Cells in the notochord and secondary neural tube secrete signals that guide somite differentiation. Within this temporary so-called “tail” are a number of such somites, a secondary neural tube, and the lower part of the notochord. The coccyx, a set of small vertebrae at the bottom of the vertebral column, provides a secure anchor for the gluteus maximus and the muscles and ligaments that form the floor of the pelvis. (We discussed Meckel’s cartilage as an example of this in the review of Your Inner Reptile and another example-the urachus-in “ Recapitulation Repackaged and Re-Applied.”) So it is with the so-called human embryonic tail. In many cases, once an embryonic structure’s job is done, it regresses and disappears.
In each segment are somites-blocks of cells that differentiate into muscles, bones, cartilage, and other supporting structures for that segment of the body or act as the scaffolding or stimulus for other structures to grow. Human embryonic development-like that of all vertebrate embryos-is organized by segments. Within this region many important things are happening, and none of those things have anything to do with swinging from trees, past or future! What about the tail? Does the human coccyx at the end of the vertebral column hang us in our rightful place on the tree of life? Or is the coccyx a perfectly designed structure that serves our needs as well as a monkey’s tail serves its need to swing in the trees? Do human embryos have a tail harking back to an evolutionary pastįrom the third to the seventh week of development the lower part of the human embryo’s curled, C-shaped body resembles a tail. Shubin travels back through presumed evolutionary history from skaters bruising their vestigial tails to monkeys swinging through trees. As we examine his parade of proofs we shall see that it is this starting assumption, not the evidence in the human body as he contends, that determines his conclusions. Shubin’s case rests entirely on the assumption that all life evolved from a primitive common ancestor. Your Inner Monkey, the final installment of Tiktaalik-discoverer Neil Shubin’s PBS miniseries Your Inner Fish, climbs to the top of the evolutionary tree of life to acquaint us with “our inner monkey” and show how an evolutionary past produced “the intelligent creatures with all our quirky flaws that dominate the planet today.”
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