A simpler explanation for orangutans walking upright like humans is that this feature evolved in a common ancestor that did not include African apes. In other words, orangutans, not chimpanzees, are our closest living relatives. This would make sense of all the similarities in sexuality, reproduction, physiology, anatomy, and behavior that are unique to humans and orangutans.

John R. Grehan
Buffalo, N.Y.

From the Nature Index

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