Contested study traces slingshot shoulders to 2-million-year-old ancestor
Baseball pitchers making big bucks to hurl horsehides are carrying on a 2-million-year-old tradition.
Upper bodies that could throw objects at high speeds appeared for the first time in a human ancestor known as Homo erectus, say biological anthropologist Neil Roach of George Washington University in Washington, D.C., and his colleagues. Nearly 2 million years ago, H. erectus hunters exploited their slingshot shoulders to throw rocks or wooden spears at animal prey, Roach’s team reports in the June 27 Nature. But at least one critic argues that skeletal changes behind modern humans’ throwing ability didn’t show up until much later.
However human throwing evolved, chimps got the heave-ho. “Adult male chimps are incredibly strong but can only throw about 20 miles per hour, one-third the speed of a 12-year-old Little League pitcher,” Roach says.
To uncover the inner workings of Homo sapiens’ ability to fling fastballs, Roach and his colleagues used a 3-D camera system to record the movements of 16 college baseball players and three athletes in other sports as they hurled baseballs, sometimes while wearing motion-limiting braces. Computerized analysis of each participant’s throwing motions indicated that energy storage and release enabled the shoulder to act much like a slingshot during fast throws.
Three anatomical traits characteristic of hard throwers today combined for the first time in H. erectus, the scientists propose. Expanded waists made the torso more flexible, realigned elbows increased energy storage in the dominant arm’s shoulder while cocking the arm, and broad shoulders boosted the power of shoulder and chest muscles.
Anatomist and fossil researcher Susan Larson of Stony Brook University in New York disagrees with that assessment. No tendons for energy storage are available at the shoulder, so human throwing probably doesn’t work as proposed in the new paper, Larson says.
In addition, H. erectus and two other early hominids — 4.4-million-year-old Ardipithecus ramidus in eastern Africa and almost 2-million-year-old Australopithecus sediba in southern Africa — had similarly aligned elbows in both arms, not in one throwing arm as observed in humans, Larson says.
Ancient eastern and southern African hominids had relatively short collarbones, suggestive of narrow shoulders unsuitable for flinging objects fast, she says. Larson regards estimates of longer collarbones in H. erectus, based on partial fossils recovered in western Asia, as unreliable. Those estimates are a centerpiece of Roach and his colleagues’ reconstruction of how throwing evolved, making the issue of when longer collarbones arose in the human lineage a central one.
Elongated collarbones and broad shoulders first appeared, Larson says, not in Homo erectus but in a European hominid — dubbed Homo antecessor by some researchers and Homo heidelbergensis by others — roughly 1 million years ago.
N. Roach et al. Elastic energy storage in the shoulder and the evolution of high-speed throwing in Homo. Nature. Vol. 498, June 27, 2013, p. 483. doi:10.1038/nature12267. [Go to]
B. Bower. Possible human ancestor in Australopithecus sediba. Science News. Vol. 183, May 4, 2013, p. 20. Available online: [Go to]
S. Larson. Evolutionary transformation of the hominin shoulder. Evolutionary Anthropology. Vol. 16, September/October 2007, p. 172. Abstract available: [Go to]
Neil Roach’s website: [Go to]
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