A cast of Australopithecus afarensis and Homo sapiens at Yale's Peabody Museum of Natural History.
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Photo by Brian Switek.
A cast of Australopithecus afarensis and Homo sapiens at Yale's Peabody Museum of Natural History.

Our Skulls Didn’t Evolve to be Punched

Hands evolved to punch faces. Faces evolved to take punches. That’s the hypothesis being bandied about by University of Utah researchers Michael Morgan and David Carrier, the pair proposing that the apparent “protective buttressing” of our skulls and hands is a sign of violent prehistoric fights where fists of fury dictated who would mate and who would exit the gene pool. It’s a great example of a just-so story.

Morgan and Carrier’s new paper, published in Biological Reviews, is a sequel to an initial paper that suggested our hands evolved as cudgels. This was more than a bit of a stretch. “The goal of this study was to test the hypothesis that the proportions of the human hand make it an effective weapon,” Morgan and Carrier wrote in the first study, but they couldn’t provide any evidence that punching was a preferred or even common mode of fighting in the past. The hypothesis rested on a post hoc fallacy of the same sort used by “aquatic ape” devotees – because our hands can be effective weapons, then they must have evolved for that purpose. No surprise that the concept of a spandrel – a trait that wasn’t molded specifically by natural selection, but is an evolutionary byproduct later co-opted for a different use – never appears in Morgan and Carrier’s considerations of pummeling fists.

But the skull paper is even stranger. Although Morgan and Carrier focused on the bludgeoning qualities of modern human hands in their previous paper, their new review suggests that our ancient relatives and forebears – the australopithecines – had faces that were molded into punching bags by natural selection. No sooner did humans come out of the trees, Morgan and Carrier suggest, than they started whaling away on each other. The trouble is that they undercut their own hypothesis, leaving only a crumpled heap of speculation.

Citing crime statistics from western countries, Morgan and Carrier write that fistfights often result in broken noses, jaws, and other facial bones. Therefore, they reason circularly, prehistoric humans that punched each other in the face should have more robust facial bones to cope with such blows. Given that early humans Australopithecus and Paranthropus – the latter often called “robust australopithecines” – had broad faces with wide cheeks and thick brow ridges, they’re obviously perfect candidates for Morgan and Carrier’s favored interpretation.

Morgan and Carrier didn’t study whether or not the hands of the early australopithecines could form a fist. Their previous work was on our species, Homo sapiens. Nor did they look for signs of broken facial bones or blunt-force trauma on prehistoric skulls, or even try to model how early human skulls would have reacted to the stresses of an incoming fist. The entire argument is simply that australopithecine skulls look like they could take a punch.

In Morgan and Carrier’s view, the heavy brows, large jaws, and flaring cheeks of the australopithecines are not signals of the way primates grow or the different plant foods they dined on, as paleoanthropologists have discerned, but were adaptations for reducing damage doled out by males as they competed for mates. There’s no evidence that australopithecines fought like this. The entire conjecture is based on sports like mixed martial arts and modern crime stats. And females don’t even figure into Morgan and Carrier’s hypothesis. Female mate choice, and why sexual dimorphism between the sexes has drastically decreased through time, is either ignored or overshadowed by the belief that we owe our most distinctive features to males walloping each other.  This is bro science – dudes pummeling each other driving human evolution.

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The skull of Paranthropus boisei. From Ungar, P., Grine, F., Teaford, MF. 2008. Dental microwear and diet of the Plio-Pleistocene hominin Paranthropus boisei. PLoS ONE 3(4): e2044

Those early humans couldn’t make the tight fists we do, though. Australopithecines – Lucy and her kin – were bipedal walkers that retained some signs of their arboreal ancestry, such as more ape-like arms and fingers. The hands and limbs of archaic hominins don’t match up with the supposedly “buttressed” skulls. More than that, our species doesn’t have the reinforced cheek bones, deep jaws, or prominent brow ridges that Morgan and Carrier cast as defensive structures. If our fists are so well-suited for punching, why have our faces lost their osteological protection? Morgan and Carrier suppose that we’re weaker than our ancestors, and therefore don’t need thick facial bones, but this runs counter to the heart of their hypothesis. If our hands evolved as weapons, then we should see a coevolution between striking hands and stout faces. Our prehistory shows no such pattern.

Saying that our hands are adapted to strike or that our skulls evolved to withstand those anatomical truncheons is fine as a hypothesis. But a hypothesis is just the initial fuel for the scientific engine. Morgan and Carrier haven’t let that experimental machinery run, instead looking to isolated tidbits of modern culture and projecting those behaviors onto our past. That’s not science. That’s storytelling.

References:

Morgan, M., Carrier, D. 2013. Protective buttressing of the human fist and the evolution of hominin hands. The Journal of Experimental Biology. 216: 236-244

Carrier, D., Morgan, M. 2014. Protective buttressing of the hominin face. Biological Reviews. DOI: 10/1111/brv.12112