This is the type of post I originally intended to publish on this website, and the only type of post, for the website was created, at the behest of my editor at Viking/Penguin, to support my book WEIT. My idea then was to post a bit of cool evidence for evolution every few weeks or so. Then things got out of hand. . . But today we are back to the original mission.
One of the pieces of evidence I use for evolution, in both my book and my undergraduate classes, is the presence of vestigial traits. And there are some nice behavioral ones. I wiggle my ears for my students, which they love, but I do it to demonstrate our vestigial ear muscles, useless in modern humans but adaptive in our relatives, which can move their ears widely to localize sounds. (Check out your cat when it hears something.)
Humans have another vestigial behavior: the “grasping reflex” (also called the “palmar reflex”). Young infants can hold onto objects with both their hands and their feet—and hold tightly and tenaciously. They lose this behavior—which is instinctive, prompted by inserting a finger or a stick in their hands or feet—a few months after birth.
While we’re not 100% sure what it represents, I’d bet that it’s a genetic holdover from our ancestry as hairier primates. (Remember: we’re the only “naked apes.”) In primate species, the young are carried about by hanging onto their mother’s fur with both hands and feet, and they keep this behavior throughout infancy. Their ability to hold on is important for their survival.
Humans aren’t hairy, and aren’t carried about by clinging to their mother’s fur. But we still, at least for a short period, show genetically-based behaviors that testify to our descent from furrier creatures.
Here are some photos of the daughter of a friend. This one shows the grasping reflex at 7 days of age. Note that she’s holding on so hard that her fingers are white!
For years I tried to persuade my friends who had infants to let them hang from broomsticks (I have a drawing of this behavior in an evolution textbook from the 1920s), so I could photograph it or make a video. But for some reason they always refused, even though I claimed that one can do this safely: just put the infant over your lap or a bunch of pillows. No dice.
But I was recently shown this video from the 1930s showing two infants “competing” to see who can hang the longest. Here are the YouTube notes:
Fragment of “Johnny and Jimmy” (twins), a silent film by Myrtle McGraw, recorded in 1932. from McGraw, M.B. (1975). Growth: A study of Johnny and Jimmy. New York: Arno Press. 
One baby makes it for only 4 seconds (what a wimp!), but the other is still hanging after 37 seconds! I love the blotting out of the genitals.
Here’s a more recent video in which the infants are suspended more humanely. The genitalic blur has also been made spiffier: it’s now a fig leaf.
This isn’t the only primitive reflex displayed by human infants. Wikipedia has a whole list of them (the foot-closing is called the “plantar reflex”), and you might amuse yourself by speculating about which of them might have been adaptive in the infants of our ancestors, and why.