In most respects, chimpanzees are physically superior to humans. Pound for pound, they are perhaps four times stronger. They are faster. They can run straight up a tree, climb and swing with an agility that is the envy of an Olympic gymnast.
But ask a chimp to throw a 90-mph fastball over a 17-inch-wide home plate -- something even the average Major Leaguer can do -- and the chimp is a chump. Chimps can throw only about 20 mph -- a speed your Little Leaguer would laugh at -- with pathetic accuracy.
The reason for the difference is anatomical, the result of critical physical changes in the shoulder, arm and torso of the species Homo erectus that first appeared 2 million years ago, a team of scientists suggested Wednesday in the journal Nature. These adaptations allow the storage and rapid release of the elastic energy that powers the arms of flame-throwing hurlers such as the Washington Nationals' Stephen Strasburg.
The changes, the researchers believe, also might have had far-reaching implications, giving early humans a singular advantage in hunting faster, stronger animals with spears and projectiles. That development might have greatly improved our ancestors' diet, providing more protein that led to larger bodies, bigger brains and the ability to roam more territory, the study hypothesized.
"It's potentially a plausible explanation for how we became such effective hunters," said Neil Roach, a postdoctoral fellow at George Washington University's Center for Advanced Study of Hominid Paleobiology and lead researcher on the study. Hunting "really did have a tremendously profound effect on who we are."
Roach and his colleagues identified three important anatomical changes behind the ability to throw hard and accurately:
Humans developed longer waists, which allows the torso to rotate separately from the hips and generate the forces that stretch tendons and ligaments in the shoulder as the arm is cocked. Their shoulders also gradually moved lower on their torsos, allowing the arm to jut out at a right angle; a chimp's shoulder puts his arm in a more vertical position.
And humans developed an upper arm bone that twists, allowing a thrower to reach back at an almost unnatural angle and store energy for the throw.
When all that stored energy is unleashed, it whips the upper arm forward in the fastest motion the human body is capable of producing, the study said.
To study their theory, the scientists recorded 20 hard-throwing baseball players at Harvard University with three-dimensional motion capture equipment. Then they put the ballplayers in stiff braces that limited their range of motion.
"What we wanted to do there was push the hard throwers back in time," Roach said in an interview from Loyangalani, Kenya, where he is conducting fieldwork.
The study is not the first to look at the human throwing motion or its impact on hunting and evolution. Charles Darwin long ago noted that humans' unique ability to throw was made possible when our ancestors stood on two legs, freeing their arms and turning foragers into hunters.
But Holly Dunsworth, an assistant professor of anthropology at the University of Rhode Island who has studied human throwing, said Roach's study advances our understanding by pairing its theory on the biomechanics of throwing with the fossil record on hunting.
Human ancestors known as hominins were "eating meat at least 2.6 million years ago and were probably hunting large prey 1.9 million years ago," according to the study. The research also suggests that throwing might have allowed our ancestors to defend carcasses from stronger competitors that had claws and fangs.
Although the study does not present a complete theory of how various human ancestors evolved into hunters, Dunsworth said, it forces us "to think of traits evolving complexly together" rather than sequentially, she said.
"It is a novel approach in paleobiology," she added.
William Hopkins, a professor of neuroscience at Georgia State University who has studied the brain's role in the evolution of throwing, also praised the study. Despite considerable research in the area, he said, "the paper, in my view, is one of the first that has tried to explain what allowed humans to go out and develop this remarkable throwing ability."
The timing of various developments millions of years ago is "a little iffy," Hopkins said, but the study is a "pretty large step in understanding physical changes in humans" that allowed them to throw so well.
Chimps generally throw randomly during displays of dominance or warning, Hopkins said. But he has occasionally seen some stand upright and throw with purpose, like humans.
Glenn Fleisig, a biomechanical engineer and research director at the American Sports Medicine Institute in Birmingham, Ala., said the paper's description of the critical aspects of throwing hard and accurately are valid, although the research "doesn't mean anything directly applicable to sports."
But Roach, in the interview, said the severe forces a pitcher places on the upper arm bone as he practices his craft result in visible changes. His findings, he said, might be able "to tell people whether they're more likely to get injured."