University of Utah biologist David Carrier theorizes about why humans, unlike most other animals, excel at both running and fighting.

For animals of many species, survival depends on the ability to fight and run. Yet, the characteristics needed for fighting — such as being strong, short, squat, and able to stand one’s ground in a shoving match — make it difficult to run long distances. And the qualities of endurance runners — such as long legs, narrow hips, and a slender upper body — make them wimpy, unstable fighters.

Thus, only a small number of Earth’s animal species excel at both running and fighting: deer, elk, certain antelope, kangaroos, and human beings.

“The fact that humans are both elite distance runners and the most violent species on the planet is a paradox,” says David Carrier BS’80 MS’82, associate professor of biology at the University of Utah. “How could a species that was highly specialized for fi ghting become one of the best distance runners among modern animals?”

After years of research into how animals move, Carrier believes he has an explanation: Deer, elk, and antelope evolved horns or antlers so they could fight while remaining lithe runners. Humans were able to evolve into distance runners only when they replaced their old weapon — the squat, brutish fighting physique of human ancestors known as australopithecines — with new ones, perhaps, says Carrier, “a stick with a point on the end, or a big bone, or a branch heavy enough and of the right length to be an effective club.”

Unlike antelope, deer, and elk, “we didn’t evolve new weapons, we invented them,” Carrier maintains.

“That allowed us to become adapted for running. Our hips became narrower, our legs became longer, and our upper body and arms became weaker.... The invention of weapons was an essential step in the evolution of Homo.”

Carrier’s theory was published last year as a chapter in From Biped to Strider: The Emergence of Modern Human Walking, Running, and Resource Transport, edited by D. Jeffrey Meldrum and Charles E. Hilton. Carrier’s chapter is titled “The Running-Fighting Dichotomy and the Evolution of Aggression in Hominids.”

Carrier’s fascination with animal locomotion started early. Born in Kansas, he and his family moved to Salt Lake City when he was 7. He spent his childhood playing with dogs and “stalking rattlesnakes” and had “a basement full of snakes, mice, hamsters, and turtles.”

At Highland High School, he wrestled, played football, and started distance running while on the track team. That sparked an interest in how animals run.

Carrier earned B.A.s in biology and geology and an M.A. in biology at the U, working with biology professorDennis Bramble. Carrier’s master’s thesis explored how the greater strength, leverage, and acceleration of young jackrabbits allow them to perform as well as adults, which have better stamina and a higher top speed.

He earned a doctorate in biology at the University of Michigan, held a postdoctoral fellowship at the University of California, Berkeley, taught at Brown University, and, in 1995, returned to the University of Utah as a faculty member.

Carrier’s doctoral thesis at Michigan dealt with the reasons lizards cannot run and breathe at the same time. That work led to highly publicized studies Carrier conducted a decade later with his wife, Colleen Farmer, a research assistant professor of biology at the U. Farmer and Carrier trained small alligators (up to three feet long) to walk on treadmills. They found that ’gators — unlike lizards — could walk and breathe simultaneously by using a rocking pubic bone to help them inhale and exhale. Some birds appear to make use of similar pelvic movements to assist their breathing. The biologists concluded that a similar way of breathing might have given dinosaurs and pterosaurs — prehistoric flying reptiles — the endurance to be active for long periods.

Carrier’s interest in how humans evolved as fighters and runners stemmed from earlier work on the tradeoff between fighting strength and running ability in dogs — specifically, pit bulls and greyhounds.

“The more we learned about the anatomy of these two breeds, the more I began to see them as a possible analogy for many of the structural differences between australopithecines and Homo,” Carrier wrote. “The short stature and broad stance of pit bulls is similar to that of australopithecines, whereas the longer-limbed and more gracile [slender] body of greyhounds is comparable to the build of Homo.”

Our ape-like ancestors, the australopithecines, were the first hominids to walk on two feet. They were short and squat, with strong forearms and lower legs, had powerful hands, forearms and feet, short legs, wide hips, and stout necks and disproportionately large heads attached to shoulder muscles that aided pushing and shoving.

Many of these characteristics have been viewed as a legacy of a life of climbing, hanging, and swinging in the trees before the australopithecines started walking upright on the ground at least 4.5 million years ago. But a key conclusion of Carrier’s research is that some climbing-related traits made our ancestors pre-adapted for fighting so that males could battle over the right to mate with females. And some australopithecine traits — such as broad hips and short legs — “cannot be associated with climbing” and only were of value for fighting, Carrier says.

He notes that australopithecine males were substantially larger and stronger than females — a characteristic known as “sexual dimorphism” — and that the size difference was greatest in the arms, which were used for fighting. Among animals, a distinct difference in male-female body size is almost always seen when males fight over access to females.

The same traits that made australopithecines good fighters “would also have made them inefficient and poor distance runners by modern human standards,” Carrier says. “If you’ve got short legs, you’ve got short strides. If you have a big, stout upper body, you have extra weight to carry. Wide hips make for an awkward, inefficient stride.”

So the evolution of distance-running ability as Australopithecus evolved into Homo “represents a dramatic change in the evolutionary path of our ancestors,” he adds.

“Apes can’t fight well in the trees. One of the reasons the ancestors of australopithecines may have become adapted to walking and running upright on the ground was because they were better fighters that way. Males who were the most stable and quickest on two legs would have had an advantage for fighting… and thus gain access to more females and leave the most offspring.”

Early members of our genus, Homo, were Homo habilis and Homo rudolfensis. Their skulls were beginning to resemble those of modern humans, but the rest of their bodies were still short and squat like Australopithecus, Carrier notes, so the modern human body form started emerging about two million years ago with the rise of Homo ergaster. (Our species, Homo sapiens, dates to about 195,000 years ago.)

Carrier suspects that ancestors of Australopithecus came down from the trees not only because savannah habitat was opening up and they couldn’t survive in the trees anymore, but also to fight.

“Apes can’t fight well in the trees,” he says. “One of the reasons the ancestors of australopithecines may have become adapted to walking and running upright on the ground was because they were better fighters that way. Males who were the most stable and quickest on two legs would have had an advantage for fighting … and thus gain access to more females and leave the most offspring.”

Carrier compiled evidence 20 years ago suggesting that humans evolved to be long-distance runners. He observed that relative to most other species, humans are elite distance runners, and he related this capacity to our exceptional ability to dissipate heat by sweating and our ability to vary our breathing pattern while running. He argued that these abilities might have evolved so that early hominids could chase prey animals to exhaustion. Carrier’s former professor, Dennis Bramble, recently refined and strengthened this theory, arguing that early humans evolved from australopithecines because they needed to run long distances — probably to scavenge for carcasses on the vast African savannah — so natural selection shaped our anatomy by favoring physical characteristics that made running easier.

Carrier contends that the invention of weapons by human ancestors may have allowed the short, squat Australopithecus to evolve into the tall, lithe Homo runner.

He says the first weapons human ancestors used against each other would have been simple implements, such as stones and tree branches employed rather ineffectively by modern chimpanzees. But with the development of weapons that could pierce skin (a sharp stick) or maximize a blow (a club), “success in physical combat would have begun to shift from the opponent with the advantage in physical strength and quickness to the opponent with the most effective weapon,” says Carrier, adding “Now the selection is not for being the strongest, but for being the one who can come up with the best weapon design, which means being the smartest and most creative.”

And as we became smarter fighters, our bodies evolved. Since the male body was no longer the primary fighting weapon as Homo evolved from Australopithecus, we developed a bigger body, longer legs, shorter and slimmer arms, narrower hips, a barrel chest, and a smaller head and gut in proportion to our body size, and the size difference between men and women dropped.

That made us “amazing distance runners, and we evolved the capacity in an incredibly short period of time — a few hundred thousand years, sometime between 2.5 million and 2 million years ago,” says Carrier.

“If I argue that humans are by nature violent animals, the concern is that such an argument will be used to justify violent behavior. Although there is some merit to that fear, if our goal is to prevent future violence we need to understand why we are violent animals—where the tendency came from and why it has stayed with us.”

“Modern humans are the most violent species on the planet,” he continues. “There is evidence that a lot of that violence is associated with male-male competition. Males are much more violent than females in our species. Males at a low social or economic status are more likely to be violent than males who aren’t — consistent with male violence being associated with competition for resources and females.

“Species where you see fatal fighting are species where the reproductive stakes are high,” says Carrier. “The stakes are high in humans because we have children that take 15 to 20 years to raise.”

Carrier’s ideas may sound similar to the so-called “killer ape hypothesis” developed in the 1950s by anatomist Raymond Dart, who claimed that the ancestors of humans evolved to be violent because those who were good at hunting had a better chance of survival. Dart’s theory ultimately died for lack of evidence and the excessively dark portrait it painted of human nature. In contrast, Carrier argues that the initial impetus for aggression in human ancestors came not from hunting but from male-male conflict over mating opportunities. He supports this suggestion with evidence that australopithocene males were much larger than females and that australopithecine anatomy was built for fighting. Carrier argues that the evolution of Homo, a species built for endurance running, was dependent on the invention of weapons that reduced anatomical conflicts between running and fighting.

Carrier says his research adds to other evidence “that suggests our high level of violence is something we evolved to be. It’s our nature.”

That evidence, he maintains, includes a pattern of modern human violence consistent with male-male competition (including the fact that men are far more likely than women to commit murder and to be murdered); the occurrence of male-male fighting among most of our closest primate relatives, the apes; and an archaeological record of systematic warfare extending back to the dawn of humanity.

Carrier now is looking at the difference in male and female body shape and size in many primates, seeking to learn more about the relationship between anatomical structure and fighting ability.

But he knows many people dislike the idea of humans as inherently prone to aggression and violence.

“If I argue that humans are by nature violent animals, the concern is that such an argument will be used to justify violent behavior. Although there is some merit to that fear, if our goal is to prevent future violence we need to understand why we are violent animals — where the tendency came from and why it has stayed with us.”

—Lee J. Siegel is a science writer for University of Utah Public Relations.