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

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.


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