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    Another Just-So Story, This Time About Fists
    By T. Ryan Gregory | January 21st 2013 03:30 AM | 1 comment | Print | E-mail | Track Comments
    About T. Ryan

    I am an evolutionary biologist specializing in genome size evolution at the University of Guelph in Guelph, Ontario, Canada. Be sure to visit

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    “It is demonstrable,” said he, “that things cannot be otherwise than as they are; for as all things have been created for some end, they must necessarily be created for the best end. Observe, for instance, the nose is formed for spectacles, therefore we wear spectacles. The legs are visibly designed for stockings, accordingly we wear stockings.”

    - Candide by Voltaire

    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.

    HumanChimpBiomechanics

    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.

    My evidence for this claim is not just the observation that other apes don’t dance and don’t have a long Achilles tendon, but also some direct measurements that I did. Specifically, I got 10 trained ballet dancers to do some pirouettes, and I measured how good their dancing was. Then I had them wear ski boots so that they couldn’t stretch their legs (thus simulating the short tendons of other great apes). And guess what? They couldn’t dance anywhere near as well.

    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?

    No?

    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.

    Morgan, M.H. and Carrier, D.R. (2013). Protective buttressing of the human fist and the evolution of hominin hands. Journal of Experimental Biology 216: 236-244.

    SUMMARY
    The derived proportions of the human hand may provide supportive buttressing that protects the hand from injury when striking with a fist. Flexion of digits 2–5 results in buttressing of the pads of the distal phalanges against the central palm and the palmar pads of the proximal phalanges. Additionally, adduction of the thenar eminence to abut the dorsal surface of the distal phalanges of digits 2 and 3 locks these digits into a solid configuration that may allow a transfer of energy through the thenar eminence to the wrist. To test the hypothesis of a performance advantage, we measured: (1) the forces and rate of change of acceleration (jerk) from maximum effort strikes of subjects striking with a fist and an open hand; (2) the static stiffness of the second metacarpo-phalangeal (MCP) joint in buttressed and unbuttressed fist postures; and (3) static force transfer from digits 2 and 3 to digit 1 also in buttressed and unbuttressed fist postures. We found that peak forces, force impulses and peak jerk did not differ between the closed fist and open palm strikes. However, the structure of the human fist provides buttressing that increases the stiffness of the second MCP joint by fourfold and, as a result of force transfer through the thenar eminence, more than doubles the ability of the proximal phalanges to transmit ‘punching’ force. Thus, the proportions of the human hand provide a performance advantage when striking with a fist. We propose that the derived proportions of hominin hands reflect, in part, sexual selection to improve fighting performance.

    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)”.

    Fig1

    2) Briefly discussed two main hypotheses that have been proposed to explain these hand proportions:
    i) This is an adaptation for gripping. Specifically, this arrangement of digits allows two types of grip: one for precision and one for power. The long digits of other apes, by contrast, are adaptations for swinging and climbing (see, e.g., Tocheri et al. 2008; Almecija et al 2010).
    ii) The short proportions of the fingers in humans are a byproduct of adaptive evolution of the feet, in which shorter toes evolved to facilitate upright walking. Because feet and hand development is regulated in similar ways developmentally, changing one has a secondary effect on the other (see Rolian et al. 2010).

    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.

    Fig2

    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).

    Fig4

    5) Measured the stresses exerted on parts of the hand in a fist or other striking positions.

    Fig5

    6) Found that:
    i) The fist delivers no more force than an open hand strike, but it does produce more “peak stress” on the target.
    ii) A fist in which the fingers are curled in tight to the palm and the thumb is crossed over the fingers reduces the stresses on the hand and wrist relative to other hand positions.

    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”.

    That’s it.

    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.

    First appeared on EvolverZone December 21st, 2012

    Comments

    rholley
    This is, albeit indirectly, derived from the author of the original Just So Stories:
    Now I’m the king of the swingers, the jungle V. I. P.
    I’ve reached the top and had to stop, and that’s what’s botherin’ me.
    I want to be a man, man-cub, and stroll right into town
    and be just like the other men. I’m tired of monkeyin’ around.

    Ooh, ooh, ooh!  I want to be like you, ooh, ooh! 
    I wanna walk like you, talk like you, ooh ooh ooh!
    You see it’s true, ooh, ooh! An ape like me, ee, ee,
    Can learn to be hu-ooh-ooh-man too!



    Robert H. Olley / Quondam Physics Department / University of Reading / England