Vestigial traits are characters of organisms which have seemingly lost all or most of their original function in a species through evolution. Some examples are wings on flightless birds, hind leg bones in whales, wisdom teeth in humans, the appendix in humans, hind leg bone "spurs" in some python species...The list goes on and on. The following is taken from biologist and author of "Why Evolution is True", Jerry Coyne:
But behaviors can be vestigial, too. One such behavior is the “grasping reflex” of human infants. When you put your finger into the palm of an infant, it will immediately and securely grasp it. The grasp is so tight that it’s sometimes hard to make the kid let go! It is said — though I have never seen this demonstrated — that up to a couple months of age a baby can hang suspended from a horizontal stick for several minutes.
The grasping reflex is evident in the feet, too. If you put your finger along a baby’s toes from the sole side, it will grasp with those toes. And when a baby is sitting down, its “prehensile” feet assume a curled-in posture, much like what we see in an infant or an adult chimp.
Why do infants show this grasping reflex, but then lose it after several months? A very plausible suggestion is that the behavior is a remnant of the grasping reflex seen in other infant primates, which they use to hold on to the hair of their mothers as they’re being carried about.
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