Wednesday, May 4, 2016

They were all my future specimens. And they died.

Without skeletal collections we'd struggle to do much evolutionary biology, especially when it comes to studying fossils.

We'd hate to let all those specimens go to waste, just languishing there in museum drawers. Sciencing them brings honor to their death. (Thanks for the new verb, Andy Weir.) But while we're learning from skeletons we can never forget that they're dead.

So although many of our samples are animals that were hunted by President Theodore Roosevelt (thanks Smithsonian!) or Major Powell-Cotton (thanks Powell-Cotton museum!), many of them, especially when it comes to human skeletons, are ones that died of "natural" causes.

You're thinking, well, duh. Well, yeah. Duh. But sometimes what's obvious still isn't so obviously important until someone goes to the trouble to very carefully consider it.

If the "osteological paradox" has already come to mind, that's probably because you're familiar with the classic paper "The Osteological Paradox" co-authored by a certain Mermaid and other former graduate school professors of mine.  Although the paper discusses issues that are more complicated and more specific than we need to hash out here, "osteological paradox" is a great term for the conundrum that scientists face when reconstructing things like health, fitness, and adaptation in past populations from the remains of the individuals who died.

Naturally, if you've been raised on "osteological paradox" thinking, it's one of the first things that comes to mind when you see a visually stunning study by my colleagues that analyzes pelvic morphology of dead individuals to reveal differing adaptive morphologies in the pelves of males vs. females.

Sexual dimorphism in the human pelvis has been known for quite some time, and it's already well-understood that the differences are largely located in the dimensions of a woman's birth canal. But this new study shows that differences are observable from birth and that women at post-reproductive ages do not retain the obstetrically-beneficial dimensions that younger women do during their fertile years. One of the arguments this new paper makes is that human female pelves are adapted to be most accommodating for childbirth during the child-bearing years. And that very well may be the case. However, these claims for adaptation, like most based on human skeletal samples, were based on women who were dead and, thus, not adapted.

In this context, the concluding paragraph of "The Osteological Paradox" is worth quoting:

"...choosing among competing interpretations of the osteological evidence requires tight control over cultural context as well as a deeper understanding of the biology of frailty and death. These problems deserve far more attention than they have received to date if we are to make sense of the biomedical consequences of the major social and environmental changes that have occurred during the course of cultural evolution."

And that could be extended to "biological evolution" as well. Maybe it has been in a later paper.

Anyway, when we're looking at dead humans with an evolutionary mindset, it's probably good to ask whether we can know if selective pressures were the same across the timespan covered by the sample. It's also probably good to ask whether environmental conditions were the same across the timespan covered by the sample. It's also probably good to sing this to ourselves as we design our evolutionary study of the human skeleton:

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