In WEIT there is a chapter on vestigial traits, defined as those traits that are evolutionary remnants of features useful in an ancestor, but now either useless or used in a different way. The paradigmatic case is, of course, the appendix, the remnant of a caecal pouch used to digest leaves and vegetation in our ancestors. 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.
One of my friends has a four-month-old daughter, and I asked her to take a picture of the grasping reflex and the prehensile foot posture for this website. Here are both in a single picture. Although the kid is somnolent, she still holds on firmly. The sitting posture of a young chimp is given for comparison.
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.
I’m not going to encourage my readers to suspend their newborns from broomsticks in the cause of evolution (they could fall, after all), but if you’re sufficiently curious and foolhardy to do this, let me know the results.
Thanks to the anonymous mom who donated her child to science.
THIS JUST IN: Photographic proof! (Thanks to commenter Wes for the link.)
Video proof (hat-tip to feelx):