic Europeans and Asians, i.e., Neanderthals in Europe and West Asia, and other hominins elsewhere (Almas in Siberia? Hobbits in southeast Asia?). This admixture seems to have occurred in two stages: an early one in the Middle East, when modern humans began spreading out of Africa, and a later one, when modern humans began to spread from Asia to Oceania (Dalton, 2010). Curiously, the admixture looks Neanderthal in both cases.

Did this admixture contribute anything useful? We still don’t know. Some academics had identified the new variant of the microcephalin gene as being probably of Neanderthal origin, but this variant is absent from the reconstructed Neanderthal genome. To date, Pääbo’s team is of the opinion that “the Neanderthal DNA does not seem to have played a great role in human evolution” (Wade, 2010).

Surprisingly, the same level of Neanderthal admixture was found in a French subject, a Chinese subject, and a Papuan subject. This seems to suggest that Neanderthal admixture happened when modern humans began spreading out of Africa, probably in southwest Asia. But why was there no further admixture in Europe, where the two populations co-existed much longer? If there had been, we would see more Neanderthal admixture in present-day Europeans, but we don’t.

There are several possible explanations:

1. Noise in the data obscures the higher level of Neanderthal admixture in present-day Europeans. The picture should become clearer once we have sequenced 100% of the Neanderthal genome, and not the current 60% (Richard Green in an interview on CBC).

2. Early modern Europeans were replaced by Middle Eastern immigrants when farming replaced hunting and gathering (Green et al., 2010).

3. In Europe, early modern humans greatly outnumbered the Neanderthals. Subsequent admixture thus had little effect (Green et al., 2010).

4. In East Asia and Oceania, ‘Neanderthal admixture’ is actually a proxy for something else. These regions had non-Neanderthal archaic populations that were probably more closely related to the Neanderthals than to modern humans. If so, ‘Neanderthal admixture’ in these regions may correspond, more or less, to admixture with other archaic humans (my idea).

5. Neanderthal genes entered modern Eurasian populations indirectly, via an intermediate Middle Eastern population that already had Neanderthal admixture (because of prolonged contact) while being fairly close to modern humans in appearance and behavior. Modern humans may thus have more readily intermixed with them but not with Neanderthals in Europe. The latter were so different in appearance and behavior that admixture would have been minimal (my idea).

I’m inclined toward the last two explanations. Explanation #4 would explain why the later admixture in Asia/Oceania seems to have been as ‘Neanderthal’ as the early admixture in the Middle East. Unfortunately, to test this explanation, we must first reconstruct the genome of archaic humans from the Asia/Oceania transition zone. This will be difficult, if not impossible, given the accelerated degradation of DNA in the tropics.

Explanation #5 is also tempting. Not long before modern humans began spreading out of Africa, the Middle East was home to a population with both modern human and Neanderthal characteristics, as attested by remains from Skhul and Qafzeh in northern Israel. These hominins first appeared in the Levant some 120,000 years ago, perhaps as part of an earlier out-of-Africa expansion into south Asia. Their technology was Mousterian and Neanderthal-like, but their anatomy was relatively modern with some archaic features (Kidder et al., 1992; Pearson, 1998). These “almost-moderns” existed on the periphery of a range centered on the African continent. Only a bit farther north were the Neanderthals of Europe and West Asia.

When the climate cooled during the Early Pleniglacial (70,000-55,000 BP), the southern limit of the Neanderthal range shifted accordingly. Neanderthals then occupied the Levant until the arrival of modern humans some 50,000 years ago (Grun & Stringer, 1991; Schwarcz et al., 1989; Valladas et al., 1987).

What happened to the Skhul-Qafzeh hominins? They probably moved farther south, perhaps to the Hejaz. Such a location would have been athwart the main line of expansion of modern humans as they spread out of Africa. Unfortunately, despite many interesting sites, there has been no dating so far of early human occupation in Saudi Arabia (Petraglia & Alsharekh, 2003).

References

Dalton, R. (2010). Neanderthals may have interbred with humans. Genetic data points to ancient liaisons between species. Naturenews, April 20.
http://www.nature.com/news/2010/100420/full/news.2010.194.html

Green, R.E., J. Krause, A.W. Briggs, T. Maricic, U. Stenzel, M. Kircher, et al. (2010). A draft sequence of the Neandertal genome, Science, 328, 710-722.
http://www.sciencemag.org/cgi/reprint/328/5979/710.pdf

Grun, R., and C.B. Stringer. (1991). Electron spin resonance dating and the evolution of modern humans, Archaeometry, 33, 153-199.

Kidder, J.H., R.L. Jantz, and F.H. Smith. (1992). Defining modern humans: A multivariate approach, in G. Bräuer and F.H. Smith (eds.) Continuity or Replacement. Controversies in Homo Sapiens Evolution, pp. 157-177, Rotterdam: A.A. Balkema.

Pearson, O.M. (1998). Postcranial Morphology and the Origin of Modern Humans, Ph.D. dissertation, State University of New York at Stony Brook, Ann Arbor: University Microfilms International.

Petraglia, M.D. and A. Alsharekh. (2003). The middle palaeolithic of Arabia: implications for modern human origins, behaviour and dispersals, Antiquity Journal, 77, 671–684.
http://repository.ksu.edu.sa/jspui/handle/123456789/5520

Schwarcz, H.P., B. Blackwell, P. Goldberg, and A.E. Marks. (1979). Uranium series dating of travertine from archaeological sites, Nahal Zin, Israel, Nature, 277, 558-560.

Valladas, H., J.L. Joron, G. Valladas, B. Arensburg, O. Bar-Yosef, A. Belfer-Cohen, P. Goldberg, H. Laville, L. Meignen, Y. Rak, E. Tchernov, A.M. Tillier, and B. Vandermeersch. (1987). Thermoluminescence dates for the Neanderthal burial site at Kebara in Israel, Nature, 330, 159-160.

Wade, N. (2010). Signs of Neanderthals mating with humans, New York Times, May 7, 2010.http://www.nytimes.com/2010/05/07/science/07neanderthal.html (read)

Comparing Neanderthal and modern human DNA

First graph: Distributions of Pairwise Sequence Differences among Humans, the Neanderthal, and Chimpanzees. X axis = number of sequence differences; Y axis = percent of pairwise comparisons. (Krings et al, 1997).

Second graph: Green: human/human comparisons; Red: human/Neanderthal comparisons; Blue: human/chimp comparisons. X axis = number of sequence differences; Y axis = fraction of pairwise comparisons. (Green et al. 2008)

Did the Neanderthal gene pool overlap with the modern human gene pool? In other words, are some modern humans genetically closer to some Neanderthals than they are to other modern humans?

The answer is ‘yes’ if we look at individual DNA sequences, as shown in the first graph (above):

Thus, the largest difference observed between any two human sequences was two substitutions larger than the smallest difference between a human and the Neandertal. In total, 0.002% of the pairwise comparisons between human mtDNA sequences were larger than the smallest difference between the Neandertal and a human (Krings et al., 1997)

It may be noted that a small fraction (0.037%) of the inter-human comparisons are larger than the smallest distance (29 substitutions) between the Neandertal and humans. (Krings et al., 1999)

We see the same genetic overlap among present-day human populations. Some Danes seem to be closer to some Congolese than to other Danes. But this overlap disappears when we compare more than one DNA sequence at the same time. The more sequences we compare simultaneously, the less overlap there is (Risch et al., 2002; Sesardic, 2010).

We can likewise eliminate the overlap between Neanderthals and modern humans if we use the entire mtDNA genome to compare these two populations:

In 2008, the first complete sequencing of Neandertal mtDNA was announced (Green et al. 2008). A complete mtDNA genome of 16,565 base pairs was extracted from a 38,000 year old fossil from the Vindija cave in Croatia. As Krings et al. 2007 had done, the authors created a graph showing the numbers of base pair differences for humans, chimps and the Neandertal when compared against humans. Because they were able to compare across the whole genome rather than a small portion of it, the differences between humans and the Neandertal was far more striking (Fossil Hominids)

This may be seen in the second graph (above).

But why must we compare entire genomes to make the overlap go away? Why should there be any genetic overlap between two populations for any stretch of DNA if the same populations show absolutely no visual overlap to our lying eyes? What’s going on here?

The underlying reason is that most of the genome has little selective value. So the selection pressure on that DNA is pretty much the same in any population, be it Congolese, Danish, or Neanderthal. Of course, once two populations become reproductively isolated, i.e., when they become different species, their DNA will start to drift apart even at genes of low selective value (because of differing patterns of random mutations). But this divergence is very slow. Consequently, it is hard to distinguish between related species that have diverged from each other only over the last 40,000 years. This is why Neanderthals and modern humans still have some overlap, even though their last common ancestor lived over 400,000 years ago.

References

Fossil Hominids: mitochondrial DNA,

http://www.talkorigins.org/faqs/homs/mtDNA.html

Green, R., A-S. Malaspinas, J. Krause, A. Briggs, et al. (2008). A complete Neandertal mitochondrial genome sequence determined by high-throughput sequencing, Cell, 134, 416-426.

Krings, M., H. Geisert, R.W. Schmitz, H. Krainitzki, & S. Pääbo. (1999). DNA sequence of the mitochondrial hypervariable region II from the Neandertal type specimen, Proc. Nat. Acad. Sci. USA, 96, 5581-5585.

Krings, M., A. Stone, R.W. Schmitz, H. Krainitzki, M. Stoneking, & S. Pääbo. (1997). Neandertal DNA sequences and the origin of modern humans, Cell, 90, 19-30.

Risch, N., E. Burchard, E. Ziv, & H. Tang. (2002). Categorization of humans in biomedical research: genes, race and disease, Genome Biol, 3, 1-12.

Sesardic, N. (2010). Race: a social destruction of a biological concept, Biol. Philos, 25, 143-162.

Are we part-Neanderthal?

How did archaic humans evolve into the different populations of Homo sapiens we see today? The answer has long divided anthropologists. Some opt for the ‘out-of-Africa’ model; others for the multiregional model.

According to the out-of-Africa model, we all descend from a small group that existed some 100,000 to 80,000 years ago somewhere in eastern Africa. This group had an advantage over other humans, perhaps a superior ability to construct mental models in all four dimensions (Dubreuil, 2008; Klein & Edgar, 2002). They thus grew in numbers and progressively replaced their more archaic rivals, first in Africa and then elsewhere after 60,000-40,000 BP. Extinction was thus the common fate of all archaic humans throughout Europe and Asia, be they Neanderthals, Neanderthaloids, or mysterious Hobbits.

This claim is disputed by the multiregional model, which asserts that archaic Europeans and Asians did not die out. In fact, they went on to provide most of the gene pool of present-day Europeans and Asians. Modern humans thus have regional differences that can be traced to longstanding continental differences among earlier hominids. These continental groups evolved in parallel from the archaic evolutionary grade to the modern one, with gene flow gradually spreading advantageous alleles from one group to another.

The multiregional model no longer has many supporters, at least not in its pure form. With the recovery of DNA from Neanderthal remains, it has become clear that any genetic continuity with modern humans must be minor at best. There is nonetheless some support for a hybrid model, i.e., our current gene pool largely comes from a population that expanded out of Africa, but with significant admixtures from archaic Europeans and Asians. This view finds backing in a recent paper by Wall et al. (2009).

Wall and colleagues compared genetic polymorphisms in subjects of European, West African (Yoruba), and East Asian descent. They first looked for gene loci whose alleles show multiple mutational differences—a sign that the polymorphism either is very old or has undergone some kind of diversifying selection (i.e., a balanced polymorphism). Among these loci, they next looked for those that lay close to other polymorphic loci whose alleles likewise show multiple mutational differences. The authors reasoned that the more the apparently ancient alleles clustered by continental origin (West African, European, or East Asian), the greater the likelihood that these alleles had entered the gene pool from local archaic groups that modern humans had encountered while spreading out of Africa.

And the results? Wall and colleagues found significant archaic admixture: 14% in Europeans and 1.5% in East Asians. Curiously, no estimate was made for the West African subjects. The authors simply state: “Interestingly, we also find evidence for ancient admixture in the Yoruba.”

Well, how much? I’d like to know because there is another estimate based on a different methodology. When Watson et al. (1997) studied mtDNA lineages in sub-Saharan Africans, 87% of the lineages seemed to originate in a series of population expansions that began some 80,000 years ago. One cluster of lineages, dated to c. 60,000 BP, defines almost all non-African humans. The remaining 13% look much older and seem to be “the relics of a less dramatic and more ancient expansion event across Africa.”

As for archaic admixture outside Africa, Wall and colleagues (like John Hawks) point to the Neanderthals and other local hominids. The admixture must have been specific to Europe and then East Asia because the European and East Asian subjects had different sets of apparently archaic alleles.

In contrast, Dienekes argues that the admixture occurred before modern humans began to spread out of Africa. The archaic alleles then persisted to varying degrees in the small founder groups that ‘budded off’ from the parent African population:

Thus, the expanding African population that eventually spilled over into Eurasia, would indeed be quite inbred and homogeneous, but its gene pool would also contain traces of the smaller, less successful African populations it had absorbed. Because of their low frequency, these traces would be more susceptible to extinction in the series of bottlenecks that led to Europeans on one side and East Asians on the other, with different sets of archaic genes preserved in either region.

I lean more to Dienekes’ explanation. If Europeans and East Asians have different archaic alleles because of separate admixture, their ancestors must have parted company before encountering the Neanderthals not long after leaving Africa (c. 40,000 BP). Yet this seems counterintuitive because the genetic distance between Europeans and East Asians (and hence their time of separation) is much less than that between Africans and non-Africans.

There is in fact a convergence of genetic, linguistic, and archeological evidence for a late split between ancestral Europeans and ancestral East Asians. A Y-chromosome study indicates that all North Eurasian peoples descend from a common ancestral population dated to about 15,000 BP (Stepanov & Puzyrev, 2000; see also Armour et al., 1996; Santos et al., 1999; Zerjal et al., 1997). The language families of northern Eurasia, particularly Uralic and Yukaghir and more generally Uralic-Yukaghir, Eskimo-Aleut, Chukotko-Kamchatkan and Altaic, share deep structural affinities that point to a common origin and not simply to word borrowing (Cavalli-Sforza, 1994, pp. 97-99; Fortescue, 1998; Rogers, 1986). Archeological evidence (characteristic lithic technology, grave goods with red ocher and sites with small shallow basins) also suggests a common cultural tradition throughout Europe and Siberia 20,000 to 15,000 years ago (Goebel, 1999; Haynes, 1980; Haynes, 1982). Finally, dental and cranial remains from southern Siberia (23,000-20,000 BP) indicate strong affinities with Upper Paleolithic Europeans (Alexeyev & Gokhman, 1994; Goebel, 1999).

Rogers (1986) places the European/East Asian split at the glacial maximum (20,000-15,000 BP). At that time, advancing glaciers and swollen glacial lakes created a barrier along the present-day Ob, thereby dividing a nomadic Eurasian population that hunted reindeer and other herbivores within a steppe-tundra belt stretching from Western Europe to Beringia.

A word of caution

Finally, a word of caution. Like any statistical analysis, the Wall et al. approach will yield a certain number of false positives. Many loci are highly polymorphic because selection has favored allele diversity, such as with balanced polymorphisms, and not because they are very old. And some will lie close on the genome to other balanced polymorphisms that likewise look older than they really are. And the alleles of these adjoining loci will often cluster by continental origin because they face different selection pressures in sub-Saharan Africans, Europeans, and East Asians—or just because of pure chance.

These false positives may also be more frequent among Europeans than among East Asians. If ancestral East Asians had budded off from an older European population, they would have taken less genetic diversity with them. They should have proportionately fewer polymorphisms and less diverse ones.

In conclusion, the jury is still out on this one. We won’t have a definite answer until we can match these continentally specific alleles with identical alleles on the Neanderthal genome now being reconstructed.

References

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Cavalli-Sforza, L.L., Menozzi, P. & Piazza, A. (1994). The History and Geography of Human Genes. Princeton: Princeton University Press.

Dienekes. (2009). Archaic admixture in modern humans? Dienekes’ Anthropology Blog.

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Haynes, C.V. (1980). The Clovis culture. Canadian Journal of Anthropology, 1, 115‑121.

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Wall JD, Lohmueller KE, and Plagnol V. (2009). Detecting ancient admixture and estimating demographic parameters in multiple human populations. Molecular Biology and Evolution (early online) doi:10.1093/molbev/msp096

Watson, E., P. Forster, M. Richards, & H-J. Bandelt. (1997). Mitochondrial footprints of human expansions in Africa. American Journal of Human Genetics, 61, 691-704.

Zerjal, T., Dashnyam, B., Pandya, A., Kayser, M., Roewer, L., Santos, F.R., Scheifenhövel, W., Fretwell, N., Jobling, M.A., Harihara, S., Shimizu, K., Semjidmaa, D., Sajantila, A., Salo, P., Crawford, M.H., Ginter, E.K., Evgrafov, O.V., & Tyler‑Smith, C. (1997). Genetic relationships of Asians and Northern Europeans, revealed by Y‑chromosomal DNA analysis. American Journal of Human Genetics, 60, 1174‑1183.