Neandertal teeth: the other shoe

The paper by Guatelli-Steinberg et al. (2005), earlier referred to here, is now available online from PNAS.

The results are basically as reported by National Geographic, finding that Neandertal anterior teeth have perikymata counts within the range of living human populations. Perikymata are microscopic ridges on the enamel surface of teeth; they mark the incremental growth of the teeth over small periods of time. The idea has been that these ridges work a bit like tree rings; they mark the amount of time that the tooth took to grow. However, as this study indicates, the formation of perikymata is not quite so simple as the addition of tree rings, and human populations actually vary substantially in the number of perikymata on their teeth.

What makes this different from earlier work (like Ramirez Rossi and Bermudez de Castro 2004) is the inclusion of an African sample. The very low perikymata count of the recent Africans significantly extends the range, which had previously been assessed in Europeans only. Thus, the conclusion here is that there is no evidence from perikymata to indicate that Neandertal development was any different from that within living human populations.

Now we can wait for the other shoe to drop:

The finding from the African population sampled here shows that some developmentally normal humans have much lower perikymata counts than others. This varies by tooth (since they don't all develop for the same time): the lower canines have the highest counts, with a mean over 150; the lower incisors have the lowest counts with a mean down near 100. Remember that these values are means; individuals in the sample must have scored lower, although the range of the sample is not reported in the paper.

With this sample, the human range encompasses the Neandertals. It encompasses all the earlier European hominids (chiefly from Atapuerca) sampled by Ramirez Rossi and Bermudez de Castro (2004), because these hominids had counts higher than Neandertals.

Let's take a look at Dean et al. (2001:628), who give values for earlier hominids. Here's a table including some earlier hominids along with the South African values from Guatelli-Steinberg et al. (2005). The current paper does not include the numbers, so I am reading estimates off the figure, but considering they are means and the important aspect is the total range, the numbers aren't critical. Lower numbers are less like the recent Europeans that were the standard before the new comparative work.

Sample UI1 UI2 UC LI1 LI2 LC
Recent South African 120 117 135 105 110 155
Sangiran 4 138
SK 27 153
KNM-ER 1590 114 127
KNM-ER 820 113
KNM-WT 15000 94 96 100 96 92 110
Paranthropus 83 85 112 78 90 103
Australopithecus 123 109 122 116 122 143

From these numbers, Sangiran 4 and SK 27 are within the range of modern human population means. So are three of the teeth of Australopithecus (i.e. A. africanus), and the remaining three teeth are pretty close, so that it seems likely the A. africanus dentition wasn't very different in its perikymata number from the range of living Africans.

The standouts are KNM-WT 15000 and Paranthropus (i.e. A. robustus). A. robustus is easy to explain: its anterior teeth are a lot smaller than ours. A lot smaller. If enamel formation rates were similar, then they ought to have taken less time to form, regardless of other aspects of somatic development.

The puzzle is KNM-WT 15000, the famous Nariokotome skeleton. Is this skeleton a normal representative of early human populations? Is it at the extremely low end of a normal range including others like KNM-ER 1590 (also a bit smaller than the mean, although probably not outside the range of living Africans)? Is it pathological?

The other shoe is the research paper that will cover all these questions.

Now it could be that these numbers really aren't comparable for some reason; I don't do perikymata, but I can tell that the counts depend on estimates of crown height and packing density, so it's not obvious that they were derived in the same way (although the papers do share one author).

But the Neandertals are far from the most interesting part of this perikymata problem. Can we tell a human from an australopithecine from these data? If so, why do some of the earliest humans have the lowest (i.e. sub-australopithecine) counts?

I think we can disregard the idea that their somatic development rates were "highly derived" in a non-human-like direction. It's not like they're Neandertals, after all.

References:

Guatelli-Steinberg D, Reid DJ, Bishop TA, Larsen CS. 2005. Anterior tooth growth periods in Neandertals were comparable to those of modern humans. Proc Nat Acad Sci USA 102:14197-14202. Abstract

Dean C, Leakey MG, Reid D, Schrenk F, Schwartz GT, Stringer C, Walker A. 2001. Growth processes in teeth distinguish modern humans from Homo erectus and earlier hominins. Nature 414:628-631.

Ramirez Rossi FV, Bermudez de Castro JM. 2004. Surprisingly rapid growth in Neanderthals. Nature 428:936-939. Full text (subscription)