extinction

Paleogenomics is changing the way we study evolution. In a number of cases, it now allows us to study extinct organisms with the same methods as we study living ones. A study last year in PLoS Biology[1] used genetic evidence from living elephants, extinct mammoths and mastodons, to reconstruct the times that these species diverged.

Woolly and Columbian mammoths

Mammoths are back in the news this week because of a paper by Jacob Enk and colleagues [2]. I think this paper represents a very nice collaboration of paleontologists (Dan Fisher, Ross MacPhee) and paleogeneticists (led by Hendrik Poinar's lab). It's refreshing to read a paper that describes not only the way that the DNA was sampled but also the age and morphological attributes of the sampled mammoths. For example:

This 60+ year old bull is exceptionally well preserved, and exhibits the classic character suite of his species, including low molar lamellar frequency (Figure S1 in Additional file 3), broadly divergent tusk alveoli, a markedly downturned mandibular symphysis, and tremendous body size. We used tusk fragments for the shotgun sequencing, and both tusk and bone samples for PCR and Sanger sequencing.

Every genetics paper should have descriptions like that. Very nicely done.

As an anthropologist, I pay a lot of attention to studies of elephants, because they are another long-lived social mammal, in some ways closer to us in population structure and dynamics than most primates. As in the case of hominins, some taxonomists have argued that we should recognize lots of fossil elephants, others question that distinctiveness. And just as we are discovering for hominins, the elephants are showing evidence for population mixture among groups once considered to be different species.

Enk and colleagues sampled the mtDNA from two Columbian mammoths and one woolly mammoth from North America. The Columbian mammoth is seen by pretty much everybody as a separate species (Mammuthus columbi) from woolly mammoths (Mammuthus primigenius), and paleontologists have thought that they diverged 1-2 million years ago. Woolly mammoths were Holarctic animals, with a range that extended from Europe to North America, while Columbian mammoths were limited to the Americas south of the U.S.-Canada border, roughly. Already other researchers have recovered dozens of woolly mammoth sequences, and their phylogenetic relations are well characterized (as shown in the paper). What Enk and colleagues show is that the two Columbian mammoths both have mtDNA sequences that belong to a single, relatively young clade that is present in woolly mammoths in Alaska and Yukon.

The simplest explanation is that the Columbian and woolly mammoths of North America were exchanging genes.

The authors also suggest the possibility of incomplete lineage sorting (ILS) -- the retention of a single ancestral clade in two isolated species. This seems unlikely given the topology of the clade within woolly mammoths, but the authors omitted the crucial test: the date of the most recent common ancestor of the mtDNA within the clade. If it's truly younger than a million years, we might easily rule out ILS.

Forest and savanna elephants

A lot more information about the variation within living elephantids has appeared within the past year. Looking at them compared to the fossil species, it's pretty clear that taxonomists haven't done well matching taxonomic levels in these groups. Here is a quote from the paper by Rohland and colleagues, who considered the genetic relationships of forest and savanna elephants in Africa.

We also find that savanna and forest elephants, which some have argued are the same species, are as or more divergent in the nuclear genome as mammoths and Asian elephants, which are considered to be distinct genera, thus resolving a long-standing debate about the appropriate taxonomic classification of the African elephants.

Forest and savanna elephants may deserve a species rank, but we might equally say that the mammoth-Asian elephant divergence doesn't merit the genus rank it has historically been given. As reconstructed in the paper, the forest-savanna elephant and Asian elephant-mammoth divergences both fall within ranges from 2.5 to 5.5 million years. Some widely-recognized mammalian genera (e.g., Homo) are younger, but most mammalian divergences in this range of times are recognized below the genus rank. Should mammoths be put into Elephas? That would probably be a better recognition of the adaptive radiation of Eurasian elephants.

One way to consider the question is by examining the pattern of speciation. With a large number of sampled loci, a far more detailed consideration of speciation can be achieved. This brings us back to a more careful examination of ILS.

We find a higher rate of inferred [Incomplete Lineage Sorting (ILS)] in forest and savanna elephants than in Asian elephants and mammoths: (FE+SE)/(AL+ML) = 3.1 (P = 4×10⁻⁸ for exceeding unity; Table 2), indicating that there are more lineages where savanna and forest elephants are unrelated back to the African-Eurasian speciation than is the case for Asian elephants and mammoths (Table 2). This could reflect a history in which the savanna-forest population divergence time TFS is older than the Asian-mammoth divergence time TAM, a larger population size ancestral to the African than to the Eurasian elephants, or a long period of gene flow between two incipient taxa. (We use upper case “T” to indicate population divergence time and lower case “t” to indicate average genetic divergence time (t≥T)).

"A long period of gene flow" would reflect a very gradual speciation event, which might argue that the two resultant species should be classified in the same genus. Or...it might suggest that the ecological differentiation actually commenced much earlier in time than the modal estimate, with later hybridization. Mammoths and Asian elephants, by contrast, seem to have a cleaner separation even though the genetic relationships are almost equally close.

We're not quite able to test these alternatives, yet, because only a single individual has been sampled from most of these species. Testing for gene flow really will require larger samples of individuals. In particular, the longer geographic distance between Asian and mammoth samples compared to forest-savanna samples may mean that population structure is hiding within this comparison. I just find it remarkable that genetics has arrived at a point where the pattern of speciation of extinct species is within reach.

The paper uses the extinct mammoth and mastodon comparisons as a frame for discussing the diversity and distinctiveness of African forest elephants. This is in a way unfortunate, because the mammoth-centric questions are probably more interesting to most readers. There's still a lot of productive biology to do there. But the status of forest elephants is a useful hook to hang a paper upon. Whether forest elephants should be given the status of a species has been a hot topic in proboscidean evolutionary biology during the past 10 years. Debruyne [3] gave a good historical review of the issues:

Indeed, when discovered by Matschie in 1900, [forest elephants] were described as either a potential species, or a regional race of Cameroon (Matschie, 1900). Matschie advocated the usefulness of hydrographical basins in order to subdivide African elephants into distinct units. He thus contributed to the profusion of new taxa to be defined by the turn of the 20th century, so that the taxonomy of the African elephant quickly became extravagant, the most meagre morphological evidence being used to acknowledge a new form (Lyddeker, 1907). Up to 22 forms of Loxodonta were described that were finally assigned either to the savannah or the forest elephant—see Laursen and Bekoff (1978) for a review. Morphologists have addressed this question for decades according to their personal taxonomic perspectives. Some have considered that, although displaying a smaller size, smaller round ears—responsible for their designation as “cyclotis”—more toenail structures on both feet, thin down-pointing tusks and a flatter back and forehead, forest elephants belong to the same species—i.e., Loxodonta africana—as savannah elephants with whom they assumed were reproductively compatible (Backhaus, 1958; Carroll, 1988; Cousins, 1996). Many cases of intermediate morphology have supported this view, which had become prevalent (Laursen and Bekoff, 1978). Conversely, the “splitter” attitude led other authors to put forest elephants apart on the basis of the same anatomical distinctiveness (Frade, 1931; Frade, 1933; Allen, 1936; Petter, 1958). More doubtful morphological characters—extent of hair-covering, color of the skin, carriage of head—have been put forward to support this division.

The problem became complicated upon recovery of genetic information. Most early phylogeography has been done using mtDNA. The deepest mtDNA clade in the African elephants defines two haplogroups, both of which are shared by the forest and savanna populations. Based on large samples of mtDNA alone, the two populations have been recently exchanging genes.

Early analyses of nuclear microsatellites indicated the opposite pattern, with relatively little allele sharing between the two elephant varieties. I became interested in the question after a paper by Régis Debruyne (a coauthor on the current paper by Enk and colleagues as well). Debruyne emphasized the great gaps in our sampling of geographic variation in African savanna elephants. Providing some additional data, he showed a very deep mtDNA clade in many forest elephants that was also in many savanna elephants. He argued that the widespread evidence of gene flow refutes the hypothesis of different biological species of elephants.

Rohland and colleagues also addressed the discordance between mtDNA and nuclear genetic variation.

Our study also infers a strikingly deep population divergence time between forest and savanna elephant, supporting morphological and genetic studies that have classified forest and savanna elephants as distinct species [13],[16]–. The finding of deep nuclear divergence is important in light of findings from mtDNA, which indicate that the F-haplogroup is shared between some forest and savanna elephants, implying a common maternal ancestor within the last half million years [21]. The incongruent patterns between the nuclear genome and mtDNA (“cytonuclear dissociation”) have been hypothesized to be related to the matrilocal behavior of elephantids, whereby males disperse from core social groups (“herds”) but females do not [13],[38]. If forest elephant female herds experienced repeated waves of migration from dominant savanna bulls, displacing more and more of the nuclear gene pool in each wave, this could explain why today there are some savanna herds that have mtDNA that is characteristic of forest elephants but little or no trace of forest DNA in the nuclear genome [13],[14],[39],[40].

The scenario may fit with the facts. It was proposed first by Roca and colleagues [4], who proposed it as a "genomic record of ancient habitat changes", which had brought the forest and savanna populations into contact across shifting hybrid zones. They reiterated the hypothesis in a later paper [5] supported with larger samples.

Further progress will require larger samples and better models. I was interested in Debruyn's account of the geographic holes in genetic sampling across the African range of forest elephants. A highly-resolved test of recent gene flow demands finding and sampling potential contact zones between two populations. Some hypotheses can be tested surprisingly strongly using only a single individual from each population. But the power of such tests depends on the pattern of inbreeding in the past. We can imagine that the ancestry of a single individual stretches through the genealogical network of a species like a cone, widening into the past. Recent events are poorly tested by single individuals.

If geographic structure is strong enough, distant populations will approximate different species in their recent genealogical connections. So the single individuals in the more recent study by Rohland and colleagues [1] carry a lot of weight.

There are many parallels here between hominin population dynamics and the elephants. Also, as I pointed out in 2006, the elephant situation helps to clarify how we should consider genetic samples from living great apes.

The past year has seen a real reversal in the race between data and analysis. For a long time, sequencing has been a bottleneck in serious analysis of population history. The genealogical connections among individuals ramify by double in every generation, so that the inheritance of a single gene reflects one possibility among countless trillions. If we can only afford to sequence a single gene, we are limited to a single sample of the genealogical links among individuals. Whole genomes give enormous samples of the genealogical history among samples. But they create their own challenges of analysis.

References

OK, so maybe it wasn't volcanoes.

Jean-Pierre Valet and Hélène Valladas in a brand new paper [1] propose that geomagnetic excursions at around 40,000 and around 32,000 years ago would have weakened the ozone layer, thereby irradiating the Neandertals with extra ultraviolet:

No special attention has been given to the geomagnetic excursions of Laschamp and Mono Lake which are synchroneous with the extinction and were the most dramatic events encountered by the Neanderthals over the past 250 thousand years of their existence. During this period the geomagnetic field strength was considerably reduced and the shielding efficiency of the magnetosphere lowered, leaving energetic particles reach latitudes as low as 30°. The enhanced flux of high-energy protons (linked to solar activity) into the atmosphere yielded significant ozone depletion down to latitudes of 40–45°. A direct consequence was an increase of the UV-B radiations at the surface which might have reached at least 15–20% in Europe with significant impacts on health of human populations. We suggest that these conditions, added to some other factors, contributed to the demise of Neanderthal population.

The paper gives all the information anyone could want about the geochronology of these events, evidenced worldwide in lava flows.

By the way, I was wondering during the volcano post why it is "lava flow" but "ice floe". I guess it's because an ice floe floats. When the ice isn't floating, as in a glacier, then it flows, too. And I don't think "floe" is a verb. The only remaining mystery: Why does a Google search for "ice floeing" bring me ads for "ice flooring"?

The paper's discussion expresses why the geomagnetic anomalies may be relevant to Neandertal extinction.

The most widespread measure of UV intensity from the public health perspective is the UV index. This is a value on a scale from zero to 11, which is a linear (not logarithmic) function of UV radiation intensity, weighted in a specific way by wavelength. The damaging UV-B waves contribute disproportionately to the UV index value. I mention this because the average UV index is readily obtained for most cities in the U.S. and Europe, and is a standard part of weather forecasts during the summer.

A 20% increase in UV-B radiation sounds dire, but it's roughly what you get by driving from Massachusetts to Virginia. It is true that melanoma rates and other complications from excess UV radiation are higher in populations who have migrated from higher to lower latitudes, increasing their exposure to UV. But these rates do not rise to a level that threatens to drive those populations to extinction.

We do worry about ozone depletion as a risk to threatened endemic populations, as they may be critically affected by new stressors. So the question is, how threatened and endemic were the Neandertals? Would this possible stressor have been enough to have tipped them over the edge to extinction?

I think we can answer these questions pretty easily. The Neandertals were a cosmopolitan population that occupied most of Western Eurasia spread across at least 20 degrees of latitude. The latest Neandertals persisted in the areas of Europe with the highest insolation. UV radiation may have been a stressor but it cannot have been decisive in their decline.

References

I'm reviewing some old viewpoints about the relationships of Neandertals and other peoples. These include mainstream opinions that persisted over decades as well as more idiosyncratic ideas. This is mostly pre-1960 stuff for the time being.

To the extent that old ideas are wrong it is no surprise: Science progresses by rejecting wrong ideas, and paleoanthropologists of the past lacked the luxury of today's data. To the extent that the ideas look familiar, they remind us that our current hypotheses in many instances echo ideas that were advanced fifty years ago or more.

Weckler's model

A bit off the mainstream was a paper published by Joseph E. Weckler [1], titled "The relationships between Neanderthal Man and Homo sapiens." Weckler was a cultural anthropologist who had done fieldwork in the American Southwest and the South Pacific [2]. He wrote only one paper on Neandertals but this received substantial attention, first published in the American Anthropologist and later revised in a simplified version for Scientific American. Weckler was very interested in the migration and dispersal of ancient populations, maybe because of his work on the ethnography of the South Pacific. He brought that perspective to the Neandertals and other ancient groups.

Weckler saw Pleistocene human population dynamics as having been directed by glaciations and geographic barriers. In general, Weckler thought that the pre-modern population had been divided into allopatric species or subspecies. These groups would have been isolated from each other much of the time, but occasionally thrust back into contact by shifts in the climate. During glacial phases, Weckler posited that Europe and Asia north of the Caucusus-Himalaya axis would have been uninhabitable. During warmer interglacials humans moved into these northern areas, where water and mountainous barriers tended to isolate them. The overall pattern was evolutionary differentiation punctuated by occasional hybridization and cultural contact between long-separated groups.

Weckler was not the first to propose that Neanderthal and modern lineages had been relatively isolated and later hybridized. The idea was widespread after the description of the Mount Carmel remains by McCown and Keith [3]. McCown and Keith themselves had favored a different explanation -- that the Skhul and Tabun remains represented a transient between a less specialized and more specialized (Neandertal-like) extreme. Others, including Carleton Coon [4] and Theodosius Dobzhansky [5], immediately favored the idea that the Mt. Carmel sample represented a hybrid population.

Weckler broadened the idea of hybridization into a general theme. He supposed that we might expect recurrent contact during second (Mindel-Riss) interglacial times in Central Asia, and repeated dispersal from India into Southeast Asia throughout the Pleistocene. Thus, hybridization between divergent groups was not a one-time affair but instead was a fundamental aspect of Pleistocene human evolution.

Interglacial population contact

This scenario faced an obvious problem: There were essentially no data to test the hypothesis of population contact at any of these earlier times. Only the third interglacial, already treated by other authors, gave the appearance of sufficient information for a test. To illustrate the plausibility of recurrent exchanges, Weckler fleshed out a third interglacial model of population contact in some detail:

Some of these pre-Neanderthal men wandered inland into Asia north of 40° during a period of warm climate. Part of this population may subsequently have been trapped north of the barrier in the general vicinity of Inner Mongolia or Sinkiang at the onset of the next glacial period. Primitive man caught in this area would have been unable to retreat directly southward because the great mountain mass that lay in that direction became frigid sooner than the lower lands to the north. Having lived where he was for hundreds of generations, primitive man might not have known he could escape the increasingly rigorous climate by moving east several hundred miles before turning south. Howell (1951:409) suggested that some of the physical characteristics of classic Neanderthal man may represent biological adaptation to a glacial climate. Coon stated in a letter to me (1953) that he has long been of that opinion. If this is so, I suggest the evolution occurred, not in Europe during the fourth glaciation, but in eastern Asia during an earlier one (Weckler 1954:1010).

This is an early exposition of the idea that Neandertals repeatedly invaded the west from a homeland somewhere in central Asia or further east. Weckler discussed the idea that these populations originated in northwestern China, but he had no good examples (as indeed there are still no such examples).

Weckler's discussion may seem confused because he accepted Zhoukoudian as an eastern "Neanderthaloid" population. His division of humanity can best be aligned along a "paleanthropic/neanthropic" distinction. Today, we might more simply state his biogeographic model as a shifting border between the paleanthropic "Neanderthaloids" and neanthropic "Homo sapiens" along a shifting Movius line somewhere in India or the Middle East, stretching to northwestern China.

A central Asian source

Teshik Tash bears much importance to Weckler's ideas, as it did to Movius, Howell, Weidenreich, and many others. To those unfamiliar with the site, an interesting place to start is my interview with Mica Glantz. Teshik-Tash is once again central to our ideas of Neandertal biogeography, with the addition of genetic evidence from the juvenile specimen from the site and others in Central Asia.

In the early 1950s, Teshik-Tash raised many of the same issues that it does today. Today, of course, Teshik-Tash is far from alone, with several sites in Central Asia bearing evidence of a local Mousterian, physical remains with Neandertal-like mtDNA sequences. There was great uncertainty about the date represented by the Teshik-Tash specimen. Teshik-Tash had a classic "Western" archaeological industry (in this case, Mousterian) and therefore evidenced long-range population contact with Europe. The East Asian fossil record was known to be very different from the west, raising the question of boundaries. Where did the Western sphere of biological influence end, and the Eastern begin?

Today Denisova Cave, embedding a highly divergent mtDNA clade in an initial Upper Paleolithic assemblage [6], presents the same issues with even greater relief.

Probably the most common interpretation of the Central Asian "Neandertal" sites is that they represent an eastward migration from the Neandertals' center of evolution in Europe. But the opposite hypothesis is an obvious alternative: that the center of Neandertal evolution was somewhere in Central Asia, and that they invaded Europe from outside. Some may see parallels for a Neandertal invasion of Europe from outside, by looking both earlier in evolution (the first Europeans obviously came from somewhere) and later (the Upper Paleolithic, the Neolithic).

Why posit Central Asia in particular as a source area, above and beyond the general idea of invasion? I thought the idea might have originated with Henry Fairfield Osborn because of his long interest in Central Asia as a center of human evolution. For Osborn, Central Asia was a source of humanity, but his "Dawn Man" idea supposed that the modern human form had long resided in Central Asia, with more primitive humans at the periphery. The idea that a Neandertal center of evolution existed in Asia is quite different from Osborn's idea, which was itself a sketch supported by little evidence. I'll have more on Osborn later.

Weckler presented his idea to address a classic problem: To many paleoanthropologists, early Neandertals appeared to be more like later human than were the later, "classic" Neandertals. Howell [7] summed up this observation as follows:

Many features of early Neanderthal morphology, both cranial and postcranial, are incipiently classic Neanderthal. However, the general morphological pattern of these early Neanderthal peoples bore a close resemblance to that of anatomically modern man, a fact which indicates again the special character of classic Neanderthal morphology (Howell 1957:332-333).

The early Neandertals were those from the third interglacial, which during the 1950's would have included those from Krapina, Ehringsdorf, and Saccopastore. Howell's description highlights the most common hypothesis: classic Neanderthals had evolved toward greater and greater specialization over time.

Weckler took a different approach: for him, the fourth glaciation Neandertals descended from already-specialized ancestors, who had existed in Central Asia:

The Asiatic migrants, probably already mixed with Homo sapiens in central Asia in the Middle East, pushed on to central Europe during the third interglacial. They may have moved northwestward from Palestine or directly westward along the north face of the barrier. In the zone of contact in western Asia and eastern Europe further miscegenation and cultural exchange probably occurred. Then, when the climate deteriorated with the onset of the fourth glaciation, the bulk of the Homo sapiens population retreated south as was its wont. This left Europe open to further Neanderthal invasion and set the stage for the modern misconception that classic Neanderthals evolved rapidly (and in a curiously regressive fashion) in western Europe during Würm I. Probably all that actually happened was that additional Neanderthals of more classic type, adjusted by previous experience to life in a cold climate, kept pushing in behind the advance guard and, by weight of numbers, blotted out the neanthropic traits the earlier migrants had acquired along the way.

Weckler proposed this scenario not long after F. Clark Howell's 1952 paper [8], in which Howell had proposed that climate isolated Neandertals within Europe during the last glaciation, leading to their increasing specialization. According to Weckler, the glaciations had not isolated Europe so much as they had wiped clean the evolutionary slate within Europe. After the last interglacial, migration from a central Asian source brought back a purer strain of Neandertal.

Out of this welter of fact and interpretation emerge the few concepts necessary to the hypothesis supported in this paper. By the end of third glacial times Neanderthal had probably developed in eastern Asia to something approximating the classic form. His numbers had probably always been small compared to developing Homo sapiens: his range was incomparably smaller, and in part of this range he had no easy retreat from glacial conditions such as Homo sapiens enjoyed. His restricted range (and possibly his sometimes severe habitat) had militated against the racial diversification that characterized the development of Homo sapiens. In spite of his cultural advances his range and numbers were probably sharply reduced during every glacial episode he had to endure. This may be why, although he stood athwart the entrance to the New World, he never expanded his range sufficiently to explore that territory. But as the climate ameliorated after the rigors of the third glaciation, his numbers increased and he did finally expand his range. For reasons not as yet ascertained he looked westward, and the lowlands north of the barrier afforded him a route to Europe.

Several strains of contemporary thought emerge in Weckler's formulation. Neandertals were always on the edge of extinction, being repeatedly driven to low numbers by deteriorating climate. Their tenuous existence did not allow them to disperse more broadly.

That old Neandertal magic

Where Weckler differed from the received view is in the way he accentuated the Neandertal positives. He wrote that the diversification of humans and Neandertals presented an opportunity to the evolution of our species. From their central Asian source, the Neandertals had acquired innovations necessary for existence in the cold north. Human colonization of these regions might be impossible without the adoption of Neandertal cultural and behavioral innovations:

The Homo sapiens groups that retreated south from Europe and perhaps from central Asia [during the glaciation] had been touched by Neanderthal magic. They may have acquired some Neanderthal physical traits, but, more important, they had achieved a new cultural outlook. They had perhaps learned the use of fire, clothing and specialized hunting techniques, and possibly of cave dwelling -- accomplishments that freed man from dependence on a mild climate and from a grubbing existence (emphasis added).

I find myself reading this on two levels. On the concrete, empirical side, Weckler would soon be proven wrong. Neandertals didn't invent fire; that was much older and more broadly shared by Middle and Late Pleistocene humans. They may have had better clothes for cold weather than contemporaries who lived further south, but the innovations of woven cloth, sewn garments, and shoes happened later. They certainly had specialized hunting techniques, but these were linked to a particular kind of social organization and technology. Later developments in both would have required new hunting (and gathering) methods. None of them lived in caves very often; their experience must have been fairly "grubbing" in either event.

But on the abstract, Weckler presents a scenario where Neandertals had something of value, cultural or physical, without which later humans would have been as successful. He had already posited biological hybridization; here he suggests a kind of "cultural hybridization" as well.

The essential idea I am suggesting is that the contact of Homo sapiens groups with "Neanderthal culture" in Asia and in Europe during the third interglacial resulted in an efflorescence of "Homo sapiens cultures" that gave rise to the Upper Paleolithic. There is general agreement, I think, that a sudden enrichment of culture is evident at the beginning of the Upper Paleolithic in Europe and that these richer and more varied cultures seem to have originated, for the most part, outside of Europe. Movius, discussing the European Upper Paleolithic (1953:171ff.), follows M. Denis Peyrony, Dorothy Garrod, and others in suggesting that different European cultures of that time may have originated in Palestine, Iran, the plains of southern Russia, and possibly Africa. All but the latter are areas where indigenous Homo sapiens was probably directly stimulated during the third interglacial by invading Neanderthal man (Weckler 1954:1016).

So why has this idea been largely forgotten? The failure of the particulars was almost complete:

Leakey claimed in the 1930's that Lower Aurignacian techniques of stone chipping were older in Africa than in Europe (1931:237-39; 1936:54-60, 161). Movius seems ready to dismiss Africa as a source of European Aurignacian (1953:171), but he doesn't dispose of Africa's claim to temporal priority. The sudden new competence Leakey claims for African Aurignacian cultures early in the fourth glaciation (1936:161) may have been the consequence of contact with Neanderthal. The stimulation may have come secondhand from Homo sapiens wanderers returning from Europe or may have resulted directly via diffusion or migration from the Middle East.

He was overreaching here. He didn't overestimate the cultural sophistication of Neandertals, although he did accentuate behaviors, like fire, that would turn out to be less special than he assumed -- older than Neandertals and more broadly shared. More critically, Weckler rested his argument on the absence of evidence for cultural sophistication in the African contemporaries of the Neandertals. But Louis Leakey's earlier claims about an "African Aurignacian" also overreached, supported by a mistaken chronology. A better understanding of the Late Pleistocene African cultural sequence would emerge only later.

When Homo sapiens had thoroughly assimilated and improved on the ideas he got from Neanderthal, he took advantage of the first interstadial of the Würm glaciation to launch forth on his initial conquest of the world. He overran Europe and pushed around the barrier into eastern Asia.... One might even hazard the guess that the reason Africa south of the Mediterranean littoral remained so backward during the Upper Paleolithic was because the Homo sapiens groups there had not had the full benefit of Neanderthal stimulation. In the new dynamics of cultural enrichment and sapiens migrations the hinterlands of Africa had become a dead end, far removed from the centers of rapid development.

I find myself wondering about the nature of "Neanderthal stimulation"....

This passage is worth examination. Most of the details have changed radically since 1954. We now know that MSA Africans had most of the tricks that Neandertals did, and vice-versa. Many MSA industrial innovations predate Mousterian or Middle Paleolithic occurrences. The complexity within Africa may itself represent a vastness of population history that we now can only guess at.

Yet the development of Upper Paleolithic cultural complexity still wants some explanation. The biological innovation of "anatomical modernity" is not sufficient to explain the cultural evolution of the Late Pleistocene -- it does not match the pattern of cultural innovation in time or space.

Bottom line

I think there was some "Neandertal magic." Middle Pleistocene humans were more isolated than present-day populations, for a longer period of time. Less gene flow made it less likely for adaptive traits to spread beyond the population where they originated. Not impossible, just less likely. So any surge of population contact caused by migration would have been accompanied by a surge of introgression of adaptive genes. The evidence for Neandertal contribution to the later gene pool of non-Africans documents one such surge of population contact, but there may well have been others.

Where genes are concerned, this is a simple matter of mathematics, discussed more fully by Greg Cochran and I in our 2006 paper [9]. Simply put, Neandertals and modern humans had comparable selection pressures for many aspects of their biology, similar adaptive responses, and the same time to adapt. Adaptive mutations are chance events, governed by demography and time. If the evolving African MSA population got many new adaptive mutations, Neandertals would have gotten nearly as many (possibly constrained by smaller population size). In a few cases, the same variants would occur in both populations by chance, but in most they would be different. These alleles should still be with us, as the extent of Neandertal contribution to our population was great enough to pick up almost all of them.

But what about Neandertal cultural traits? These were the real focus of Weckler's argument, and here I think the question is very difficult to resolve today. Cultures are ephemeral. As we know from history, if we choose a beginning and end point a few hundred years apart, it can be difficult to show the continuity of cultural information even within a single place.

With the transition from Mousterian, through Châtelperronian into Aurignacian in France and northern Spain -- a place where we have relatively dense archaeological documentation -- we are nevertheless talking about time gaps of hundreds of years. I'm skeptical that we're in a position to test the hypothesis of cultural exchanges across these time periods.

We're in a better position to test the hypothesis of stasis. If genetic exchanges happened in the absence of culture change, that would tell us something very relevant to the relation between gene flow and demographic contact. Likewise, persistent stasis of different cultures in adjacent areas tells us something about the absence of information flow. A kind of regional stasis, over thousands of years, seems to have been the norm in MSA and Middle Paleolithic contexts, and it's not a pattern that we are well-placed to understand without a better understanding of the limits on information exchange. Some of those limits may, in these ancient populations, have been biological constraints. So I'm less confident that we will be able to understand the cultural consequences of Neandertal contact.

References