Violence may have played a larger role in our evolution than we thought. By studying the faces of australopiths, our first fully bipedal ancestors, researchers show how the strongest parts of their skull were places that were most likely to be punched. Those features may have minimized injury during hand-to-hand fights.
The earliest members of the genus Australopithecus lived around 4 million years ago. They had distinctly robust faces, and for the past several decades, researchers contended that these features were functionally related to their diet of hard objects, like nuts or tough plants.
On the other hand, University of Utah’s David Carrier and Michael Morgan argue that the hand proportions that allow us to clench our hand into a fist evolved as a result of selection to make the hand a weapon. "The australopiths were characterized by a suite of traits that may have improved fighting ability," Carrier says, "effectively turning the delicate musculoskeletal system of the hand into a club effective for striking." And as the primary target, the head would have undergone evolution, making it more robust with protective buttressing.
To find evidence for their "protective buttressing hypothesis," the duo reviewed literature on topics ranging from the hominin fossil record to modern assault injuries. To identify primary targets of interpersonal violence, they looked at injuries sustained during hand-to-hand fighting in untrained modern humans: The majority were inflicted to the face. Then they compared skulls and reconstructions of gorillas (pictured above) and chimpanzees, Australopithecus afarensis, Paranthopus boisei, Homo erectus, and Homo sapiens (pictured below).
"What we found was that the bones that suffer the highest rates of fracture in fights are the same parts of the skull that exhibited the greatest increase in robusticity during the evolution of basal hominins," Carrier explains in a news release. These features -- which include larger, muscular jaws and more robust cheekbones and eye sockets -- appeared in the fossil record at approximately the same time our first bipedal ancestors evolved the right hand proportions for forming a fist. Taken together, their findings suggest that many of the facial features that characterized early hominins evolved to protect the face from fracturing when struck.
Their hypothesis might explain why our species exhibits pronounced sexual dimorphism in the strength and power of the jaw and neck. The bones with the greatest increase in robusticity "are also the parts of the skull that show the greatest difference between males and females in both australopiths and humans," Carrier says. "In other words, male and female faces are different because the parts of the skull that break in fights are bigger in males."
The work was published in Biological Reviews this week.
[Via University of Utah]
Images: David Carrier (top), reconstructions were supplied by Skulls Unlimited (middle)