In a much-discussed paper published in this week’s Nature, a team of scientists unveiled the findings of their study of the human throwing motion. The study’s authors, a group that includes Daniel Lieberman, author of The Evolution of the Human Head, sought to uncover how it is that humans are so much better at throwing than other animals. Adult chimpanzees, for example, are much stronger than humans but their tosses top out around 20 mph. Young humans can easily exceed that rate, and some major league pitchers are routinely clocked at over 100 mph.
By making 3-D recordings of athletes, Lieberman et al were able to break down the throwing motion in a way that’d never been done before. Analysis of that footage revealed the crucial part played by the human shoulder, the ligaments and tendons of which are able to store elastic energy as an arm is cocked in preparation to throw and then release it in the arm’s ensuing forward motion.
Human shoulders are broader than our primate cousins’, and our shoulder blades are located along the flat of the upper back, explains Daniel Lieberman, a physical anthropologist at Harvard University and a co-author of the study. The shoulder blades of chimps on the other hand, are positioned more along the side of the body, he says. “If your shoulders are more vertically oriented—like in a chimpanzee—then the muscles can’t generate that much power.”
“Our earliest ancestors also had chimp-like shoulders,” Lieberman says. But evidence from two fossils suggests that may have changed with Homo erectus, a species that lived about 2 million years ago. The fossils Lieberman and his colleagues examined had shoulders resembling a modern day human’s.
Homo erectus shared two more features with humans that Lieberman and his team think crucial to the ability to throw with power: a slender, flexible waist, which allows us to twist the torso in relation to our hips and legs, and a twist in the shape of the humerus, the bone that connects the shoulder to the elbow. These features “evolved bit by bit,” Lieberman says, “but they appear all in a package in Homo erectus.”
He and his team think the ability to throw also gave this ancestral species an evolutionary advantage. “We don’t have fangs. We don’t have claws and massive paws,” Lieberman notes. Without such built-in weapons, the ability to hurl objects with force and precision must have allowed Homo erectus to become a more efficient hunter.
Lieberman’s The Evolution of the Human Head is one of the 100 significant works we’ve chosen to highlight as we celebrate the 100th anniversary of the founding of HUP. Over at our centennial site, where we’ve been posting excerpts from these 100 titles, you can read a section of Lieberman’s book that really makes clear how this new study of human throwing ability fits with his work on the head and on running.
From the book:
At some point, members of the human genus became better at acquiring and using energy differently than other primates. It is reasonable to speculate that this shift was partly if not largely a byproduct of early Homo’s successes as a trekker (perhaps a trend that began in Australopithecus), as an opportunistic forager, and most importantly as a carnivore. H. habilis apparently had access to meat, but H. erectus certainly became an excellent hunter, thanks to a suite of derived features. These features, some of which may be adaptations for running and hunting, and others of which may have been exaptations of other functions, were important because they enabled early members of the genus Homo to range widely to acquire reliably plenty of high-quality food. This shift in ranging and diet not only permitted selection to make teeth smaller but also made more energy available to hominins, setting in motion other selective forces. Most important, more energy probably allowed females to wean infants earlier, lower their interbirth intervals, and have more babies that survived to become adults. In addition, more energy allowed individuals, both males and females, to grow and fuel bigger bodies and bigger brains. In turn, bigger bodies and brains would have ramped up the selective pressure to lower interbirth intervals.
Once one commits to the strategy of acquiring lots of energy in order to spend it on reproduction, then natural selection can promote a positive feedback loop. Other factors that improved the ability of H. erectus and then H. heidelbergensis to acquire high-energy resources included food processing, cooking, the invention of stone tools and projectile technologies, and probably also behaviors that are hard to document in the fossil or archaeological records, such as cooperative hunting, food sharing, language, and tracking prey. In turn, these behaviors led to higher fitness, which helped promote further advances—many of them cognitive or behavioral—in ways to exploit and use energy.
So: foraging plus running plus hunting with projectiles (enabled by evolutionary adaptation of shoulder?) equals meat equals energy equals bigger bodies and bigger brains that many thousands of years later we can use to figure out how we threw things way back when. Evolution is the jam.