Notable: Selection on the human chin

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Notable paper: Pampush, James D. (2015) Selection played a role in the evolution of the human chin. Journal of Human Evolution (in press) doi:10.1016/j.jhevol.2015.02.005

Synopsis: Pampush examined the angle of inclination of the mandibular symphysis in 123 primate species, finding that the apparent rate of evolution of the mandibular symphysis in the human lineage was unusually high (77 times faster than the average across the primate evolutionary tree). This argues that the human chin has evolved under selection rather than random genetic drift.

Interesting because: It’s strange that people all over the world today have a bony bar on the front of their mandibles. Hardly any Middle Pleistocene humans and no other primates have such a trait. Anthropologists have used the chin as a marker of modern humans, arguing that any mandible bearing a chin must be one of our close fossil relatives. They equally argue over the definition of the chin itself. But we really don’t know why the trait became common in Late Pleistocene people. It’s very clever that Pampush uses the primate phylogeny to establish a rate distribution for mandibular symphysis morphology, because instead of examining only the maximum rate of change by genetic drift, this distribution considers humans in comparison to the constraints on evolution in other species.

Wait, isn’t this obvious? Actually, it’s surprisingly difficult to demonstrate that a morphological trait has been under selection in the fossil record. One problem is that genetic drift can cause almost any pattern of evolutionary change, in principle, as long as (a) the population was fairly small, or (b) the change took a long time to happen. Human evolution took millions of years, and the population sizes of our ancestors were a lot smaller than today’s population, so drift was more powerful in the past. Another problem is that we tend to notice the traits that changed a lot, but don’t tend to notice the ones that stayed the same. In other words, our attention is biased and this bias makes us likely to ascribe changes to selection just because they are changes. To test the hypothesis of genetic drift, we need to consider the probability that a given amount of change could happen by chance over the long span of time involved. Clearing that bar is very difficult.

You make it sound like this study doesn’t answer the question. Here’s the thing: With natural selection, there must be some connection between the trait and the organism’s survival or reproduction. How did a chin make people survive more, or reproduce more? Showing that selection must have influenced the evolution of the trait doesn’t show that the trait itself was important—change in the mandible may have been a side-effect of selection on the face, or the skull, or the dentition. We can speculate about whether chinny humans had a reproductive advantage—chins are sexy, bruh—but we lack a good test of the hypothesis.