Male faces are different from those of females because men's skulls evolved to take a punch -- delivered by another male in competition for mates and other resources, a new study proposes.
University of Utah biologist David R. Carrier says he believes the cheeks, jaws, brows, molars and other male skull parts evolved into more robust forms to minimize injuries during fights over females or food.
Writing in Biological Reviews, Carrier and the study's co-author Dr. Michael Morgan suggest individuals who could better take a punch had better odds of dominating, mating and surviving.
"These characteristics that make the face resistant to injury don't necessarily make the male more attractive," Carrier said. "What they do is allow other males and also females to judge that individual's strength and fighting ability."
The researchers, analyzing data gathered from hospital emergency wards, said facial areas most frequently injured in forceful confrontations between two individuals include the jaw, eye, cheek and nose regions.
"Jaws are one of the most frequent bones to break -- and it's not the end of the world now, because we have surgeons, we have modern medicine," Carrier says. "But four million years ago, if you broke your jaw, it was probably a fatal injury ... You'd just starve to death."
The researchers, in an earlier journal article, had argued the human hand also underwent evolution based on the need to compete, developing the ability to form a fist to deliver more energy in a blow delivered during a fight.
They note that close relatives of humans, including chimpanzees, are unable to form fists.
Some scientists who had been critical of the hand evolution suggestion have also criticized the latest skull study.
Suggesting an evolutionary adaptation by matching traits to possible environmental circumstances is insufficient to prove an evolutionary pressure drove a change, says David C. Nickle, an evolutionary biologist from Merck Research Laboratories in West Point, Pa.
Other factors could have driven skull evolution, he suggests, like stronger molars needed to chew and consume a more varied diet leading to stronger jaws or other craniofacial changes that were the result of the evolution of a larger brain.
"Heads that evolved to withstand impact can be seen in animals such as the bighorn sheep, not in humans," he says. "I think Carrier and Morgan's argument is akin to arguing that human speech evolved so that humans could more effectively lie to each other."
"What I find most objectionable is that this type of research does a disservice to the general public by supplying a very wrong understanding of human biology, and more generally, human evolution," Nickle says.
