In some ways, chimpanzees seem less primitive than humans. (AP Photo/Martin Meissner)

A new study puts the burden of humanity's evolution on our shoulders. Like, literally on our shoulders. By examining the shoulder blades of two early human Australopithecus species, researchers believe they've found further evidence that humans and apes shared an ape-like ancestor.

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Humans are, of course, most closely related to the great apes of Africa -- chimps and bonobos, specifically. That's already established. But while we know that these animals are the last we shared a common ancestor with, we've definitely taken a different route in our evolutionary journey. In fact, humans have some features that seem more "primitive," or more like the monkeys that came earlier in primate lineage, than the analogous features on apes.

That's led some scientists to suggest that our common ancestor was actually more like a monkey, allowing humans to retain some of these primitive features -- like a spine more suited for looking down at tools than for hanging from branches -- while evolving, by chance and by nature of our similar habitats, some of the same advanced adaptations as apes. 

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But according to the new study, published recently in the Proceedings of the National Academy of Sciences, it's safe to assume that the simpler answer is the right one: Humans and apes did descend from an ape-like ancestor. It's just that humans got a little turned around along the way.

Human shoulder blades, built more for labor than for climbing, seem superficially more similar to those in monkeys than in apes. But by analyzing 3-D scans of shoulder blades from humans, early-human ancestors, apes, and monkeys alike, the researchers concluded that human shoulders do have enough in common with those on apes to have evolved from the ape model. And the early-human species studied followed a logical progression from ape to human. In a way, the human shoulder worked backwards, moving from the ape's structure back to a better version of what monkeys have.

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According to the study authors, our journey from swinging through the trees to making tools and hunting was a long, slow one. But over time, our success with the latter made us give up the former.

"These changes in the shoulder, which were probably initially driven by the use of tools well back into human evolution, also made us great throwers," study author Neil T. Roach, a fellow of human evolutionary biology at Harvard University, said in a statement. "Our unique throwing ability likely helped our ancestors hunt and protect themselves, turning our species into the most dominant predators on earth."

One day, if we're lucky, we'll get the chance to study the common ancestor herself. But until then, scientists will just have to keep piecing together the sparse fossil record of human evolution in the hopes of filling in the blanks of our history.

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