Theory suggests wrinkling of wet digits evolved for a reason | Hacker News Reader

Theory suggests wrinkling of wet digits evolved for a reason

nature.com

43 pointspixdamix28 comments

Not everyone is gripped by the new theory. "This hypothesis is unjustified", says Xi Chen, a biomechanical engineer at Columbia University in New York. Chen thinks that the wrinkles have a simpler cause: when fingers are immersed in hot water, the blood vessels tighten and the tissue shrinks relative to the overlying skin. This contraction causes the skin to buckle. "It's a classic mechanics problem," he says.

Ugh. Explaining the mechanism behind a behavior is not an argument against its purpose!

Scientist A: I think polar bears evolved white fur so they'd be camouflaged in the snow.

Scientist Crazy Pants: No silly, polar bears have white fur because their hair follicles contain keratin and are hollow.

Scientist A: I'm going to kill you.

I completely see your point, and in many cases I agree with you. But there is sometimes a tendency for evolutionary biologists to assign deeper reasons to things that are just a consequence of physics. Occam's razor and all that.

In this particular case that fact that the phenomenon is not present when the nerves are severed does present a pretty relevant argument against simple biomechanics.

Surely if it were that, then fingers should wrinkle faster?

If I stick my hand in water then it gets wet immediately, but doesn't wrinkle for... what, fifteen minutes? And that's warm water -- cold water (like practically all the naturally-occurring water in the world) takes a lot longer.

And then, I can dry my fingers much faster than I can un-wrinkle 'em. Surely the circumstances in which an ancestral human would:

a) Spend fifteen or twenty minutes in the water, then

b) Need to pick up something vitally life-preserving within the next few seconds

would be sufficiently rare that it probably wouldn't have been a major selection pressure?

Unless it wasn't selected for in humans. Or any great apes. But perhaps in a much older mammalian ancestor. And in humans it simply has not been selected against, which is why it's simply not as good or useful as it used to be. Sort of like wiggling your ears.

What was wiggling our ears for? Attracting mates? :]

Echo location, just like a dog or any animal with long ears.

It doesn't seem like you understand how this would be a useful. This is not an explanation for slipping in the water and wrinkling to climb back out, it's for spending long periods, perhaps for fishing or during the wet season, and still being able to grab and manipulate objects.

Sorry, I am missing your point. In the absence of wrinkling, what is stopping me from being able to grab and manipulate objects?

I can do that without wrinkling, even in water.

There's probably a cost to having wrinkly fingers which is why they don't wrinkle immediately.

But if enough of our ancient ancestors fell into bodies of water surrounded by steep, slippery rocks, then the 15-minute delay for gecko fingers to set in would be worth the wait. As long as they could tread water or hold their heads above it long enough, the grippy fingers might have really helped.

Edit: I like katovatzschyn's reasoning even better: http://news.ycombinator.com/item?id=2718662

Scientists have known since the mid-1930s that water wrinkles do not form if the nerves in a finger are severed, implying that they are controlled by the nervous system.

That is the amazing part, if it's not a merely mechanical process. A bit further down they mention poor circulation also impeding wrinkling, so it may have something to do with control of capillary blood vessels.

Your hands also don't get wrinkled in water with the maximum possible amount of salt dissolved in it, no matter how long you stay there. It seems like this must somehow be important.

I think that may well be purely mechanical or actually osmotic.

Articles like this satisfy some tiny general interest (wrinkles! neat!) but then squander any good will by predisposing the public to completely misunderstand evolution.

The trait may stick around if it's useful, but it didn't evolve "for a reason."

"had ended up neatly filling a need"?

Passive voice is discouraged grammatically, however evolution is pretty much the embodiment of a passive voice.

I would argue that it (unlike all the other things that did not evolve) did evolve for the reason that it was useful. It's not like all of a sudden there was someone that had wrinkly fingers and that "stuck around because it was useful". In all likelihood, there were many incremental changes that were progressively selected for and the "reason" for that selection is that it was useful.

It seems to me that it's perfectly acceptable language to say that a cause is the "reason" for an effect, even if it's not possible to assign intent.

Your comment reminds me of Feynman responding to the question of why magnets attract each other (http://www.youtube.com/watch?v=wMFPe-DwULM) which, while inscrutable from a physical point of view, seems not to be a fruitful way of approaching questions. There are always levels of deeper understanding, but you have to take it one level at a time.

> it's perfectly acceptable language to say that a cause is the "reason" for an effect, even if it's not possible to assign intent

Sure, but that's not the case here. While existing traits are selected for based on fitness (which has absolutely no meaning outside of environment, another rant), new traits (whether wrinkly fingers in one leap or in several small ones) are evolved by chance.

Evolution has no intent, no direction. Each individual trait has no reason other than, "Hey, I'll try this out!"

In a society where a good chunk doesn't even believe evolution is an accurate model, I think it's helpful to be precise with the language to minimize misunderstanding which further inhibits the idea's acceptance.

the problem is, scientists find it very handy to talk in terms of directed evolution simply because it tightens up the language and everyone knows what it's shorthand for. (where by "everyone" i mean other scientists)

When I grip something with wrinkled fingers, I get the impression that the contact surface compresses and smooths out the wrinkles. So they don't act like treads. If I grip a glass with wrinkled fingers and pour water over my fingers, I would expect the water to go around the contact surface - not be channeled through the finger wrinkles.

For this hypothesis to make more sense, I would expect to see "micro-wrinkles" not the large compressible ones which I actually do.

You're misunderstanding how tread works. It is active only as it is pressed down; it allows water to rush away from under the contact patch more quickly and easily as pressure begins to be applied. It does not actually increase mechanical grip, and in fact serves no purpose once full contact is established. So, it does not matter if the treads get flattened by full pressure- it has already done its' job.

That's really interesting. Never thought about why my fingers do that!

I think this hypotheses is a little silly though.

You think the hypothesis is silly? Why?

I don't think it's silly, but I do think that it will be extremely difficult to test. People that have damaged nerve endings of the fingers probably are not good grippers, even if their fingers are wet. The control and the feedback is simply not there.

People that have damaged nerve endings of the fingers probably are not good grippers, even if their fingers are wet

I guess you actually meant "even if their fingers aren't wet"? But why do you think it would be extremely difficult to test this hypothesis? All that would be needed is to get a large enough group of people and test their gripping ability on the same surface when their fingers are wet and when they aren't.

Well not exactly. I'd imagine that the wet surface will always have lower gripping ability. You want to know if the wrinkled fingers have better grip on a wet surface than unwrinkled fingers. You could wet the fingers and immediately test the grip (before wrinkles form), and then soak them in water until they wrinkle and do it again.

Sorry about the word weirdness, waking up.

The article states: Scientists have known since the mid-1930s that water wrinkles do not form if the nerves in a finger are severed

Would such a person not have good wet-grip because:

A) They don't have the ability to form wrinkles

B) They can't grip well because they can't feel their fingers

That's the point I was getting at.

I did not know you could spell tyre with a 'y'. Is Nature a British publication?

Yup, that’s British spelling. That’s why the bicycle that got tired joke never works on Facebook if you’re from there.

I wonder when my revulsion at touching paper or other skin with wrinkled fingers came into play?

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