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Thursday, November 18, 2010

Tooth formation rates - what do species comparisons really mean?

A paper just came out in PNAS, by Tanya Smith and others, in which they estimate tooth-crown formation times in a large sample of modern humans (n=>300 individuals), a modest sample of Neandertals (n=8), and a poor sample of 'fossil Homo sapiens' (n=3). Teeth form by the periodic deposition of enamel (the hard, white part visible in teeth in the mouth) and dentin (forms the tooth root and internal part of the crown). These periodicities are fairly regular (though variable), thus allowing researchers to estimate how long it took for teeth to develop. As previous studies have shown, Smith and colleagues find that Neandertals formed most of their teeth faster than modern humans.

Growth and development are part of an organism's life history strategy, and so the observation that Neandertals (and other fossil human species/lineages) form their teeth faster than modern people suggests that perhaps they 'lived faster' and died younger than us. It has also been used as evidence that Neandertals are a different species from modern humans.

But I don't know how well the latter taxonomic argument works. Along these lines, I wish the authors had discussed the meaning of the estimated crown formation times for their fossil 'modern' humans (Qafzeh 10 & 15 from Israel ~100 thousand years ago, and Irhoud 3 from Morocco ~160 thousand years ago). The boxplot summaries of crown extension rates (above) show that Neandertals are, indeed, generally fast relative to the large modern sample. However the fossil-modern humans (asterisks, which I've circled in red) show a bizarre, not easily interpretable pattern. For the central upper incisors (I1), fossil-moderns are either within the Neandertal range or an outlier at the high end of the human sample. For the lower second incisor (I2) the two fossil-moderns are either waaaaaay above the human range, or a little below it -either way it's outside the human range. In addition, the sole fossil-modern lower first molar has a lower rate than the modern sample - suggesting an even slower development time. Only the fossil-modern canine formation time fits comfortably within the range of modern humans. Given this wide range of variation in tooth crown formation times in the very small sample of fossil-modern humans, I don't think we can use this information to make taxonomic arguments.

I think these dental histology studies are very interesting, but I don't know how much stock we can put in any taxonomic interpretations of them. That Neandertal teeth form faster than modern humans' is old news, and the discussion focused solely on the neandertal-modern human comparison. It's too bad that the really interesting part of the paper - the variation in formation time displayed by the fossil-moderns - got no discussion.

The paper
Smith TM et al. 2010. Dental evidence for ontogenetic differences between modern humans and Neandertals. Proceedings of the National Academy of Sciences, in press.

Tuesday, November 16, 2010

Unwarranted zeal: Melvin Moss on modern methods

I'm doing some reading on the study of craniofacial growth, I stumbled across this poignant quote from Melvin Moss, from a seminar on "New Techniques in Processing and Handling Growth Data":
"This is very beautiful. It is neat, it is modern technology, and it is fast. I am just wondering very seriously about the biological validity of what we are doing with this machine." (Moyers & Krogman, eds: p. 326)
Reference
Cranio-Facial Growth in Man. RE Moyers & WM Krogman, eds. 1967. New York: Pergamon Press.

Thursday, November 4, 2010

Atavisms: talk about old school

This month's Current Biology has a "Quick Guide" segment by Brian Hall on atavisms: the occasional and random appearance of ancestral traits in individuals of species that no longer have that trait. Examples Hall provides are vestigial hindlimbs (legs or fins) occasionally found on dolphins or snakes, which evolved from animals that did have limbs limbs.

This is wild, because it implies that part of the ancestor's developmental program has been furtively retained in its descendants, but this program generally never gets carried out. But every now and again a mutation may arise that causes the ancestor's developmental program come alive all Franken-style. Nuts!

Here's a crazy hypothetical example: the axolotl is an evolutionary abomination, a salamander in a state of arrested development. It's basically a salamander that terminates development in what would otherwise be the larval stage of any other salamander. This is a nice real-life example of heterochrony (changes in the timing and rates of developmental events). Here, it's a adult descendant that resembles the juvenile form of the ancestor ("neoteny"). (photo credit: John Clare, axolotl.org)

Wouldn't it be wild, then, if the there was an axolotl in whom the ancestral full-salamander developmental plan was completed, resulting in an accidental salamander?! And then you could try to select for this atavism, possibly breeding peramorphic-atavistic-salamander axolotls ("salamander axolotls" for short)! If grad school doesn't work out, this'll be my Plan B.

Poll: If you could have any atavism, what would it be?

Reference
Hall BK. 2010. Quick guide: Atavisms. Current Biology 20: R871.