A recent study revealed how tool use among ancient humans helped shape modern day shoulders.
Humans split from our closest African ape relatives from the genus Pan between six and seven million years ago, but modern humans still posses traits similar to more distant relatives, such as bonobos, the University of California, San Francisco reported. These contradictory characteristics have caused scientists to question whether the last common ancestor of modern humans and African apes were similar to modern chimps and gorillas, or had their own unique look.
"Humans are unique in many ways. We have features that clearly link us with African apes, but we also have features that appear more primitive, leading to uncertainty about what our common ancestor looked like," said Nathan Young, assistant professor at UC San Francisco School of Medicine and lead author of the study. "Our study suggests that the simplest explanation, that the ancestor looked a lot like a chimp or gorilla, is the right one, at least in the shoulder."
The researchers believe the ancient shoulder changes could help tell the story of early hominin behavior, such as reduced climbing and increased tool use. The shoulders of African apes are characterized by "a trowel-shaped blade and a handle-like spine that points the joint with the arm up toward the skull," giving the animals an advantage when climbing or swinging through trees. On the other hand, the scapular spine of monkeys and humans is pointed more downwards, which could aid in tool making and high-speed throwing. The researchers set out to understand whether humans got this trait from a primitive ape or more modern ape-like creature.
To make their findings, the researchers compared 3-D models of shoulder bones fromearly hominins and modern humans against "African apes, orangutan, gibbons and large, tree-dwelling monkeys." They discovered modern humans' shoulder blades shared the lateral orientation with orangutans and the scapular blade shape with African apes.
"Human shoulder blades are odd, separated from all the apes. Primitive in some ways, derived in other ways, and different from all of them," Young said. "How did the human lineage evolve and where did the common ancestor to modern humans evolve a shoulder like ours?"
To answer these questions the researchers looked at two early human Australopithecus species (A. afarensis and the younger A. sediba) as well as H. ergaster and Neanderthals. They found australopiths werean an intermediate link between African apes and humans. Past research has suggested Australopithecus was capable of advanced tool use.
"The mix of ape and human features observed in A. afarensis' shoulder support the notion that, while bipedal, the species engaged in tree climbing and wielded stone tools. This is a primate clearly on its way to becoming human," said Zeray Alemseged, PhD, senior curator of Anthropology at the California Academy of Sciences.
The shoulder development of early humans made us great toolmakers and fastball throwers, but also left us more prone to shoulder injuries. The researchers believe these findings could help understand which people are more likely to suffer a shoulder injury.
"We could potentially use information about the shape of an individual's shoulder to predict if they have a higher likelihood of injury and then recommend personalized exercise programs that would best help to prevent them," Young said. "For a baseball pitcher, depending on your shoulder shape, you might want to emphasize some strengthening exercises over others to protect your rotator cuff."
In the future, the researchers hope to analyze variability in the shoulder blade of modern humans and the genetic sequences that led to these changes to gain insight into what factors cause modern individuals to be more likely to suffer a rotator cuff injury.
"Once we understand how the shape of the shoulder blade affects who gets injured, the next step is to find out what genes contribute to those injury prone shapes," said Terence Capellini, assistant professor of human evolutionary biology at Harvard University. "With that information, we hope that one day doctors can diagnose and help prevent shoulder injuries years before they happen, simply by rubbing a cotton swab on a patient's cheek to collect their DNA."
The findings were published in a recent edition of the journal Proceedings of the National Academy of Sciences.
