Stick your fingers in water and your fingertips will soon start to wrinkle. There’s a common belief that this happens because the tips absorb water, but that can’t be right. Since the 1930s, we’ve known that the wrinkling process is under nervous control—if you sever the nerves in your finger (don’t try this at home, kids), the wrinkles won’t form.
But neurobiologist Mark Changizi has an intriguing hypothesis about the origin of pruney fingers—they’re an adaptation that allows us to grip wet surfaces. Like the rain treads on tyres, when pressed down, pruney fingers create channels that let water drain away, allowing them to make better contact with damp surfaces.
I wrote about this hypothesis back in 2011, when Changizi first proposed it. Now, an independent team from Newcastle University has found some support for it. In a simple experiment, they showed that people can pick up wet marbles more quickly if their fingers are wrinkled after a 30-minute soak in warm water than if they are dry. And this advantage only applied to the wet marbles—both wet and dry fingers were equally good at picking up dry marbles.
The new study has picked up a lot of coverage, but it also raises some interesting questions about how to test evolutionary explanations. So far, all of the evidence for Changizi’s idea comes from looking at modern human fingers. How much can such structural studies tell us about function? How much, if anything, can the what tell us about the why? If modern human fingers grip wet marbles well, and form patterns that resemble rain treads, does that tell us anything about the origins of such patterns or are all such explanations merely just-so-stories?
T. Ryan Gregory, an evolutionary biologist from the University of Guelph, has been a vocal critic of studies like these, including the dubious paper on fists and fighting that I covered recently (here’s his post). I engineered a debate between Gregory and Changizi over the pruney fingers hypothesis on Twitter, and I think it’s a fascinating case study in how to think about evolutionary hypotheses.
Here’s a Storify of their exchange, with some commentary from me for people who aren’t familiar with the issues being raised.