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
"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
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
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."
team think the ability to throw also gave this ancestral species an evolutionary advantage.
Dan Lieberman is an anthropologist at Harvard University and an author on the new study.
: As you cock your arm back, you store up huge amounts of elastic energy in the muscles and the other tissues that cross the front of your shoulder.
: If your shoulder were more vertically oriented, like in a chimpanzee, your muscles can't generate the same amount of power.
says the earliest human ancestors also had chimp-like shoulders.
But that changed about two million years ago.
That's when the ancestral species Homo erectus came into the picture.
: They widened and broadened their shoulders significantly in the genus Homo.
CHATTERJEE: Now, the authors were only able to study two fossils, so they can't say for certain exactly how human-like these Home erectus shoulders were.
But being able to throw with power and precision, Lieberman
says, must have been advantageous to our ancestors.
If you consider humans, he
: We don't have natural weapons.