life history

Can ‘ape-like’ actually be ‘human-like’?

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I’m reading up on life history in Homo erectus for a few projects I’m working on, and something’s just caught my eye. A 2012 issue of Current Anthropology presents a series of papers from the 2011 symposium, “Human Biology and the Origins of Homo.” This issue is full of great stuff, and to top it all off, it can be accessed online for free! (here’s the JSTOR link)

Gary Schwartz has a paper here recounting what is known (or as he stresses, what is still largely unknown) about growth and life history in early Homo. Dental evidence accumulated over the past 30 years has pointed to a rapid (ape-like) life cycle for fossil hominins, in comparison with a slow, long and drawn out human pattern. But much of the evidence against a human-like pattern is somewhat indirect. For instance, Holly Smith (1991) has shown that there’s a pretty tight relationship between brain size and age at first molar (M1) eruption in Primates:

M1 crancap

Fig. 1 from Schwartz (2012). “Bivariate plot of ln M1 emergence age in months (y) versus ln cranial capacity in cubic centimeters (x) for a sample of anthropoids.” The hominins and humans are the open shapes, to which I’ve visually fitted the red line.

It’s a very high correlation (r=0.98). This means that armed with simply an animal’s cranial capacity, which is fairly easy to estimate given complete enough fossils, one can estimate with a bit of confidence its likely age range for M1 emergence. With brain sizes between apes’ and ours, fossil hominins can be estimated to have erupted their M1s at younger ages than us. Many subsequent studies of tooth formation, based on the microscopic remnants of tooth development, have supported these inferences. So presumably, faster, ape-like dental development could be extrapolated to mean ape-like body growth rates and other aspects of life history as well.

But although this is a tight relationship, there are deviations. As Schwartz notes in the article, and others have noted before, high correlations found when examining large interspecific groups (e.g., primates as a whole) often break down when the focus is on smaller groups of more closely related species (e.g., just apes). Based on the relationship figured above, humans are expected to erupt M1 around 7 years of age, but nearly all humans erupt M1 closer to 6 years (hence the open diamond for humans is below the regression line). What hominins appear to share in common with humans is a younger age at M1 eruption than expected for primates of their brain sizes (the red line I’ve added to the figure).

Hominins’ faster dental development and eruption may be ape-like in absolute terms, but eruption ages may be human-like when their brain size is taken to account. As with many life history variables, the significance of this similarity (if anything) is difficult to ascertain.

Open wide for open access: chimpanzee tooth eruption

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Two anthropology papers came out yesterday in advance print at the Proceedings of the National Academy of Sciences. I’d like first to draw your attention to the fact that they’re open access – normally such scientific papers are behind a paywall, but these two can be obtained by anyone (well, anyone with internet). One is about the chronology and nature of Acheulean technology at the 1.7-1.0 mya site of Konso in Ethiopia. The other, and the subject of this post, is about life history in wild chimpanzees from Uganda.

Tanya Smith and colleagues analyzed behavior of chimps and photographs of chimps’ erupting first molars (“M1”) to determine a] the age at which these events happen in the wild (in this population at least), and b] whether M1 eruption is tightly linked with other important life history variables, such as the adoption of adult foods, as had previously been claimed. What an adorable study – check out figure 1 from the paper (right):

Figuring out age at M1 eruption in wild, healthy chimps is important because there has been debate about whether wild chimps actually erupt their teeth at as young of ages as they do in captivity – not natural conditions. This question has recently been investigated in a skeletal sample of wild chimps of known age, from Tai forest in Cote d’Ivoire (Zihlman et al. 2004, T Smith et al. 2010), but somehow these studies raised more questions than they answered (e.g. BH Smith and Boesch 2011). So TM Smith and colleagues decided to further address this question with photographic evidence of living, arguably healthy chimps. I’m kicking myself in the ass because I had this exact same idea a few months ago but had a bit too much on my plate to tacklet it at the time. Life.

Anyway, Smith and pals showed found that M1 eruption occurred anywhere from 2.8-3.3 years of age in their sample of 5 cuddly infants, consistent with estimates from captivity. I have to say I’m a bit surprised it wasn’t later (but what fun is science if it’s not surprising?). Of course, this is based on 5 infants from one population, so it could stand to be reinvestigated in other chimp populations, as the authors note; variation is, after all, key for evolution and a key problem for evolutionary biologists. Maybe I’ll get another crack at a photo-based eruption study after all…

Smith et al’s second task was to see how well age at M1 eruption coincided with other life history variables – this is supposed to be an important event, alleged to coincide with cessation of weaning and the adoption of adult foods. Moreover, since a mother is no longer nursing her infant, M1 eruption “should” also be roughly contemporaneous with a mother’s return to estrus cycling and subsequent re-pregnancy. Many infants were observed to begin eating adult-like foods prior to M1 eruption, around 3 years. Unexpectedly however, infants also nursed for a while even after M1 eruption. In fact, time spent nursing actually increased for a brief period around 3 years of age, possibly because their mothers’ milk was not as nutritious as at younger ages.

Now, what interests me most about this are possible implications for my research on the evolution of growth and life history. Many researchers have argued that extinct hominids, like the australopithecines, would have grown up relatively rapidly like apes, rather than slowly like humans. This claim has been based pretty much entirely on dental development, until my dissertation research. There, I’ve shown that one hominid, Australopithecus robustus, probably experienced greater jaw growth than humans prior to eruption of the M2. Now, if this hominid erupted its teeth as fast as apes, and grew more than humans, this implies really really high growth rates for A. robustus (that is, if we can extrapolate from the jaw to the overall body size).

ResearchBlogging.orgI’ll be working a bit more on this latter point in the near future. In the mean time, let’s hear it for bioanthro dominating open access today!

References
Smith BH, & Boesch C (2011). Mortality and the magnitude of the “wild effect” in chimpanzee tooth emergence. Journal of human evolution, 60 (1), 34-46 PMID: 21071064

Smith TM, Smith BH, Reid DJ, Siedel H, Vigilant L, Hublin JJ, & Boesch C (2010). Dental development of the Taï Forest chimpanzees revisited. Journal of human evolution, 58 (5), 363-73 PMID: 20416929

Smith, T., Machanda, Z., Bernard, A., Donovan, R., Papakyrikos, A., Muller, M., & Wrangham, R. (2013). First molar eruption, weaning, and life history in living wild chimpanzees Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1218746110

Zihlman A, Bolter D, & Boesch C (2004). Wild chimpanzee dentition and its implications for assessing life history in immature hominin fossils. Proceedings of the National Academy of Sciences of the United States of America, 101 (29), 10541-3 PMID: 15243156

Tooth formation rates – what do species comparisons really mean?

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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.