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Shoulder shape in humans and apes gives picture of our last common ancestor

Contrary to popular belief, humans did not evolve from apes; rather we share a common ancestor with them. Until recently we didn’t know much about this common ancestor. Scientists speculated that it would have resembled an ape much more than a human or other primate but due to the fact that several human features appear quite primitive and resemble other animals like orangutans, doubt has been cast on this idea. Fortunately, we now have concrete evidence to support the hypothesis that the last common ancestor was much more like an ape than any other primate.

By studying the changing shape of scapular bones, scientists from UC San Francisco were able to get a good picture of what our common ancestor with the chimpanzees would have looked like. The team created a 3D simulation of human and ape scapulas and tested several different hypotheses to see which made the most sense. Measurements came from several humans, chimpanzees, bonobos and gorillas, which are very closely related to chimpanzees and orangutans, which are less closely related.

The best explanation for the shapes was a gradual change. This is consistent with the results from previous studies which estimate that the lineage that lead to humans diverged from that of apes about 6 to 7 million years ago, plenty of time for slow change in scapular shape. The study also showed that the last common ancestor resembled apes much more than it did modern humans. This suggests that the ape lineage has changed very little in the past 7 million years while the human lineage has undergone significant evolution.

Pictured below are the some of the stages of evolution in scapular shape starting with the last common ancestor and ending in humans. In the starting shape, the scapular spine, which is the part protruding from the bone, is almost vertical pointing to the skull of the ape. There can be seen a slow progression towards a more vertical pointing spine.

The progression of shoulder blades from African ape (top left) to human (bottom right). Also included are scapula images of Australopithecus afarensis and Australopithecus sediba early hominids as well as  Homo ergaster and Homo neanderthalensis, two other human species. In grey are predicted ancestral shapes. Credit: Nathan Young
The progression of shoulder blades from African ape (top left) to human (bottom right). Also included are scapula images of Australopithecus afarensis and Australopithecus sediba early hominids as well as Homo ergaster and Homo neanderthalensis, two other human species. In grey are predicted ancestral shapes. Credit: Nathan Young

The events that caused these changes can be easily explained by the lifestyles of early humans as they migrated out of forests and started using tools. While the vertical scapular spine would have been best for climbing, a lateral one would have been much more effective in using tools, especially when it came to throwing stones. It would have allowed early humans to store much more energy in their shoulders allowing for high speed throwing; a very useful tactic when hunting for food.

The team also took into account where some of the earlier hominids and humans fit in. Not surprisingly, they fit in with the images in the picture. The more closely they were related to us humans, the more their scapula resembled ours.

About Harry H

Harry H

Harry is currently studying biology and chemistry in University and hopes to go to grad school for evolutionary biology. He enjoys writing about sciences and sports and is a big fan of hockey and soccer. Some of his other interests are reading and rock climbing.
Contact Harry: harry.h@youthindependent.com