Neandertals didn't disappear before 40,000 years ago

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 mezmaiskaya, 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 Pinhasi:2011, 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 Higham:2006. 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 Blockley:2008 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 Gravina:2005 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 Martinez-Moreno:2010 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.