Head on over to Lab Rat and check out the latest instalment of Carnival of Evolution!
This past May, I was fortunate enough to take part in a conference in Venice, Italy, which was a retrospective on the legacy of famous paleontologist and author Stephen Jay Gould 10 years after his death. The choice of Venice as the conference venue was a nod to one of Gould’s most famous and influential papers, “The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme“, which he co-authored with Richard Lewontin in 1979.
In the paper, Gould and Lewontin (1979) drew an analogy between the “spandrels” (or, if you’re overly pedantic about architectural terminology, “pendentives“) in Saint Mark’s Basilica in Venice and features of organisms that are often assumed to represent adaptations whose current functions reflect the reason for their origin. The spandrels in St. Mark’s Basilica house elaborate mosaics depicting important Christian iconography, and represent one of the most stunning aspects of the cathedral. However, housing religious imagery is not why the spandrels were created in the first place. As Gould and Lewontin (1979) point out, spandrels are the inevitable result of resting a domed ceiling atop walls with arched doorways. Once they exist, spandrels can be co-opted for a function such as artistic decoration — but whatever this current function may be, it does not, by itself, explain the origin of the structures. Many traits of organisms, rather than being the result of adaptive evolution under selection for their current function (if any) are, in fact, biological “spandrels”.
The tendency to conflate current function with historical origin is one of the things that Gould and Lewontin saw as a pervasive problem in the evolutionary biology of the 1970s. They also considered the general tendency to focus almost exclusively on adaptationist explanations for individual traits to be misguided. Not only did authors not properly consider alternative, non-adaptive explanations, they presented only weak evidence in support of adaptive “just-so stories” or simply moved on to a new adaptive tale if evidence was found lacking for a particular explanation.
Some subsequent authors have charged that Gould and Lewontin (1979) presented a straw man, and did not provide a fair assessment of what members of the so-called adaptationist camp actually did or said. Nevertheless, I think many biologists feel that theirs was a very important contribution, at least as a warning for how the study of trait evolution could go awry. I would take a stronger position, and argue that not only was the Gould and Lewontin (1979) paper needed at the time, but that it is still needed today. And nowhere is this more true than in studies of human behaviour and anatomy, particularly because these tend to be widely covered in the science media. Evolutionary psychology takes a large amount of flak from evolutionary biologists and others for its perceived tendency to present “just-so stories” or to make extraordinary assumptions about the social and physical habitats of hominin ancestors. But this is not limited to studies of human minds by any means.
I recently discussed one example, the widely-reported claim that the dimensions of human hands evolved in large part to allow the formation of fists. In that post, I outlined several major problems with the underlying assumptions and the (very weak) data that were presented as supporting evidence. I won’t re-hash these points here, but many of them apply equally to the even more recent claim that a human trait is the product of adaptive evolution: namely that the wrinkling of our fingers and toes when submerged in water is an adaptation for gripping in wet conditions.
The idea was first put forward by Mark Changizi and colleagues of 2AI Labs in Boise, Idaho. Changizi has a PhD in applied math and primarily studies neurobiology, but in this case his focus was on why fingers wrinkle when they’re soaked. Specifically, Changizi et al. (2011) wondered whether the wrinkled patterns that form on the fingers after prolonged soaking have a function similar to the treads on tires, namely for channelling water and improving grip in wet conditions. Based on the idea that wrinkled digits might actually represent water channels, they looked at photos of 28 wrinkly fingers from 13 hands that they found online. They then compared these wrinkle patterns to the channels of rivers in a mountainous drainage system and to the treads on car tires and found that they are similar in that they branch and diverge from the middle of the finger.
In addition, the authors noted that the wrinkling of fingers is not simply a result of osmotic changes when hands are soaked, and that there is evidence that the process is linked to the action of the autonomic nervous system. Or, at least, wrinkling does not occur if there is nerve damage in the hand. In fact, physicians use wrinkling in water as a test of neural function in the limbs (Wilder-Smith 2004; Barneveld et al. 2010). So, the evidence to this point was a) wrinkling seems to be under nervous control, and b) the wrinkles look like drainage systems or tire treads.
This hypothesis has gained more attention recently with the publication of a follow up study by Kareklas et al. (2013) in the lab of Tom Smulders in the Institute of Neuroscience at Newcastle University. Kareklas et al. (2013) wanted to test the idea that wrinkling improves grip in wet conditions. To do so, they had 20 volunteers soak their hands in 40°C water for 30 minutes and then had them move marbles of different sizes from one container to another, with the source container either filled with water or dry. They compared this to the performance of the volunteers in the same task when their hands had not been soaked and therefore their fingers were not wrinkled.
Here is the entire Results section of the paper:
In short, soaking your hands in 40°C water for 30 minutes will give you a 12% advantage in picking up round, smooth objects if they are underwater.
Not surprisingly, given that it is about a familiar yet curious human trait, the science media have reported this study widely.
Now, as you may have guessed, I don’t find this hypothesis compelling at all, for many reasons. The first is that it is based on the thinnest of datasets. Photos of 13 hands with finger wrinkles that resemble drainage channels. And a small advantage in manipulating wet marbles. That’s it for the data.
I also don’t buy the conceptual arguments, for example Changizi’s claim that there are infinitely many possible patterns for wrinkles to form, and yet we see the one that is consistent with rain treads. I strongly doubt that there are more than a few possible arrangements for wrinkles, given the properties of human skin and the shape of human digits. Moreover, it looks to me like there is substantial variability in wrinkle patterns even within the tiny sample of their study. Would all of these channel water effectively? We don’t know, because no effort was taken to demonstrate that wrinkles actually displace water. (I have my doubts about this. Treads are hard, fingers are soft. Tires rotate, fingers don’t).
Finally, I am not convinced that the link to the central nervous system implies that this trait is functional. Vasoconstriction is involved (Wilder-Smith and Chow 2003a,b), and this can be controlled by the nervous system, but that alone does not mean that the trait evolved as an adaptation rather than as a spandrel. Perhaps the effect is simply a byproduct of other nerve signal changes that are functional in response to hot water immersion. I don’t know, but neither does anyone else because such non-adaptive hypotheses have not been considered. There certainly seems to be some cross-wiring going on, since reduced wrinkling ability is correlated with congestive heart failure (Kamran et al. 2011) and heart rate variability (Win et al. 2010).
There is also this:
The photo above shows a pretty typical wrinkle pattern for a hand that has been submerged in water (in this case, for 120 minutes in 13°C water). What sets this hand apart from the ones examined by Changizi et al. (2011) is that it belongs to a dead person. That is to say, wrinkling occurs, with the same pattern and at a similar rate, even when the subject is deceased (Weber and Laufkötter 1984; Reh 1984).
From Reh (1984):
If finger and toe wrinkling is an adaptation for gripping in the wet as claimed, we might also ask:
I don’t think these observations provide support for the idea that finger wrinkling is an adaptive response to being wet, or at least they are not what I would expect to be the case if this were such an adaptation.
In any case, the bigger problem with the wrinkled gripper ape hypothesis, the punching ape hypothesis, and other such hypotheses is that they are not based on solid evolutionary thinking. More specifically, the authors of these hypotheses do not seem to work through the assumptions that would have to be true for these traits to be adaptive in the way that they propose. Did an ancestral population actually have variation in this trait, and is it heritable? Did these ancestors really submerge their hands and feet regularly? Did individuals with more wrinkles actually have more children than individuals with less tread-like wrinkles?
Many biologists favour the adaptationist approach to studying trait evolution. That is, they begin with ideas about why a certain trait might be adaptive and then proceed from there. Others (increasingly, myself included) may argue that there must be evidence that the trait is adaptive in the first place before such hypotheses are pursued. This may be a matter of preference or training or philosophical bent more than anything. However, it should be clear either way that there are good and bad ways to test adaptationist hypotheses.
Here’s the bad way:
Here’s a better way:
Getting the data for this second list is much more difficult than for the first, but without them any conclusions about adaptive evolutionary history remain subject to the criticisms laid out several decades ago by Gould and Lewontin (1979).
Cales, L., R.A. Weber, and T.X. Temple (1997). Effects of water temperature on skin wrinkling. J. Hand Surg. 22A: 747-749.
Changizi, M., R. Weber, R. Kotecha, and J. Palazzo (2011). Are wet-induced wrinkled fingers primate rain treads? Brain Behav. Evol. 77: 286-290.
Gould, S.J. and R.C. Lewontin (1979). The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme. Proc. R. Soc. Lond. B 205: 581-598.
Kamran, H., L. Salciccioli, and J.M. Lazar (2011). Reduced water induced skin wrinkling in congestive heart failure. Clin. Auton. Res. 21: 361-362.
Kareklas, K., D. Nettle, and T.V. Smulders (2013). Water-induced finger wrinkles improve handling of wet objects. Biol. Lett., in press.
Reh, H. (1984). On the early postmortal course of “washerwoman’s skin” at the fingertips [in German]. Z. Rechtsmed. 92: 183-188.
Tsai, N. and S. Kirkham (2005). Fingertip skin wrinkling — the effect of varying tonicity. J. Hand Surg. 30B: 273-275.
Van Barneveld, S., J. van der Palen, and M.J.A.M. van Putten (2010). Evaluation of the finger wrinkling test: a pilot study. Clin. Auton. Res. 20: 249-253.
Weber, W. and R. Laufkötter (1984). Stage classification of washerwoman’s hands at experimental time intervals [in German]. Z. Rechtsmed. 92: 277-290.
Wilder-Smith, E.P.V. and A. Chow (2003a). Water-immersion wrinkling is due to vasoconstriction. Muscle Nerve 27: 307-311.
Wilder-Smith, E. and A. Chow (2003b). Water immersion and EMLA cause similar digit skin wrinkling and vasoconstriction. Microvascular Res. 66: 68-72.
Wilder-Smith, E.P.V. (2004). Water immersion wrinkling: Physiology and use as an indicator of sympathetic function. Clin. Auton. Res. 14: 125-131.
Win, S., L. Salciccioli, H. Kamran, P. Baweja, M. Stewart, and J.M. Lazar (2010). Water immersion-induced skin wrinkling is related to heart rate variability. Cardiology 116: 247-250.
Humans are unique among the great apes (of which we are one) in various ways. One of them is our possession of a very long Achilles tendon, which is quite short in chimpanzees, gorillas, and orangutans.
Most anthropologists will tell you that this likely evolved as an adaptation for upright running, since it provides a great deal of spring in the stride. Or perhaps it’s just an inevitable byproduct of lengthening the lower leg. Yeah, sure, there’s evidence to support these hypotheses and everything. But I have another idea: I think the Achilles tendon evolved for dancing.
Think about it. Every human culture has some form of complex dance, but other apes don’t. And humans use dance as a way of attracting mates, displaying physical prowess, strengthening group bonds, and so on. So, we can imagine an advantage among our ancestors in being able to dance.
Therefore, I conclude that dancing was a significant factor in the evolution of human leg anatomy. Sure, running might also be important, but dancing is a big part of the picture. Pretty compelling, eh?
Well, what if I did pretty much the same thing in order to claim that human hands evolving for making fists?
Why, then this would happen!
This is a sample of the attention that has been gained by a recent paper on the subject.
Put more simply, the authors did the following:
1) Observed that the proportions of the digits are different in human hands as compared to those of other apes. Specifically, “In comparison to other apes, humans have short palms and fingers (i.e. digits 2–5), but long, strong and mobile thumbs (i.e. digit 1)”.
2) Briefly discussed two main hypotheses that have been proposed to explain these hand proportions:
3) Suggested a third hypothesis, namely that the proportions of our fingers evolved as an adaptation to making a well-supported fist, which allowed us (males, anyway) to punch each other with greater force and lower risk of injury.
4) Got 10 trained martial artists to hit a heavy bag equipped with accelerometers in order to measure the force delivered by forward, overhead, and sideways strikes using a fist versus an open palm. (For you karateka out there, this is a punch/tsuki, palm strike/teisho, and hammer fist/tetsui).
5) Measured the stresses exerted on parts of the hand in a fist or other striking positions.
6) Found that:
7) Argued that a gripping hand could have evolved in several ways that would not have been compatible with a buttressed fist, and yet this is the hand shape that evolved.
8) Concluded that “the geometry of a fully buttressed fist provides a clear explanation for the specific skeletal proportions of the human hand”.
This is no different from the imaginary scenario about dancing and the Achilles tendon that I presented at the beginning. Both that imaginary study and this all-too-real one suffer from a number of serious flaws in both methods and logic. Let me outline just a few.
1) Assumptions about ancestral populations. If you want to claim that our hand proportions evolved as an adaptation (i.e., by natural selection) for pugilistic functions, then you need to postulate several things. One, that there was heritable variation in this trait among individuals in the ancestral population. Two, that individuals with slightly shorter fingers than those with slightly longer fingers made better fists OR that the change in finger length happened all at once in some mutant individuals. Three, that this ability to make better fists was actually important in affecting reproductive success. And finally, that this selective pressure was strong enough and sustained enough to result in the evolution of specific hand proportions over many generations. Obviously, the authors present absolutely no evidence to address any of these major assumptions. (At the very least, they could have quantified the extent of variation in human finger lengths among living individuals and correlated this with fist-making capability).
2) Unrepresentative sampling. Accepting for the sake of argument that 10 individuals is a sufficient sample size for a biomechanics study, there remains a major issue: namely, that most people don’t naturally punch with a properly buttressed fist. In fact, one of the first things that martial arts students need to be taught is how to make a fist. This study used trained martial artists, not people who punch using instinctual or intuitive hand positions. Indeed, the very fact that people need to be taught how to make a fist speaks against fist-making being a driving force in human hand evolution. If it were, there would also have to be a corresponding behavioural adaptation to actually make such a fist when hitting opponents.
3) Risk of injury remains high even with a buttressed fist. One of the reasons that martial artists practice open-handed strikes is that they are less likely to injure themselves when connecting with a solid target. Indeed, there are some punching-related injuries that are so common that they are called “boxer’s fracture” (fracture at the neck of the 4th or 5th metacarpal) and “boxer’s knuckle” (sagittal band tear). (I have suffered both, and they suck). To my knowledge, there is no “slapper’s fracture”.
4) Confusion of function and effect. Given its proportions, the human hand is capable of forming into a strong fist. It can also be used to play piano or type on a keyboard. We can also use it to shake hands or give each other the finger. It can throw and catch a ball. It can even be used to accentuate or replace spoken language. None of these is likely to have been a significant factor in shaping the evolution of the hand. Rather, these are functions that have arisen after the evolution of the hand’s current anatomy. This is a common theme in evolution. Structures evolve for one reason (adaptive or not), and then are co-opted into new functions. The results of this study are equally compatible with the notion that the human hand evolved its current proportions for some other reason, and then people figured out the best way to hit things with it. In fact, this strikes me as a much more plausible interpretation of the observations.
5) What about women? The hypothesis put forward in this paper is that sexual selection in the form of male-male combat contributed to the evolution of the human hand’s unique properties. Why, then, do women also have very similar finger proportions? One possible explanation is that hand development is rather constrained, so that adaptive changes in the hands of males carried over to changes in female hands as well. This is not unlike the best current explanation for why men have nipples: men have nipples because women need them, and they arise very early in development before sex-specific differences appear. But highly constrained hand development is a problem for the notion that hands evolved gradually for fighting.
6) Insufficient consideration of alternative (and much more plausible) hypotheses. As I noted previously, one of the major hypotheses for the evolution of human hand proportions is that it is a byproduct of the evolution of foot morphology for bipedal locomotion. Or the human hand could have been shaped directly by natural selection for gripping and tool use. Or some combination of these — perhaps developmental correlations with foot evolution gave the hand its general proportions, which were then refined by selection for specific functions. It seems obvious that tool use played an important role and that this would have exerted significant selective pressures in ancestral hominins. It would also be fully compatible with the fact that both males and females have the shortened hand proportions that we observe. Although the authors of the paper do mention these hypotheses, they do little more than pay them lip service. However, the fighting fist hypothesis adds no additional explanatory power.
To sum up, this is a paper that presents a small dataset of biomechanical analyses. It used an inappropriate sampling of subjects, and the only conclusions that can be drawn from the data are that the fists of trained martial artists are buttressed better than other arrangements of the hand. There is absolutely no information that is relevant to the question of why the human hand evolved as it did. (Note that this was not published in an anthropology or evolutionary biology journal). Moreover, to connect these observations with the evolutionary origin of human hand morphology requires some very unrealistic assumptions and a rather poor grasp of how one actually studies trait evolution.
The most impressive thing about this study is that it managed to gain so much attention with so little substance.
You may recall that I was an Associate Editor of the journal Evolution: Education and Outreach from 2007-2009. I also edited the first “special issue” of the journal, on the subject of eye evolution, and I wrote a number of papers for early issues of the journal.
You may also remember that I resigned from the editorial board of the journal when the publisher, Springer, stopped making the journal available free online. I felt that this went against the intent of the journal, which from the outset was to make high-quality but accessible articles available to scientists, educators, and interested members of the public.
Well, I am very pleased to announce that Springer is planning to return to an open-access model for the journal in January 2013!
For me, this also means that I will begin writing articles for the journal again in 2013.
I have also agreed to re-join the editorial board, this time as Senior Handling Editor. So, friends and colleagues, you can expect me to begin soliciting papers from you in the new year.
All in all, this is great news for evolution educators.
The complete series of talks is now posted from the conference on Stephen J. Gould’s Legacy: Nature, History, Society, May 10-12, 2012 at the Istituto Veneto di Scienze, Lettere ed Arti in Venice, Italy.
Telmo Pievani – Ten years without Stephen J. Gould: the scientific heritage
Alessandro Minelli – Individuals, hierarchies, and the levels of selection
Elisabeth Lloyd – Gould and adaptation: San Marco 33 years later
T. Ryan Gregory – A Gouldian view of the genome
Giuseppe Longo – Randomness increases biological organization
Marcello Buiatti – Biological complexity and punctuated equilibria
Ian Tattersall – Steve Gould’s intellectual legacy to anthropology
Guido Barbujani – Mismeasuring man thirty years later
Are you an excellent Canadian student who is interested in completing a PhD? Know someone who fits that description? If so, please note (and/or pass it on). I will be happy to discuss possible projects with qualified candidates to work in my lab.
Again, this is for Canadian students only.