Though something like kneecaps can be traced back 400 million years, to the Devonian period in frogs.
He offers a Darwinian explanation that “individuals who just happen to have sesamoid bones at their knees” happened to run better and thus left more offspring.
“We know almost nothing about what the kneecap did when it first evolved, when both the tendons that held the bone and the bone itself were thinner and not as well developed,” paleontologist Brian Switek wrote in his recent book Skeleton Keys.Ross Pomeroy, “Four Fascinating Facts About Kneecaps.” at RealClearScience
More and more, that sort of explanation begins to sound like what we say when we don’t really have more specific information. Especially now that we are starting to get more specific information. That said, Skeleton Keys: The Secret Life of Bone sounds like an interesting read.
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See also: Cartilaginous Skeleton Not Necessarily More “Primitive”
Complex Skeletons From 550 Mya (“Earlier Than Realized”)