Muscle markings, chimpanzees, and Neandertals

Earlier, I pointed to my new article in Slate, about chimpanzee strength compared to humans. For anthropologists, I thought I might point to a passage in one of John Bauman’s articles (1926:7-9), which raises a point I remember well from graduate school:

The last question raised by the strength of the chimpanzee seems to have been completely overlooked in the past. All anatomists place reliance upon the relative development of the various muscle attachment ridges and pits on the bones as a trustworthy indication of the strength of the owner.
Yet anyone who will take the trouble to compare carefully the crest of the ilium of the chimpanzee with that of the human being will notice that the muscle attachment roughnesses are very markedly less prominent in the former than in the latter, yet Suzette's pulls have clearly demonstrated an immense superiority in strength of the lumbar region in the ape. Also with regard to long sustained action, a short time spent in the anthropoidal posture will convince any person that this posture calls for more taxing long sustained action of the lumbar muscles than does the erect posture of the human being.
We certainly can not look to man's erect posture for an explanation of the smooth sharp rim of the hip bone in the anthropoid ape, why then do the usually so reliable muscle attachments fail here to correctly indicate relative strength? The discrepancy is an extremely pronounced, not a trifling one, moreover.

Bauman then generalized to Neandertals:

And finally, how about those interesting Neanderthal men? We customarily base our estimate of their probable strength upon the degree of prominence of their muscle attachments as observed in the fossil bones-but should not the above consideration incline us toward caution in this class of inferences, particularly when the subjects are an ancient race known to have approximated closely to the anthropoidal type in their anatomy--as well as impel the comparative anatomist to a thorough investigation into the reason for this strange discrepancy.

The hip is not a great example, because of the architectural difference between the human and ape pelvis. Still, we can make the same general observation about other bones like the humerus, on which muscle attachments don’t convey the information about relative chimpanzee strength.

Pronounced muscle attachment sites are not evidence that Neandertals were weak; within the context of recent hominids the rugosities and robusticity of bones are probably good indicators of muscle mass. But with some increasing evidence for evolution of muscle functional properties on the human lineage, I hesitate to assume that any Pleistocene human muscles interacted with their bony attachments in exactly the same way as ours.

The muscle attachment issue may be especially confusing in Australopithecus, where there is a substantial contrast within species between small individuals like AL 288-1 and larger individuals – the Maka humerus comes to mind. Here’s the comparison, from White et al. (1993):

Maka (top) and AL 288-1 humeri, from White et al. 1993

Maka is the heavily-crested humerus at the top, Lucy’s slender and smooth humerus underneath. Straightforward mass difference? Or difference in activity pattern? Or both? This is not such an unusual comparison considering the variability in living hominoids, including people. But it illustrates well the kind of range of muscle marking and cresting that existed in fossil populations. The changes during human evolution would have happened upon this underlying pattern of broad variation.

We can probably assume it’s not the functional properties of muscle differing between these two specimens. But what about between Australopithecus and Homo? Or Neandertal to recent human? We already know there are differences in muscle function among human populations, in part corresponding to alpha actinin-3 allele frequencies. Genetics may be starting to make the “expensive tissue” story come down to muscle instead of gut reduction – if I’m going to make predictions, I would say that MYH16 will not long be alone as a gene corresponding to human muscle reduction.

References:

Bauman JE. 1926. Observations of the strength of the chimpanzee and its implications. J Mammal 7:1-9. JSTOR

White TD, Suwa G, Hart WK, Walter RC, WoldeGabriel G, de Heinzelin J, Clark JD, Asfaw B, Vrba E. 1993. New discoveries of Australopithecus at Maka in Ethiopia. Nature 366:261-265.