This morning a colleague sent to the department, with a snide comment, an article that just came out in Biology Letters, Wet-induced finger wrinkles improve handling of wet objects. At first glance it looked like a joke or a hoax, but I opened the text anyway. On the second glance, after reading the first paragraph of the paper, I learned a new fact. It turns out that wet wrinkles are not simply a mechanical result of skin staying wet for too long. It’s actually an active response of the body mediated by autonomic nervous system. In other words, it looks like an adaptation designed by evolution for a specific purpose.
On the third glance, I noted that the second author of this paper is actually someone I know, and a respected member of the scientific community. In fact, he is a colleague at the Evolution Institute, the parent organization for this Forum.
On the fourth glance I read the complete paper. Fortunately, it is short and quite well written (in scientific writing the two properties are closely related).
However, as I earn much of my salary teaching undergrads scientific writing, I cannot resist doling out some advice to the authors of the article. You guys have gone too far in trying to appear scientific and overloaded the paper with ‘scientese.’ The first author is probably a graduate student, as they are particularly prone to this error.
Just consider the first paragraph:
The glabrous skin on human fingers and toes forms wrinkles in response to immersion in water. This wrinkling was previously thought to be the result of osmotic swelling of the stratum corneum, the outermost layer of skin . However, more recent evidence has shown that it is in fact due to a reduction in the volume of the fingertip pulp, caused by vasoconstriction, which in turn is controlled by the autonomic nervous system [2–5]. In combination with the mechanical properties of the glabrous finger skin, this reduction in pulp volume results in the typical pattern of ridges and valleys on the tips of fingers and toes . The dependence of finger wrinkling on the autonomic nervous system has led to the use of finger wrinkles as a clinical indicator of autonomic function [2,6–9].
What the hell is ‘glabrous’? Are my fingers ‘glabrous’? It turns out they are, and it is not some kind of a horrible skin disease. It simply means (according to Merriam-Webster dictionary) “smooth; especially: having a surface without hairs or projections.” The origin is Latin glaber, meaning ‘bald.’
I won’t comment on the rest of it. However, it worked its magic on me (well, I have to read and grade much, much worse…). It got the main idea across, which is that it is likely to be an evolved adaptation, which suddenly makes it interesting. And I don’t want to beat too much on the authors, as the rest of the paper is better. And as I said, it is admirably short (just three pages).
Actually, on the fifth glance, it was the second sentence in the abstract that did the trick: “The formation of these wrinkles is known to be an active process, controlled by the autonomic nervous system” (sorry about spending so much on the writing aspect, I sometimes obsess on these issues).
Also, as I learned on subsequent research (but could have realized simply by thinking about it) only the skin of fingers and toes wrinkles up after prolonged immersion in water.
The authors did a very simple experiment: soaking their subject’s fingers in water to get them wrinkled up, or not. Then the experimental subjects had to grab a bunch of glass marbles with their right hand, pass it through a hole to the left hand and place them in a different container. It turns out that wrinkled fingers made the job of handling wet marbles much easier. So there you have it. It is indeed an adaptation.
So the first glance was wrong. In fact, I’d like to nominate this paper for the ‘Anti-Golden Fleece’ award.
For those of you too young (or too old…) to remember, the Golden Fleece award was handed out by the late (and unlamented) senator William Proxmire. He particularly liked to give it to various scientific projects funded by the NSF. As blogger Orac wrote,
What they [Golden Fleece Awards] did, more than anything else, was to take cheap shots at worthy scientific projects that could easily be made to sound ridiculous to the scientifically ignorant. In other words, when it came to science the Golden Fleece Awards were anti-intellectual and anti-science to the core, more akin to demagoguery than anything else.
So, in my opinion, the paper on wet wrinkles deserves the Anti-Golden Fleece award. First, it apparently was very cheap to produce. I would be surprised if the authors spent on this research much more than £200, if that. Second, and even more importantly, it has a number of very interesting implications.
One of these implications that immediately struck me was the connection to Omega-3 oils. The article doesn’t make this connection, but to me it was obvious, probably because the diet of early humans has been much on my mind lately (take a look here and here).
It has become increasingly clear that Omega-3 fatty acids are absolutely essential for normal development and functioning of Homo sapiens. Recent research in British juvenile prisons showed that giving inmates Omega-3 oil supplements dramatically reduced their anti-social behaviors. Evidence accumulates that getting enough Omega-3 oils is critical for normal development of children. My parents made me drink horribly tasting fish oil, which I hated them for, but now I am grateful.
What does this have to do with wet wrinkles? Well, it turns out that the main sources of Omega-3 oils are various kinds of seafood. But how did our species become dependent on this critical source of nutrients, which we cannot synthesize and must get from food? Various people proposed that early modern humans must have evolved in, or near an aquatic environment. One line of evidence for it is that we have naked skin. This is a typical evolutionary development in aquatic mammals (think dolphins or seals).
Another one is our heavy dependence on Omega-3 oils that are most easily obtained from aquatic food sources. This means that early humans must have spent a lot of time in water, and ability to handle wet, submerged objects dexterously was highly evolutionary advantageous. If this is right, wet wrinkles evolved as a way for us to efficiently harvest mussels and other shellfish, or perhaps grab those fish full of yummy and healthful Omega-3 acids!
Reminds me of the ridges on a gecko’s toes enabling it to climb walls.
Well, toes like fingers also wrinkle up, apparently to ease walking over wet stones.
It is unlikely that humans spent much time in water. And of course they did most of their early evolution on the savannas of Africa. That we lack hair is probably primarily do to it providing freedom from ticks and similar insects coupled to the ability to use animal skins and campfires to keep warm. Hair on our face is probably an adaptation to northern climates. Baldness is probably related to the necessity to synthesize vitamin D in winter time coupled with the ability to make hats for the real cold days. This begs the question as to why women have no hair on their face. I suspect this is related to the necessity to be sexually attractive, since a woman’s face is what is primarily attracted to.
That’s actually the big question – whether we evolved in the savannah or moved into them after evolving in a marine/lacustrine environment. Also, all other savannah mammals did not get rid of their fur to free themselves from ticks. But aquatic animals routinely lose hair.
‘since a woman’s face is what is primarily attracted to.’
I suspect that other parts of the body are more important than face…
many theories out there… for me the most reasonable is the out of Africa version, however, with some modifications: The replacement of body hair with sweat glands made it possible to go hunting at mid day. upright walk minimized the sun exposed area at midday. both adaptions went hand in hand. the niche “persistence hunting at mid day”, had the advantage that predators like lions were resting in the shadow of trees, while the prey collapsed after prolonged exposure to heat due to the lack of controlling body temperature by sweat glands. Evolutionary, sweat glands are modified hair cells. Many people argue that white skin is an adaption to produce Vitamine D in regions with limited access to sunlight. It could, however, be that pigment based protection from sunlight is not under selective pressure in these regions. Consistently, almost all vitamin D in humans is produced in retinal cells. I do not think it has to do with baldness is an adaption to vitamin D production but rather an adaption to persistance hunting at midday and handling very high temperatures.
While the omega 3 idea is good and valid, I would like to add one idea: surfaces after rain can also be quite slippery. Especially when under pressure (fleeing from predators by climbing wet trees or rocks) I see place for selective pressure..
I tend to agree with Paul Grok that most of his points have some affect. However, I do not agree with him that tree trunks are unreasonably slippery when wet or even most rocks. I have never chased antelopes or been chased by lions on a grass land anywhere. However I have harvested clams out on the Raritan Bay, Staten Island using a very heavy wooden pole clam rake, and do not recall any difficulty at all gripping it.
The point I wanted to make is that wrinkled skin is not necessarily an adaption to obtain aquatic food sources. imaginary scenario: If a group of small apes is fleeing from a predator (in this special case after rainfall). it is more likely that the one ape slipping will fall prey to the predator – thereby leaving the gene pool. I am not saying that wrinkled skin is not useful to obtain aquatic food. I am just saying that the selective pressure to get more grip on wet surfaces can also be found far away from open water. By the way – I haven’t read the original papers referred to in this study – how about other animals? do for example mice also get wrinkled skin on their hands and feet? and if so, would it be an homologous or analogous adaption? anyone?
Good points all. Certainly, wet trees are not slippery at all. On the infrequent occasions when it rains in the savannah, walking on wet grass does not present much of a problem. So it seems to me that living in a marine or lacustrine (lake) environment is where you get the most adaptive advantage for wrinkly fingers and toes.
It would be relatively easy to find out if the skin of other animals wrinkles up after immersion – if you work in the zoo, for example. That would be a very interesting test of this whole idea.
Upright walk has many other, more important advantages than reducing exposure to the sun. It freed hands for throwing stones and later spears. Bipedalism is also a surprisingly efficient method of locomotion, if you are interested in endurance, rather than sprinting speed.
Maybe humans didn’t, but who is to say whatever the specie lead to us did or did not, maybe back in the day when we shared a common ancestor with chimps half of them went into water to hunt the other half up on trees, leading to human ancestors being water dwellers for a significant enough period for us to loose our hair, and get erect spines.
Doesn’t it make sense that humans are erect walkers as opposed to chimps because human ancestors walked in water, meaning to keep your head up over water you would need to be as tall as possible if wading into deeper waters, while chimps climbed trees resulting in them having no need to walk erect.
Maybe during the ice age when Africa was much wetter there were many lakes and inland seas where we dwelled, but as they dried up human ancestors had no choice but to leave the water, resulting in the evolutionary changes being carried over onto land.
Why on earth would humans evolve to walk erect on the savannah when it is such a blatant disadvantage, and easier to climb trees if you already had that ability.
1. Being erect in a savana predictors can easily spot you.
2. Good luck seeing a camouflaged predator that is shorter than the savannah plants, crawling to creep up on you.
Just a thaught
I suspect that the chief advantage to walking erect is to free one’s hands for tools. However being able to see prey or predators at a great distance is a big advantage also. This last is probably the reason why many dinosaurs evolved walking erect. I realize that dinosaurs are portrayed often with their bodies held horizontally, but this is incorrect because their center of gravity is between the hips and head. If they failed to walk erect, they would have toppled forward and had to break the fall with their huge heads. Humans, of course, break their falls with their arms. Humans can walk with their bodies held horizontally, of course, by pushing their hips behind their feet, but I doubt if they ever did except for short periods of time.
As I wrote above, bipedalism is a surprisingly efficient form of locomotion. Ostriches are great runners, and they are bipedal (as a number of other running birds).
long distance running + projectile weapons + collective defense and offense = superpredator
I made an experiment today. Indeed wet wrinkles improved my grasping ability big time, especially with tiny objects.
It never occurred to me to ask why it is that we don’t have wet wrinkles all over the body after soaking in a hot tub. But that is how it is.
One theory is that humans went through a major bottleneck about 70000years ago. Humans survived in a small area in the southern cape of South Africa. The location of one of the longest inhabited caves is in a town called mossel bay…translated it means mussel bay and was known for its shellfish. even today you can see the placed where stone aged men set mini pools to capture fish left by the retreating tide. The people in mossel bay tell me that the are has the most equitable climate in the world with the least temperature overall variation.
I am not sure they are correct about the active mechanism. Fingertips continue to get more and more wrinkled after death; it is a common way of estimating how long a corpse has been in the water. I remember reading about this in a forensic pathology manual, and it had photos of progressive stages of wrinkling.
Interesting. Can you find the reference? Also, many autonomous nervous functions continue after death, so this is not a definitive proof it’s a passive mechanism.
It was a book I read in third grade, so it would be difficult to find. I don’t think nervous function can explain why fingertips continue to wrinkle for up to ten days after death, if I remember correctly. I’ll try to find a more recent reference.
Here’s something I found: http://www.dundee.ac.uk/forensicmedicine/notes/water.pdf
Interesting, although pretty grim. But if it is true that skin wrinkles in the water after death, it seems to overturn a major assumption of the study Biology Letters article. On the other hand, why is it that only finger and toe skin that wrinkles up?
Apparently, the underlying structure is unusual: there are “ridges” where the outer layer is firmly attached, but between those ridges there are areas with a kind of connective tissue that soaks up water easily.
It is true that other savanna animals did not lose hair. However humans were able to keep warm on cold nights by using animal skins and sometimes camp fires. And bare skin was an advantage under the noon day sun as well. As for other parts of the body than the face being sexually attractive, the only part I know of are the breasts and they can be noticeable even when wearing clothes. Indeed, it is almost impossible to tell for sure if someone is female if the face is not visible. Even breasts are a doubtful clue in tribes that use dairy cattle, where women’s breasts tend to be smaller.
Your: Omega-3 oils … the diet of early humans …
Oh dear — your picture actually listed “canola oil” as one of the sources of omega-3?!? (Canola is actually rape-seed oil — they changed the name to a version of ‘Canadian oil’ because who would buy “rape-seed oil”!) And can you imagine ANY early humans having the industrial-grade processors needed to make “oil” (or, as we in the primal/paleo world like to call it: industrial lubricant!) out of rape seeds?! Great article and all — but the devil is in the details and this is a really BAD detail!
I actually did not say it, the text came with the picture. Which is not great, I admit. I really wanted something to illustrate fish-derived oils, but couldn’t find a good image for that – do you have a suggestion?
Classical Chinese peasants cooked with rapeseed oil. Earlier humans might well have eaten the seeds. Even if it is slightly toxic, early man had to eat many slightly toxic things to get by. I wouldn’t rule out rapeseed or similar omega-3 rich oily seeds being eaten in prehistory.
There is a broad hypothesis around humans developing in aquatic environment
Thanks. Generally a good summary, but they don’t mention the hankering for Omega-3 oils. In any case, the extreme Aquatic Ape Hypothesis is not persuasive to me. Some sort of middle ground that points to a period of human evolution in the coastal areas of South Africa and utilization of marine resources, all supported by archaeology, is the one that seems to make the most sense.
Much early human migration (out of Africa) is thought to have used coastal routes. Is this too late for extra omega 3 to have become a factor then?
This is a good point. Modern humans spent hundreds of thousands of years in coastal environments of southern Africa, like the famed Blombos cave. If the ‘southern route’ hypothesis is correct, then they slowly (across tens of thousands of years) spread along the east coast of Africa to the Middle East. That’s a thousand of generations, a lot of time for evolution.