A wrinkly hypothesis

The aquatic ape hypothesis was first proposed 70 years ago by German pathologist Max Westenhöfer. The hypothesis has more recently championed by Elaine Morgan, most notably in her book The Aquatic Ape. But the hypothesis has not drawn a lot of attention in the literature and has been dismantled in various places (here's one that's pretty good). Essentially the hypothesis interprets certain features, such as human hairlessness, as adaptations to an aquatic environment.

In 2011 Changizi et al. published a paper arguing that water-wrinkled fingers are an adaptation to life in aquatic environments. A new paper, just published, purports to test this hypothesis. Kareklas et al. had test subjects soak their hands in 40°C water for 30 minutes. Then they got the subjects to move marbles from a source container that was either filled with water or dry. The performance of the wrinkly handed participants was compared to a control group that performed the same task without soaking their hands.

Their results showed that the wrinkly finger group completed the marble moving task 12% faster when the source container was filled with water. There was no difference between the two groups when it was dry. They argue that their study shows a clear advantage to having wrinkly fingers when manipulating submerged items. But, I find the experiment completely underwhelming as support for wrinkly fingers as adaptations.

Most obviously, participants had to soak their hands in 40°C water for 30 minutes in order to obtain the small advantage. If wrinkled fingers are important for manipulating submerged objects, it seems to take an inordinately long time for fingers to wrinkle. Moreover, prior research has shown that lower temperatures, which are more likely to be encountered by our ancestors, result in slower and less exaggerated finger wrinkling (reviewed in Wilder-Smith 2004).

Marbles are also particularly small and slippery when compared with almost all conceivable objects that a paleolithic primate would be interested in picking up. It would be far more compelling if they had shown that the performance advantage remained when other objects were manipulated underwater. Given the very small advantage for marbles, I strongly suspect that the advantage would disappear for a vast array of other items.

The proponents of the aquatic ape hypothesis will probably incorporate the new study into their lists of evidence for a close association with with water in our ancestors. But, like most of their evidence, it is little better than plausible speculation dressed up as a compelling theory that deserves attention. It's a great way to get your ideas attention in the popular press, it's a horrible way to do science. The inoculation for such adaptationist nonsense is, as always, Gould and Lewontin 1979. 

Kareklas, K., Nettle, D., & Smulders, T. (2013). Water-induced finger wrinkles improve handling of wet objects Biology Letters, 9 (2), 20120999-20120999 DOI: 10.1098/rsbl.2012.0999 

Changizi, M., Weber, R., Kotecha, R., & Palazzo, J. (2011). Are Wet-Induced Wrinkled Fingers Primate Rain Treads? Brain, Behavior and Evolution, 77 (4), 286-290 DOI: 10.1159/000328223 

Wilder-Smith, E. (2004). Water immersion wrinkling Clinical Autonomic Research, 14 (2), 125-131 DOI: 10.1007/s10286-004-0172-4