Late Pleistocene

The science press has its own synchronized cycle, like brain waves, and being in Rome seems to make me into a misfiring neuron. Here it is tomorrow, and there's this story about Neandertals all being dead before modern humans showed up, which for Americans is now yesterday's news. Unless you take the paper NY Times, of course, in which case you probably haven't read it yet.

The occasion for the article is a paper reporting new radiocarbon dates for one of the specimens from Mezmaiskaya, a site in the Russian Caucasus.

The site and the date of the Mez 2 burial

Excavations by Golanova and colleagues have recovered two burials of young children from this site. One of them (Mez 1) has been the subject of much research. Based on some skeletal features and a partial sequence of its genome, the skeleton is a Neandertal child. A sample of one of its ribs was taken for radiocarbon dating, reported in 1999, and yielded a direct AMS date of 29,000 BP. This was out of sync with the other dates from the surrounding level of the site.

Ten years ago, Milford Wolpoff and I suggested that the skeletal features by themselves weren't very convincing, and a recent date (apparently out-of-sync with the surrounding archaeological layer) might signal an intrusive Upper Paleolithic burial [1], despite its Neandertal mtDNA sequence. Now we can look at a large part of the genome of this individual, which is very much like the Vindija Neandertals. In 2005, Skinner and colleagues reported ESR dates from Mezmaiskaya, concluding that layer 3 (including the Mez 1 burial) dates to between 60,000 and 70,000 years ago. By that time, those authors were discussing the inconsistency of the recent 29,000 date for the rib, compared to much older dates (>35,000 BP) for an overlying Mousterian layer. They expected the underlying layer 3 to be much older, and found that to be true of the ESR date estimates.

The second child burial, Mez 2, comes from layer 2 of the site, which is also Mousterian but younger than the first burial. A date around 40,000 years for the Mez 2 infant is basically what was expected six years ago by Skinner and colleagues:

Infant 2 was found in a pit introduced from Layer 2 into Layers 2A and 2B(1). Its’ age therefore is probably about 40 ka. Since the precise surface from which the pit was dug is unknown, this should be considered a maximum age.

That left open the possibility that the burial might be younger.

In the new paper [2], Ron Pinhasi and colleagues report that a sample taken from the infant itself dates to 39,700 +/- 1100 radiocarbon years BP, which calibrates to between 42,960 and 44,600 calendar years BP. The new date confirms that the burial happened relatively soon after the deposition of the surrounding dated bones.

The authors additionally report many other date estimates for faunal materials from the site. These form a pattern in which most are consistent with a relatively narrow range of date estimates, but a few are outliers. One of the important conclusions from the outliers is that contaminated carbon is hard to get out of a sample, even with the advanced ultrafiltration performed by the Oxford lab. The conclusion is narrowly interesting and solid, and it's very important to iron out such inconsistencies -- compare, for example, my 2008 post on the Gorham's Cave chronology.

So what is the big deal?

What does the paper say about the dates of other sites?

Here's where things get interesting. The paper includes this passage in its discussion:

The critical reanalysis of directly dated Neanderthal and AMH fossils from across Eurasia, taking into consideration pretreatment histories and redating results (5), supports our findings in the Caucasus and highlights the lack of reliably dated Neanderthal fossils younger than ∼40 ka cal BP (Fig. 3). Contrary to traditional arguments for up to 10,000 y of coexistence, these data suggest that Neanderthal extinction across Western Eurasia, including the Caucasus, was probably a rapid process, and that coexistence with AMHs, when it occurred, may have been of limited duration.

and this in its abstract:

Our results confirm the lack of reliably dated Neanderthal fossils younger than ∼40 ka cal BP in any other region of Western Eurasia, including the Caucasus.

That last part is a pretty strong statement. No reliably dated Neandertal fossils anywhere after 40,000 years ago?

I thought that was so surprising that I corresponded with the study authors today. One distinct advantage of being in Rome is that I'm synchronized with Europeans, so Tom Higham was able to write back with some of his thoughts. The authors' doubt in the later dates for Neandertal specimens is genuine; their experience is that the newer treatments to remove recent contaminating carbon from samples is eliminating Neandertal dates under around 40,000 years.

A systematic revision of the radiocarbon chronology of late Middle and early Upper Paleolithic Europeans has been underway for several years. This has been an important story, and I've written about it several times (my "dating" category hits most of the posts). I think I once told a journalist that this was the most underreported story in paleoanthropology.

In 2006, Higham and colleagues reported that dates obtained for the Vindija G1 Neandertals, at 29,000 BP, were too young by some 4000 years [3]. That result is listed in the current paper as "doubtful" becuase it did not employ the latest purification strategies. That helps to show that the current paper is "equal opportunity" -- past results from the Oxford Accelerator unit are not immune to doubt. But it is hardly confidence-raising. If we cannot trust radiocarbon determinations made in the last five years, why should anyone submit further samples for testing?

Personally, this was my reaction to the paper: don't grind up any more human bone until the radiocarbon community is unified about sample processing techniques. Let them work it out on the fauna.

The paper lists 15 direct dates on Neandertal specimens younger than 40,000 calendar years BP (some of them multiple samples from single skeletal remains). It lists all 15 of these as doubtful because they do not employ the latest techniques. That is a point emphasized by Higham also (and reflected in several past papers): these date determinatinos are not trustworthy given what we know about sample contamination by recent carbon-14. The Oxford group has put out several papers on this problem. One of the most useful is by Blockley and colleagues [4] because it introduces the device of using the Cantabrian Ignimbrite ash horizon as a marker to compare dates -- dates below the horizon should be consistent, whereas a large sample of actual date estimates include many that are far too young.

At any rate, this is where the story in the linked news article (and others) comes from. University College Cork issued a press release in conjuction with the paper's early edition release in PNAS.

Direct dating of a fossil of a Neanderthal infant suggests that Neanderthals probably died out earlier than previously thought. Researchers have dated a Neanderthal fossil discovered in a significant cave site in Russia in the northern Caucasus, and found it to be 10,000 years older than previous research had suggested. This new evidence throws into doubt the theory that Neanderthals and modern humans interacted for thousands of years. Instead, the researchers believe any co-existence between Neanderthals and modern humans is likely to have been much more restricted, perhaps a few hundred years. It could even mean that in some areas Neanderthals had become extinct before anatomically modern humans moved out of Africa.

This is the lead of the press release. I think that the claim makes up only a minor part of the paper (which is really a results paper about Mezmaiskaya). It is clearly interesting and provocative, but I think the paper's results by themselves do not justify the claim. In the case of Mez 2, a skeleton that the excavators expected to be 40,000 years old, actually turns out to be 40,000 years old. No surprise. There were no incorrect radiocarbon assessments of this specimen, and the apparently wrong assessment of the Mez 1 infant (at 29,000 years old instead of beyond radiocarbon range) has not been corrected here.

Were Neandertals really extinct by 39,000 years ago?

Now, in one sense, the survival of Neandertals after 40,000 years ago is not terribly important. Africans mixed with Neandertals, and as far as we can tell (an issue my lab is addressing now) the mixture is not preferentially within Europe. That argues for a West Asian interaction of the population, and it remains to understand why the ancestors of Europeans did not interact more than other populations. Probably a good hypotheses is that today's Europeans derive most of their ancestry from outside Europe during the last 10,000 years. If the Neandertals did not persist within Europe long during the Upper Paleolithic, that provides another alternative.

But to say that we doubt a particular kind of information about dates is not the same as saying that Neandertals did not exist after 39,000 years ago.

Direct dates on Neandertal bones are far from the only evidence of their persistence in Europe. Dozens of sites are dated by radiocarbon on fauna or charcoal. These dates themselves may be subject to the same critique as applies to the human bone. But there are not 15 of them, there are many, many more.

For example, Gravina and colleagues [5] list 24 AMS dates for Châtelperronian contexts that are 36,000 BP or less. Calibration of dates for this era adds more than 3000 years or more to the calendar years represented by a radiocarbon date, so these are dates likely less than 40,000 years. They may be contaminated by recent carbon (and indeed a few are outliers below 32,000 BP), but if so some of them are remarkably consistent.

Martínez-Moreno and colleagues [6] give a recent review of the Middle-Upper Paleolithic transition in Iberia. They list several sites with late Mousterian industries later than 34,000 radiocarbon years BP, even without counting the contentious examples (like Gorham's Cave) that arguably are later than 30,000 BP.

I would not be happy assuming that every Mousterian site is a Neandertal site, not even in this limited geographic context. There is too much technical overlap, and sometimes small samples of artifacts, to be definitive about such an association. Technology is not biology. Neither would I be willing to assume that late Neandertals are entirely Neandertal -- we see no genetic evidence of African mixture into this population in the Vindija or El Sidron genomes, but these are older than 45,000 years. Who knows what a 35,000-year-old Neandertal in France or Spain (or Croatia) would look like genetically? But only Neandertal remains have thus far been associated with Mousterian and Châtelperronian in France and Iberia. Several sites have stratified Middle to Upper Paleolithic transitions with dates after 40,000 calibrated years BP.

So from the Neandertal point of view, I think this is largely a non-story. There remains substantial question about the pattern of appearance of the post-Neandertal population, as I've extensively discussed here. When we consider the Caucasus, we are still working to understand the timing and mode of the later Neandertals and early Upper Paleolithic people. But there's really no serious challenge to the idea that Neandertals existed in Western Europe after 40,000 years ago.

Or if there is, it'll be out of sync with what most of us think we know.

References

Bornean and Sumatran orangutans are the most highly divergent subspecies within any of the living species of great apes. The two farther apart even than chimpanzees and bonobos, which are good biological species. The time of the Bornean-Sumatran orangutan divergence as estimated from mtDNA is around 3.5 million years ago.

This is old enough that many primatologists consider the two populations as separate biological species. The species distinction is supported by some aspects of morphology, but as yet we have no good nuclear DNA information about the extent of divergence. In chimpanzees, nuclear genetic comparisons suggest a relatively recent founding of one subspecies and recurrent gene flow between the others, despite high mtDNA divergence between the subspecies. So information from across the genomes of Bornean and Sumatran orangutans may be necessary to substantiate the hypothesis of long isolation suggested by mtDNA.

Within Borneo, different local populations of orangutans have strong genetic differentiation, with few shared mtDNA haplotypes among them. A new study by Natasha Arora and colleagues [1] has provided further detail about these relationships within Borneo. Based on earlier work, they expected to find high population differentiation within Borneo, and that is what they found:

[O]ur analyses revealed high and significant mitochondrial differentiation, with populations within currently recognized subspecies generally displaying as much differentiation as those between subspecies. Of notable interest is the great extent of subdivision and lack of reciprocal monophyly for the morphologically recognized subspecies P. p. morio and P. p. wurmbii. MtDNA haplotype sharing is uncommon and for populations separated by rivers occurs only in two instances: (i) for SA and GP and (ii) for the northern and southern populations across the Kinabatangan river. In both cases, very recent common ancestry could explain the incomplete mtDNA lineage sorting. For North Kinabatangan (NK) and SK, Jalil et al. (27) proposed an expansion from a recent common refugium further west in Mount Kinabalu, as posited for other Bornean species (46, 47, 49). DV, with its low haplotype diversity, might also be the result of a recent range expansion. GP is located proximally to the Bangka–Belitung–Karimata–Schwaner divide, from where orangutans are presumed to have dispersed to the rest of Borneo (12) and where we might expect a rich haplotype diversity. However, the presence of only one mtDNA haplotype shared with populations further east suggests that the current population in GP is recent and/or underwent a severe recent bottleneck. This and other local bottlenecks make it impossible to reconstruct a colonization of Borneo through the southwestern “choke point” (52).

They were able to confirm the relatively strong differentiation of Bornean populations by examining nuclear microsatellites. These do not give a great indication of the time period over which the populations may have developed their differentiation, but the microsatellites do document the relative lack of allele sharing between the populations, attesting a history of low gene flow in the recent past. The populations they identify as strongly differentiated do not correspond entirely with the subspecies recognized along morphological lines, but there are strongly differentiated populations here.

The "news" aspect of the paper is the one unexpected observation: the mtDNA ancestor of Bornean orangutans lived relatively recently, only around 176,000 years ago (with a range of error stretching from 72,000 to 320,000 years ago. The data in the study do not allow us to distinguish whether this was a time when the Bornean population may have been founded, or whether instead the mtDNA lineage spread through pre-existing populations. The authors pursue the hypothesis that Bornean orangutans were limited to a refugium sometime during the early Late Pleistocene:

Assuming that orangutans arrived in Borneo around the same time as gibbons and macaques, the recent coalescence of Bornean orangutans could be explained by a bottleneck through a severe rainforest contraction. Such a bottleneck would have had a more dramatic impact on the mtDNA structure of orangutans compared with other species as a result of their low densities and slow life histories (18) as well as habitat requirements.

The comparison with gibbons and macaques is necessary because both have substantially deeper mtDNA coalescence times within their Bornean populations. If the forest had been substantially reduced to a small area where orangutans could survive, we might expect the other primates to reflect this event -- and they don't. Nevertheless, a grab-bag of climate change scenarios appear next:

Geomorphological and palynological data indicate the presence of dryer, more open vegetation in southern and western Borneo during the last glaciation (2, 41), and by extrapolation also during other glaciations (but c.f. refs. 42, 43). Climate change was especially severe during an extended cold period within the penultimate glaciation between 130 and 190 ka (44, 45), which occurred approximately at the time of mean coalescence of Bornean mtDNA haplotypes. More recently, the last Toba eruption approximately 74 ka resulted in a short, albeit signi␣cant, decrease in regional temperatures, ensued by a 1,800-y cold stadial (9, 10). Our data do not provide clear signals to make conclusive statements about potential Toba effects. Nonetheless, the coldest period of the penultimate glaciation (44, 45) was more prolonged than the cold period following the last Toba eruption, suggesting more severe effects of the former on the extent of rainforest across Sundaland. In any event, suitable rainforest habitat for orangutans should have existed in certain regions in Borneo where a refugium population survived the dry glacial conditions.

A coalescence time of 176,000 years ago does not point to a short-duration bottleneck that began 74,000 years ago. If orangutans in the Middle Pleistocene of Borneo had high genetic differentiation, a crash would have to have been very severe -- eliminating all but one small regional population -- to have effected the present distribution. Still, the great uncertainty in the actual coalescence time leaves open many possibilities, and the refugium hypothesis in the general case is worth testing, even if the Toba eruption in particular cannot explain the data.

Given the uncertainty about the habitat structure of the now-submerged areas of Sunda, we may also want to consider the hypothesis that the present orangutans arrived recently on Borneo from mainland Southeast Asia. Even if orangutans had lived on Borneo during the Middle Pleistocene, they may not have been the current orangutans. Or even better, they may have been Neanderorangs -- an initial population that was genetically swamped by migrants arriving from elsewhere. The deep Sumatra-Borneo divergence means that the Bornean population was probably not recently derived from Sumatra, but that's a very restricted source compared to the Late Pleistocene distribution of orangutans across mainland and island East and Southeast Asia.

Some other animals walked from Sumatra to Borneo repeatedly during the Pleistocene, including humans. In the human case, we know that a large fraction of the genetic ancestry of Bornean and Javan people was derived from Asia within the last 100,000 years -- in other words, Late Pleistocene gene flow. The movement of genes may have happened in the context of a dispersal of Asian (or ultimately, African-derived) populations into island Southeast Asia. The paper includes some discussion of other primate species:

For instance, the south Bornean gibbon Hylobates albibarbis and the Sumatran–Malaysian gibbon Hylobates agilis have a TMRCA of 1.56 Ma (36), and Bornean and Sumatran pig-tailed macaques have one of 3 to 4 Ma (37). By contrast, the Bornean–Sumatran common ancestor of both the silvered langur(39) and clouded leopard (40) is much more recent than that of orangutans, gibbons, and pig-tailed macaques, probably because of a higher ␣exibility in habitat use.

The pig-tailed macaque divergence time is more or less the same as the orangutan divergence; the others are more like the time range for human dispersals into island Southeast Asia. We can add to the primates a few other medium-sized mammals; for example, clouded leopards are highly differentiated between Sumatran and Bornean populations, and their mtDNA divergence occurred sometime after 3 million years ago.

There may be no contradiction between the recent mtDNA common ancestor and the high degree of population structure in Bornean orangutans; the mtDNA could have been selected. We really would want resequencing of a lot more loci in these orangtuan populations, for which we may not have to wait too long. Mitochondrial DNA is convenient in many ways, including its greater sensitivity to restricted population size and higher mutation rate. But the intrinsic variance of a single gene system under genetic drift is so high that this disadvantage probably outweighs all advantages for reconstructing population sizes.

At any rate, the orangutans now provide an additional case where the subspecies-level history of hominoids is more complex than depicted five or six years ago. Uncovering these kinds of dynamics highlights the need for better modeling of demography and dispersal within a geographically widespread species. Isolation-by-distance and long-lasting subspecies are well-defined models, but when they are refuted, we have a lack of well-defined alternatives.

References

Well, I already snarked on the science headlines that have been claiming volcanoes "wiped out" the Neandertals. Some variation of this story, swapping in a different Neanderkiller, has been circulating since around 1890. But is there any truth to the headlines?

(see UPDATE below)

The source of the story is a paper in the October issue of Current Anthropology, by Golovanova and colleagues [1]. The paper reviews the chronology of Mezmaiskaya Cave, a site occupied by Neandertals and successive Upper Paleolithic peoples, in the Russian Caucasus. This site produced the skeleton of an infant, from which DNA evidence has been recovered. As Golovanova and colleagues describe, the deposit additionally contains volcanic ash from two eruptions that happened around 40,000 years ago.

The latter of the two eruptions appears to coincide with a long abandonment of the site:

Hominin occupation of Mezmaiskaya Cave changed dramatically after the later volcanic eruption represented in layer 1D. This eruption was probably more powerful than that in layer 2B-1. Layer 1D has a thickness up to 0.7 m and in some areas is composed of a relatively clean sediment lacking any inclusions (fig. A10). Limestone fragments, bones, and lithic artifacts are absent, and even pollen grains are rare. Pollen data show that extreme deterioration to a very cold and dry climate occurred in this time period....A chemical analysis of layer 1D indicates that the volcanic ash apparently derives from an eruption in the Kazbek volcanic province that occurred around 40,000 years ago. Because no Neanderthal specimens or MP lithic industries postdate layer 1D at Mezmaiskaya, this eruption seems to have significantly disrupted the ecological niche of local Neanderthals, possibly resulting in their rapid disappearance in this region.

The Kasbek volcanic province is in the Caucasus, so we're talking about a large eruption relatively local to the site. This is the sort of event you might well expect to have a strong impact on a dispersed hunter-gatherer population. The Middle Paleolithic people (presumably Neandertals) might have locally declined in numbers, or they might have moved on. The region need not have been abandoned entirely; a new population might have entered the area without using the same site. In this case, when new people began to use the site much later, the newbies were using an Upper Paleolithic industry.

A relatively local effect of volcanism in the Caucasus is one thing, but the extinction of Neandertals across western Eurasia is quite a bit more. How does the paper go from local event to a regional extinction?

The local eruption was the second event to leave ash in the Mezmaiskaya sequence. The first was a different eruption from Mt. Elbrus, which had a smaller impact than the second, as discussed below.

At issue in the paper is the possible coincidence of the second eruption and consequent abandonment of the site with a much larger volcanic event in Italy:

The CI [Campanian Ignimbrite] eruption from the Phlegrean Fields, southern Italy—the largest eruption documented in the Mediterranean region during the past 200,000 years (Wohletz, Civetta, and Orsi 1999)—drastically impacted European ecosystems. The most recent numerical (⁴⁰Ar/³⁹Ar) age determinations for CI eruption vary from to BP and cluster around 40,000 BP (Fedele, Giaccio, and Hajdas 2008:839).

This eruption produced the CI in Italy and Y5 tephra in Central and Eastern Europe and Eastern Mediterranean (Fedele et al. 2003; Fedele, Giaccio, and Hajdas 2008). High-altitude clouds of volcanic ash from this eruption had a significant effect on global climate. The resulting ash fall covered km2 of land and sea (fig. 1), and the Y5 tephra layer accumulated in the Eastern Mediterranean as far as Cyprus—more than 1,500 km from its source (Mussi 2001:191). The Y5 tephra is also identified in the EUP sequence at Kostenki in the Middle Don River in Russia (Holliday et al. 2007). In Eastern Europe, the ash layer varies from 1–2 cm in the eastern limit (between Penza and Rostov) to 5–8 cm in the west and southwest (southern Ukraine and Moldova) and averages about 3–4 cm (Laverov et al. 2005:51). Obviously, the area affected by this ash fall was much larger than the documented Y5 tephra sites.

Golovanova and colleagues propose the hypothesis that the climate effects of this CI event caused the demise of the Neandertals:

Our new data provide support for the hypothesis that the MUP transition in western Eurasia coincides with one of the most globally significant volcanogenic catastrophic events in the recent history of the earth. The large and coeval volcanic eruptions (from an unusually large CI eruption in the Apennines to a smaller eruption in the Caucasus) had a sudden and devastating effect on the ecology and forced the fast and extreme climate deterioration (so-called volcanic winter, perhaps comparable to the effect of nuclear winter) of the Northern Hemisphere in the beginning of Heinrich Event 4. We guess that this catastrophe likely may have both drastically destroyed the ecological niches of Neanderthals, possibly resulting in the mass death of hominins and prey animals and the severe alteration of foraging zones, and caused Neanderthal depopulation from Central Europe to the Caucasus.

That's a very clearly stated hypothesis. A volcanic eruption initiated climate effects that the regional population of Neandertals could not survive.

However, Golovanova and colleagues include in their paper several critical facts that run against this hypothesis:

1. The Mezmaiskaya sequence itself shows Middle Paleolithic people returning and proliferating after a large relatively local eruption. The Elbrus eruption apparently left ash in layer 2B-1, with a low density of bones and a very low frequency of bison compared to caprids. The excavators interpret the layer as a very low-intensity use of the cave. The pollen evidence suggests a "cold, dry climate". In other words, the paleoclimate and faunal evidence are both consistent with the hypothesis that the eruption had effects on Neandertal populations in the Caucasus. But then the Neandertals apparently returned in force:

The intensity of site use increased, however, during the accumulation of the upper MP layers 2A and 2 when the climate become cool and wet. Although the lithic industry changed slightly after the environmental crisis of layer 2B-1, it still remained typically MP Eastern Micoquian. Skeletal and mtDNA evidence indicates that Neanderthals produced both the earlier and the later MP industries at Mezmaiskaya (Briggs et al. 2009; Golovanova et al. 1999; Green et al. 2010; Ponce de Leon et al. 2008). Thus, the late MP environmental crisis at the cave had repercussions for local Neanderthals but did not cause a break in the continuity of occupation or technology.

That makes it seem pretty unequivocal. Neandertals survived and effectively adapted to at least one volcanic event in this area. That eruption did not kill them off, and it did not leave the area devoid of Neandertals in a way that facilitated a "modern human invasion."

It was only after the second volcanic event that Middle Paleolithic people declined at the site.

At issue is whether this second event was coincident with the CI eruption. The ash in the Mezmaiskaya sequence is not from the Y5 tephra, it is attributable to a much nearer source. I do not fully understand why the authors attribute this second event to the same time as the CI event; the time between layers 2 (terminal Mousterian) and 1C (early Upper Paleolithic) appears to have occupied a few hundred years, between 32,000 and 34,000 radiocarbon years BP. Calibration will move those dates older by a few thousand years (I discussed radiocarbon calibration a few years ago). But I think the CI eruption, around 40,000 years ago, doesn't fit well with this later event. It might fit with the earlier eruption, in my view, as Elbrus lava flows include 40,000 BP.

In any event, I think the associations of either local volcanic event with the larger CI event is at best uncertain. The record at the site makes it pretty clear that Neandertals were effectively adapting to the changing local climates and faunal abundance that coincided with the first eruption.

2. The initial Upper Paleolithic of Kostenki had appeared before the Y5 tephra was deposited. I wrote about the identification of this Y5 tephra at Kostenki a couple of years ago ("An earlier initial Upper Paleolithic at Kostenki"). As my post indicated, the identification of the ash layer with the Campanian Ignimbrite event suggested an earlier date for the initial Upper Paleolithic on the Russian Plain.

From the standpoint of the Neandertal volcanic winter hypothesis, this sequence of events is a problem, which Golovanova and colleagues discuss:

In any case, with or without the Kostenki addition, the few CI-bearing sites show that this eruption could have also extinguished the first wave (Proto-Aurignacian) of EMH expansion into Europe (Fedele, Giaccio, and Hajdas 2008): “At all key sites, where sedimentary resolution is good, the CI tephra directly seals archaeological layers that contain assemblages of the MUP mosaic, often variants of ‘Aurignacian’-like or so-called Early Upper Paleolithic Industries. … The layers above the CI tephra, where they are not culturally sterile, contain later and often much later properly defined Upper Paleolithic industries” (841). Thus, the CI-bearing sites demonstrate clear evidence of the break in habitation and culture change—a whole gamut of archaeological attributes for population replacement.

The volcano is supposed to explain the MUP transition, but occurs earlier than the MUP transition in some areas, but later in others. Golovanova and colleagues propose an ad hoc hypothesis to account for this mismatch: some early Upper Paleolithic modern humans were also wiped out.

Many researchers might find this idea tempting. It might, for example, explain why the (few) skeletal remains of the earliest Aurignacian people have such a high proportion of Neandertal features. We could propose that the initial Upper Paleolithic represents a degree of population mixture that later populations do not; the discontinuity between them could have been caused by climate extremes.

But we don't need climate or volcanism. Later Upper Paleolithic people retained similarities to Neandertals, which reduced in frequency over time. This is most readily explained by continued gene flow into a sparse European population from West Asia. A volcano-induced climate catastrophe is superfluous: It doesn't add to the explanation of a sustained genetic transformation of Europe that continued through the later Upper Paleolithic and Neolithic.

3. "Catastrophes" are not rare. The record of climate change during the last glaciation shows frequent strong oscillations. Some of these occurred at the same time as known eruptions, and so might be associated with them, but most climate oscillations have no obvious cause. Up to 40,000 years ago or so, the Neandertals survived them all. They survived the Toba event, largest eruption by volume in the Pleistocene, with no evidence of ill effects.

The "intensity of occupation" of archaeological sites naturally fluctuated for many reasons. In Paleolithic contexts, sites were almost never inhabited continuously. We usually don't know why a local population returned more often to a site, or why the later population may have used the site less often, but those changes in pattern will make a big difference to the occupation intensity. It's not enough to show that a fluctuation in occupation intensity was coincident with an eruption or climate event -- such coincidences are inevitable even when "occupation intensity" changed randomly.

What role volcanoes?

Bad things happened in the past. Many of those bad things -- megadroughts, volcanoes, asteroid impacts, flesh-eating bacteria -- probably killed a lot of people.

But our ability to find the effects of these death-dealing events is a lot more limited than you might assume. Less than a thousand years after the Black Death, how many signs of it are still evident today? To the exceedingly clever, who know where to look, there are a few. If we discount historical records, which do not exist for the Pleistocene, and limit ourselves to very small samples of bone and stone remains, it becomes very difficult to demonstrate this widespread epidemic, which reduced the population of some parts of Europe by up to half.

Most Paleolithic sites document exceedingly low-intensity use of an area by ancient people, and have gaps of thousands of years. The hope of finding a single event with a short duration is near zero, unless it affected many sites in the same way.

The extinction of a widespread group of hominids would be one kind of event we might test. In the current example, I think the data point to a clear conclusion: Not all Neandertals were killed, starved, or slowly declined due to the effects of any single volcanic eruption. Too many of them clearly survived the time of the large eruptions, and the available archaeological indicators suggest that their populations tended to recover after climate extremes had been reached. They were very resilient to climate change, more than many other mammals.

It's not possible to rule out that one or more eruptions may not have had important effects, even ones that may have devastated some local populations. This is possibly the case at Mezmaiskaya. Nor is it possible to exclude the hypothesis that climate changes of greater and greater amplitude may have stressed their populations, contributing to the Neandertal demise.

That's one of the returning frustrations of the archaeological record. An event might have been a major tragedy in human terms, but essentially invisible to us today. Meanwhile, the large-scale dynamics of human populations, including speciation and extinction, do not appear to fit the record of catastrophic eruptions. I don't see that as the end of the story, but a more interesting prologue to our understanding of ancient human dynamics.

UPDATE (2010-10-16): I received a note from Golovanova and Doronichev, kindly pointing out a serious error in my post. I had misread their paper -- I described it as supporting a coincidence of CI with the first ash evidence at Mezmaiskaya, but the paper clearly argues that the CI event was "coeval" with the second ash, in layer 1D of the site.

I have extensively updated the part of the post that refers to the CI eruption.

I'm skeptical that the CI and Kazbek ashfalls could have happened near the same time, because the latter seems by radiocarbon evidence to be 2000 or more years later than 40,000 years ago. But the ESR dates are arguably consistent with the idea that the two eruptions coincided. I wouldn't push a chronology argument very far, not without a list of calibrated radiocarbon and ESR/TL dates from the relevant eruptions. But the multiplicity of events helps to reiterate the basic point that geological events happened, and fluctuations of site intensity happened, and it will take a coincidence across many sites to correlate the two.

References