But suddenly, Rocky was on the upswing. He had drawn the mighty Drago’s blood. “You see!” Balboa’s manager hollered at him. “He’s not a machine! He’s a man!”

Drago, looking dazed, expressed disbelief at Rocky’s facial fortitude. “He’s not human,” Drago murmured. “He’s like a piece of iron.”

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Actually, according to a study by biologist David Carrier and physician Michael H. Morgan published this week in Biological Reviews, Balboa was less iron than a product of evolutionary adaptation. The male human evolved to look the way he does, scientists say, because such bone structure confers a high degree of protection during a fight. Balboa’s endurance was more science than machinery — though growing up in Philadelphia probably didn’t hurt.

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The new theory both hints at our species’ profoundly violent past and challenges the long-held contention that our facial bone structure and strong jaws derived from our diet. The thinking was that contemporary and ancient bone structure emerged to “maximize strength” for the consumption of hard-to-crush items such as nuts, the study said.

But this compelling explanation said fisticuffs spawned our facial bone structure, a phenomenon that’s even more pronounced in men.

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“When modern humans fight hand-to-hand the face is usually the primary target,” said co-author Carrier, a biologist at the University of Utah. “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 evolution…. These bones are also the parts of the skull that show the greatest difference between males and females…. In other words, male and female faces are different because the parts of the skull that break in fights are bigger in males.”

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For proof, look no further than the australopith, a hominid that had wide, flat, sturdy cheekbones. It roamed throughout eastern Africa, and looked like it could take quite a punch. Its “abrasive diet” was thought to be the evolutionary origin of its skull shape, but it may also be that the sturdy facial build “allowed energy from an upward strike to the jaw to be transferred from the lower jaw to the skull,” the study said.

“The australopiths were characterized by a suite of traits that may have improved fighting ability, including hand proportions that allow formation of a fist; effectively turning the delicate musculoskeletal system of the hand into a club effective for striking,” Carrier told his university news service. “If indeed the evolution of our hand proportions were associated with selection for fighting behavior, you might expect the primary target, the face, to have undergone evolution to better protect it from injury when punched.”

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Adding more credence to the theory, the study said, is that vital facial features — the flattening of the face, the widening of the cheek bones — occurred at the same time our ancestors evolved hands that could make a fist. “Together these observations suggest that many of the facial features that characterize early hominids may have evolved to protect the face from injury during fighting with fists,” Carrier said.

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Carrier said the study’s results were surprising. He hadn’t expected that the modern punch evolved at the same time as the modern skull, he wrote in an e-mail to The Washington Post.

Another surprise: We’re less able to take a hit now than we were before. “For the past 2 million years, the face of Homo has gradually become less robust,” Carrier wrote in an e-mail. “This is almost certainly due to several aspects of selection acting on our species.”