The radiocarbon dating paper without a radiocarbon date

Nature this week released two papers about European archaeological sites that come near the end of the Neandertals and beginning of the archaeological transition to Upper Paleolithic industries. Here, I'll devote some attention to the first, by Tom Higham and colleagues [1], which discusses the morphology and dating of the maxilla fragment from Kent's Cavern, England. The paper claims that this is the oldest modern human specimen in Western Europe.

The earliest anatomically modern humans in Europe are thought to have appeared around 43,000–42,000 calendar years before present (43–42 kyr cal BP), by association with Aurignacian sites and lithic assemblages assumed to have been made by modern humans rather than by Neanderthals. However, the actual physical evidence for modern humans is extremely rare, and direct dates reach no farther back than about 41–39 kyr cal BP, leaving a gap. Here we show, using stratigraphic, chronological and archaeological data, that a fragment of human maxilla from the Kent’s Cavern site, UK, dates to the earlier period. The maxilla (KC4), which was excavated in 1927, was initially diagnosed as Upper Palaeolithic modern human1. In 1989, it was directly radiocarbon dated by accelerator mass spectrometry to 36.4–34.7 kyr cal BP. Using a Bayesian analysis of new ultrafiltered bone collagen dates in an ordered stratigraphic sequence at the site, we show that this date is a considerable underestimate. Instead, KC4 dates to 44.2–41.5 kyr cal BP. This makes it older than any other equivalently dated modern human specimen and directly contemporary with the latest European Neanderthals...

One thing you won't see in any of the reporting on the paper: There is no new radiocarbon date for the maxilla.

I must admit, I was completely confused by the paper and had to read the entire thing several times! The first time, I was so busy concentrating on how they obtained their new "date estimate" that I completely missed the one sentence indicating that there is no radiocarbon result.

The supplement gives more details. The radiocarbon dating of faunal specimens from the stratigraphy led the authors to suspect that a 1989 date for the maxilla (30,900 +/- 900 BP) was too young. One woolly rhino and two other bones above the maxilla, over a depth of around a meter, yielded radiocarbon dates around 6000 years older than this. So they went to redate the maxilla, but didn't get enough collagen to obtain a result:

To explore this further, permission was obtained from Torquay Museum to obtain a small sample of dentine from the right P3 of the KC4 specimen for another direct date. The tooth was extracted from the maxilla and carefully sampled at the ORAU so that the external hole could not be seen from the exterior once the tooth had been replaced. Only 89 mg could be drilled due to the small size of the tooth. This produced 0.4% collagen after ultrafiltration pre-treatment, but the total amount extracted was too small for a reliable AMS measurement, so the sample was not dated (Table S2).

So, if they didn't get a radiocarbon result from the maxilla, why are they reporting that this is the earliest modern human in Western Europe?

What they did do: They used the radiocarbon dates on the fauna, and the depth of those faunal specimens in the stratigraphy, to interpolate a date for the maxilla in the absence of radiocarbon information. The Nature paper is simply reporting this interpolation model.

We can look at Figure 3 of the paper to get an abbreviated picture of AMS dates for early Aurignacian human specimens in different parts of Europe. The new Kent's Cavern maxilla date is way out of this distribution.

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Figure 3 from Higham et al. [1]. Original caption: " Comparison of direct radiocarbon determinations of AMH bones from European Palaeolithic sites with the KC4 model age. Calibrated using the INTCAL09 curve12. Brackets under the distributions represent the 68.2 and 95.4% probability ranges, respectively. The PDF derived from the Bayesian modelling of KC4 (Model age of the maxilla, in red) is earlier than the original direct date from Kent’s Cavern (OxA-1621) and all others, and overlaps the start of the age range of the earliest European Aurignacian, which is widely accepted as being linked with the earliest AMH. Ultrafiltered collagen radiocarbon dates are indicated with red text; non-ultrafiltered dates are in black. Asterisks denote duplicate dates on the same human bone. The Oase date is a mean of two determinations, one ultrafiltered and one not.

The red distribution is the new model date for the maxilla, way earlier than any other specimen. The gray distribution indicated for Kent's Cavern is the 1989 date, with a calibration model applied to it.

The archaeological association of the maxilla is very weak, as summarized by Higham and colleagues:

The maxilla was found in 1927 at a depth of 10 ft 6 inch (3.23 m) beneath a key ‘granular stalagmite’ used as a datum during excavations undertaken between 1926 and 1941 by the Torquay Natural History Society. Below it were found two blades similar to those discovered in Aurignacian industries, and deeper still were found two blades that resemble those from Initial Upper Palaeolithic industries of the Lincombian–Ranisian–Jerzmanowician complex, which are tentatively associated with Neanderthals.

Such as they are, these associations permit a much later date and do not preclude an earlier one. They are certainly not enough to speak of a date for "Early Aurignacian" on this basis, there is no diagnosis of the industry here.

You can see why I found this so irritating. Here's a paper trying to make a big splash, by establishing the claim in the literature that we have Aurignacian-associated modern human remains earlier at Kent's Cavern than anywhere else in Europe. The reported date estimate is a clear outlier compared to human remains everywhere else. And although there is a radiocarbon estimate, that is ignored (possibly for good reason) in favor of a model that doesn't include it, because radiocarbon gave a date younger than the paper claims, by seven millennia or more.

I'm not saying the authors could have done better with the material they had available. Sometimes we don't get definitive results, and that's expected in paleoanthropology. I just think it's bizarre that Nature would put such press behind a dating paper with no date.

UPDATE (2011-11-07): A couple of people have contacted me, confused by the apparently very ancient dates for other Early Upper Paleolithic sites in the figure. The figure reports calibrated dates, not radiocarbon dates. I have noticed a trend over the last several years to reporting and picturing only calibrated dates instead of the actual radiocarbon determinations. I think this is a very negative development, because it creates confusion between the calibration model and the source of the data. We see how confusing that presentation can be in this paper, where a result that does not come from radiocarbon data is pictured alongside calibrated dates without any distinction between the two.

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