dating

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

Michael Balter reports on a new radiocarbon calibration called INTCAL09. The calibration curve purports to provide a calendar age calibration up to 50,000 years ago for AMS radiocarbon dates.

Balter's report gives a good account of the basics. The atmospheric concentration of carbon-14 varied over time, so that organisms from some ancient times started with a higher proportion and other times started with a lower proportion. The radiocarbon dating technique depends on knowing this initial carbon-14 proportion. But we can only figure this out by comparing the present carbon-14 proportion in things whose ages we know -- like tree rings. Before 25,000 years ago, good non-radiocarbon chronologies are hard to come by, so up to now there has been no good calibration curve.

More recently, however, thanks to new and more accurate data from foraminifers, corals, and other sources--plus some fancy statistical treatments that help predict which way data gaps bend the curve--the INTCAL group has been able to resolve most of the discrepancies. "It took the group quite a while to come together and agree," says INTCAL team leader Paula Reimer, a geochronologist at Queen's University Belfast in Northern Ireland. But the new data, combined with what Reimer calls a "real sense of necessity" among team members to resolve the debates, won the day.

I'm skeptical when I see calibrated dates because they seldom report the calibration error. I like "fancy statistical treatments" that actually report their error. The entire reason a calibration model like INTCAL09 looks good is that it represents only one component of variability within a large set of separate chronometric datasets. The "debates" are more or less about whether that component is time, and if not what other factors must be controlled. Resolving the debates doesn't mean that the model will reduce the error associated with calibrating a given date -- it (hopefully) means that calibrated dates will be unbiased.

In principle, calibration is good because it facilitates comparison between radiocarbon and other dating methods, like OSL or ESR. It also gives a more accurate view of the temporal scale of events -- the radiocarbon chronology compresses the period between 40,000 and 10,000 years ago into 25,000 radiocarbon years instead of 30,000 calendar years. It makes a difference, if for no other reason, because it makes the initial Upper Paleolithic look more rapid than it really was.

Julien Riel-Salvatore ruminates on similar issues ("Paleolithic radiocarbon legerdemain")

The really bad dating problem happens at points where the atmospheric carbon-14 declined. Some declines occurred with nearly the same rate as actual decay of the carbon-14. A younger sample may then up with the same carbon-14 proportion as an older sample, with no way to tell between them. (I discussed this problem as applied to initial Upper Paleolithic-era dates in "Radiocarbon fudgery".)

Because different datasets vary in their results, apparent declines in atmospheric carbon-14 seem more common in those individual datasets than in the model that reflects their common features. The atmospheric carbon should be better reflected by the model -- after all, there's only one atmosphere, so these datasets should reflect the same value.

But any single series of dates ought to have temporal stochasticity more like an individual dataset. When we take dates from bone collagen -- which is not one of the kinds of data with chronological controls -- there ought to be a separate, source-specific error that we can't control by a calibration model.

Does it matter? I think we should assume the resolution of a 40,000-year-old calibrated radiocarbon date is no better than 3000 years. And in some cases more -- depending on the atmospheric trend. If one date is 3000 years earlier than the other, I think there's a very good likelihood that the earlier date really did happen first.

Too conservative? I'd like to see somebody run the numbers on it.

The story of the New Zealand rat bones is a bit deeper than the press reports (e.g., this AP report). The main idea is that the rat radiocarbon dates support an initial habitation of New Zealand that was relatively late, around 1200 AD. That's not a big surprise, since no human archaeological site or remains have been found to have earlier dates.

I don't have any opinion about New Zealand prehistory, really. It seems to me that the rats are a very good source of evidence, because their population growth is potentially much much faster than human population growth. If rats arrive on an island, there's a good chance of finding them early. I could imagine that humans might escape leaving archaeology for some time. I doubt very much that they could remain invisible for over a thousand years, but that depends on the intensity of archaeological research. But rats are not going to stay invisible. When you have extinct predators who ate rats, and they leave rat bones in their feces that you can sample, and none of those rat bones are more than 800 years old, well that's a sign.

So what's the real story here? The Oxford Radiocarbon Accelerator Unit keeps changing sample preparation protocols! These changes have brought in a number of new ways to take contamination and recent carbon out of the sample. I noted the redating of Vindija G1, which was based on a new sample preparation method using filtration to purify collagen from the bone. At the time, this was one among several new methods attempting to improve the accuracy of AMS dates. The cumulative effect of the advent of AMS dating, coupled with these later improvements, has added substantial precision to our knowledge of Europe during the last 40,000 years, as I reviewed here. Tom Higham, who was behind the new dates in the New Zealand paper, also worked out the Vindija G1 redating.

The problem is that every new sampling method raises the prospect that a lot of currently accepted dates are actually wrong. That is what has happened in the case of the New Zealand rats. The rat case demonstrates the depth of the problem: Holdaway (1996) presented seven AMS dates on rat bones whose confidence intervals are significantly older than 1000 AD (calibrated), two that are significantly older than 500 AD. The present study by Wilmhurst et al. must claim that all those rats were contaminated with old carbon.

Since the half-life of carbon-14 is 5730 years, an elevation of more than 500 years in a date represents a very substantial deficit of carbon-14 -- on the order of five percent of the maximum amount. Such deficits might be possible, either due to conditions after burial or consumption of marine carbon by the animals during their lives. But in his original study, Holdaway closely considered these effects:

Potential sources of error include the addition of 'old' or reservoir carbon to the bone gelatin before death in the diet, or after deposition via unremoved humics or diagenetic processes in carbonate sedimentary environments, especially for small specimens.

Dietary influences were not apparent. Two individuals of known death date give calibrated ages that include their death dates. In addition, ¹⁴C dates on bone gelatin from two herbivorous birds (equilibrium carbon consumers) are not significantly different from those on rat bones from comparable levels. Humic contamination is unlikely, most being removed by gelatinization, but must still be considered fro earlier 'collagen' dates. Environmental carbonates were removed by an acid pre-wash, eliminating carbonate contamination. Measured ages were not related to whole-sample mass.

Longer-term diagenetic changes do not appear to have a significant effect. Samples of moa eggshell (species unknown) and bird bone from close proximity in sediment enclosed by two undisturbed volcanic tephras give indistinguishable ages.... These materials were prepared using different treatments. Finally, a rat dentary excavated from beneith the Taupo Tephra gives an age of 1,775±93 yr BP. In addition to the radiocarbon age being consistent with that of the covering tephra, the bone's position beneath the undisturbed layer provides independent evidence that Pacific rats were established in the North Island before the Taupo eruption (Holdaway 1996:226).

Yes, you read that right. He had a rat under a well-dated volcanic tephra.

The current paper claims that all the oldest dates for rat remains have come from a single lab, all before a single date:

Subsequent dating of Pacific rat bones sampled from both laughing owl (32) and archaeological sites (33-35) failed to duplicate the early series of old rat bone dates (35-38). The most telling criticism of the original dates is that they fall into two distinct groups according to when the bones were processed in the same dating laboratory (22, 36, 37) (see Fig. 1). The early series of rat bone dates processed in 1995 and 1996 are all older than the oldest-dated archaeological evidence (1280 A.D.), but all bones dated after 1996 are younger (36, 37) (Fig. 1). Moreover, some rat bones from archaeological assemblages that were processed in 1995 and 1996 are significantly older than consistent dates on diverse materials from the same stratigraphic contexts (34, 35). Critics argued that this unusual bimodal distribution of ages according to when the bones were processed was due to inadequate pretreatment of small bones (33, 35-37). It has also been argued that some of the old 1995-1996 rat bone dates are older than their "true" age because of dietary uptake of carbon depleted in ¹⁴C (e.g., refs. 39-40).

Well, there you have it. The argument has to be that the dates are wrong due to the different sample preparation methods. The "dietary carbon-14" argument can't be the explanation, because some of the more recently dated samples ought to show the same deficit, and they don't. I personally don't see how they deal with the rat under the tephra -- they don't address the question. The only possibility that makes sense with their argument is that the samples were technically processed in a way that led to older dates.

Again, I have no opinion about New Zealand settlement. The recent chronology proposed here sounds reasonable to me, but mainly because people in a massively expanding population shouldn't remain archaeologically invisible.

I just want to point out how much our knowledge of the archaeological sequence depends on the technical details of dating methods, known only to a small number of researchers. To be sure, technology advances. But we have thrown out an awfully large number of radiocarbon dates in the last few years, due to small but important changes in methods. And the New Zealand case shows that this problem is not confined to the upper limits of AMS dating, where the preserved carbon-14 fraction is at its lowest. In the European case, the biggest problem has been supposed Aurignacian specimens that turned out to be Holocene in age.

This raises the obvious question: how much weight should we give to current date estimates?

References:

Wilmhurst JM, Anderson AJ, Higham TFG, Worthy TH. 2008. Dating the late prehistoric dispersal of Polynesians to New Zealand using the commensal Pacific rat. Proc Nat Acad Sci 105:7676-7680. doi:10.1073/pnas.0801507105

Holdaway RN. 1996. Arrival of rats in New Zealand. Nature 384:225-226. doi:10.1038/384225b0