Convergent Evolution and Primate Origins

Hooray, the third post in a short amount of time about convergent evolution! Just found a paper in press in Journal of Human Evolution, where authors demonstrate that the convergent evolution in primates and some birds suggests that the earliest primates initially filled a visual-predator niche, and that the bony covering surrounding the haplorhine primate (tarsiers, monkeys, apes, and people) eyes serves to protect the eyes from the adjacent chewing muscles.

The authors note that one hallmark of primates are their forward-facing, convergent orbits (fancy name for where the eyes sit in the skull). Back in the 1970s Matt Cartmill suggested that this helped increase the visual acuity of the earliest primates, who used their sharp sense of vision to hunt for small insects, probably at night. So to test such a “nocturnal visual predation hypothesis,” they compared the orbits of 103 species of birds. Lo and behold, strigiform birds (you may know them as owls, I know them as delicious), which are visual predators and mostly nocturnal, have the most orbital convergence.

In addition, haplorhines are unique among primates in having a “post-orbital septum,” or a bony plate separating the orbit from the temporal fossa behind it (though the tarsier’s septum is incomplete). Just as haplorhines are unique among primates, so too are owls among birds, in having a large, bony projection behind the eye. It has been suggested that this morphology serves to keep the temporalis muscle from pushing against the eye during chewing. To test this hypothesis regarding the function of bony protection behind the eye, the authors dissected a few owls. As predicted, the dissection showed the owls’ projection deflects the path of the adjacent jaw muscle.

I think this study is neat for two reasons: first, it’s another great example of the pervasiveness of convergence in evolution. In spite of how diverse animal forms are, they often have very similar features, which makes it likely that diverse animals will evolve similar traits or behaviors in similar ways, under selection. If the ancestral condition in birds and pre-primates was to have eyes that don’t face forward, and there was selection to increase visual acuity for predation, then it looks like each group had no evolutoinary ‘choice’ but to move their eyes forward.

Second, the study demonstrates the utility of the comparative method to test functional hypotheses. While all primates have forward-facing eyes, modern primates are so diverse that it is hard to say why exactly such a feature evolved in the first place. By seeing why the feature evolved in certain birds, we can infer why it evolved in our earliest ancestors.

Menegaz R and Kirk EC. Septa and processes: Convergent evolution of the orbit in haplorhine primates and strigiform birds. Journal of Human Evolution, in press.

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