How modern is "modern tooth development"?

Regular readers of the blog will remember previous occasions when I have written about dental development in fossil humans. I am by no means an expert on the topic of dental development. I don't use a scanning electron microscope, or micro-CT equipment. I can recognize perikymata and striae of Retzius, but I've never counted them. I am perfectly willing to accept the idea that other people count them accurately, and even that they can determine their periodicity (that is, how many days of development each line represents).

In 2005, Guatelli-Steinberg and colleagues showed that the variation in perikymata counts for the anterior teeth of different human populations is more extensive than the differences between living people and fossil humans. I discussed that paper at the time. The perikymata counts in modern human populations are so variable, that the variation in sample means encompasses almost all fossil humans. As I noted, there are few fossil exceptions -- KNM-WT 15000 being the most important. What's worse, the variation among living people encompasses most australopithecine teeth.

To me, this was the end of the story of tooth development and maturation rates in early humans. Modern human variation encompasses most australopithecines? End of story.

So I was surprised to see last week's paper by Tanya Smith and colleagues (2007) claiming that the Jebel Irhoud 3 dentition was the earliest example of "modern" human dental development. It seems pretty clear from Guatelli-Steinberg's work that there is no modern human pattern of enamel formation.

The paper deals with this problem in a surprising way. It just doesn't talk about any of the work showing extensive variation among living people!

Still, the data are clearly there, reported in Table 2, where it is obvious that there is no significant difference between Neandertals and the modern samples. Moreover, there is no significant difference between Neandertals and Jebel Irhoud 3, except for the lower canine perikymata number, which is even more different between JI3 and the recent Africans!

The real story of the paper seems to be that Jebel Irhoud 3 has an unusually long period of enamel development compared to most recent people, and also compared to Neandertals and other early humans. But since humans vary in these traits between populations more extensively than fossil Homo, this observation demands an adaptive explanation, not a phylogenetic one.

References:

Smith TM, Tafforeau P, Reid DJ, Grün R, Eggins S, Boutakiout M, Hublin J-J. 2007. Earliest evidence of modern human life history in North African early Homo sapiens. Proc Nat Acad Sci USA (online early) doi:10.1073/pnas.0700747104

Lampl M, Mann A, Monge J. 2000. A comparison of calcification staging and histological methods for ageing immature modern human specimens. Anthropologie (Brno) 38:51-62.

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. doi:10.1073/pnas.0503108102

Guatelli-Steinberg D, Reid DJ, Bishop TA. 2006. Did the lateral enamel of Neandertal anterior teeth grow differently from that of modern humans? J Hum Evol 52:72-84. doi:10.1016/j.jhevol.2006.08.001

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)