Saturday 8 September 2012

You Gotta Hand It to Them: From Handedness to Humanity in One. No, Two. No! Seven Inferential Leaps!

All in a row...Regourdou 1's lower front tooth crowns viewed labially.
It has to be said: no outrageous leap of reason is beneath the editors of PLOSONE, who, in their zeal to hasten publication of research to meet the instantaneous demands of the information age, choose to publish papers that could readily have waited months or years to reach their audience, and, no doubt because of the haste with which they are published, reviewers of which tend toward an uncritical, and unprofessional-seeming, credulity.
     In this case, it's the breath-taking intellectual journey of Volpato et al. from scratches on the anterior mandibular teeth of the Regourdou 1 Neanderthal to language ability comparable to that of modern humans. Thanks to Bob Muir for his good natured challenged to me to say something about the latest plonk from PLOSONE. So, I read it. And here is my response.
     O. M. G.! Sorry. Disdain attack. I'll be fine in a minute.

Boiled down, the authors are ostensibly presenting novel evidence for handedness among the Neanderthals, in the form of scratches on dental enamel. That's it! That's all there is. Possibly useful information. But even their foundational inference is open to question, because it's based on an unwarranted assumption. As a result, the follow-on inferences lose cogency, one misstep at a time. A good number of inferential leaps occur in this paper. I'll enumerate them here.
     Leap number one. Volpato et al. examined the labial surfaces of the six lower anterior teeth of an adult Neanderthal. They found scratches that they attribute to accidental contact with a sharp stone [presumed to be an artifact]. But only those scratches on the incisors count as artifact-induced damage--those occurring on the canines must, according to the authors, be environmental, rather than of cultural or behavioral origin, since they are shorter and less frequent. [I would guess that this could easily be due to the canines' location deeper in the mouth and the possibility that whatever is scratching the teeth during life was positioned more toward the front of the mouth.]
     Leap number two. The preponderance of the surface scratches, when viewed from the front, are oriented with left end lower than the right. With no explanation, the authors [again] presume that the course of the sharp object must have been from left to right from the Neanderthal's point of view--in other words, from upper left to lower right--rather than the other way around.
     Leap number three. Based on the assumption that the scratches were made by a stone tool or tools, and the further assumption that they show a preferred oritentation, and that therefore this stands as evidence of the Neanderthal's preference for using the right hand to wield the sharp object, the authors conclude that the scratches must have occurred while the Neanderthal was in the act of cutting off a hunk of [presumably] meat after stuffing a morsel in the oral cavity. For this, they simply cite C.L. Brace's characterization of a documented modern human practice as the 'stuff and cut' technique, and paste it onto their narrative about a Middle Pleistocene hominid. [Why the Neanderthal didn't just bite off a chunk, as you or I would, the authors don't explain. I guess they were trying to be genteel in a ravenous, trogolodytic sort of way.]
     Yet, even if the genteel Neanderthal was doing so, I have to wonder where the authors got the idea that a sharp edge used to assist cutting would ever have come in contact with the teeth [except in the case of a thoroughly uncoordinated individual]. As it's easy to see from the photo of an inhabitant of northern North America, the cutting is most often achieved outside of the mouth. So, ethnographically, I think the authors have this idea very wrong. Of course, if a rude, or uncoordinated, or malicious neighbour were to have bumped the Neanderthal's right hand in the act of cutting, the stone tool might have found its way past the meat and the individual's lips and come in contact with the dental enamel. I really can't imagine that happening all that often. Can you?
Human brain function expressed in
terms of the brain real-estate
associated with
different body parts. Our brains are
primarily there to operate our
hands, followed by our mouth
and tongue, eyes, limbs, and so on.
Language is in there somewhere,
but I believe that it's the result of
homology and thus not directly
tied to any of the structures and functions
for which the brain evolved prior to
its advent.  
     Leap number four. By analogy to modern humans, the authors aver that the [now] right-handed Neanderthal must have had a genetically based neurological asymmetry such as that observed in the here and now. [Frankly, I've never heard a good explanation as to why, in view of the lateral brain asymmetry documented in our species, right-handedness is seen to be governed by the left-hemisphere, with no good evidence that a mirror image of that symmetry is responsible for left-handedness. To my mind this weakens the authors' argument that handedness in a Neanderthal is evidence of the same degree of lateralization as that occurring in modern humans.]
     Leap number five. Again by analogy to modern humans, the brain lateralization inferred by the authors is tied to the notion that brain lateralization in us is predominantly due to the brains preference to house the language capacity in the opposite side of the brain from the right hand. [Again, I'm afraid this raises the question 'What about left-handed people?']
Kebara 2 hyoid
     Leap number six. The Kebara Neanderthal hyoid bone is evidence of linguistic ability [and not just a quasi-modern morphology that may have nothing at all to do with language].
     Leap number seven. Taken together, all six of the above leaps converge on the conclusion that Neanderthals had linguistic capabilities. Quite an intellectual journey! No?
Homology demonstrated. These
structures all have the same
embryological origin--they are the
same skeletal elements in each
kind of animal. They've all been
re-shaped and re-purposed for
different functions. All locomotory,
it's true, but all very different.
     No. And I say no if only because, as any evolution scientist will tell you, no genetically determined physical or neurological trait in an extant organism can be seen as anything other than the result of selection acting on those same structures in the species' evolutionary history. Our lungs, for example, did not arise in evolutionary history so that we could breath air. In fact, they arose for completely different reasons, but were gradually transformed to perform the function that they do in mammals. I'm talking about the concept of homology, and it is a pillar of evolutionary theory. It's simply wrong to view any structure that's visible in the fossil record and formally similar to that occurring in modern humans as evidence that similar somatic or neurological functions or behaviours were present in the fossil form. This goes for brain morphology as much as it does for biomechanics or any other facet of past life that can be observed in the skeleton. It's why I cannot view the emergence of the modern skeleton at almost 200 kya as evidence that modern humanity arose at the same time.
     In the illustration of homology up above, it's plain to see that the front limbs of vertebrates have evolved for very different purposes, even though the skeletal elements themselves are the same--i.e. they have the same embryological origin, and thus they all express the same skeletal elements. Even within these groups there is considerable inter-specific variation in these structures. This is why I'm very wary when similar structures in the hominid past are unequivocally treated as having the same function as they do in modern humans.
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  1. I agree this is one of the more absurd articles I've read recently. It is difficult to imagine what sort of artifact would make tiny scratches like that. Obsidian tooth picks?

    Seems far more likely they are due to diet, but what? (eating rodents 'alive and kicking' perhaps), or post depositional processes (?).

  2. Thanks, KB! An obsidian toothpick would do the trick. So would grabbing onto a deer's antlers with your teeth while trying to slit its throat. I wonder how long a list we could come up with...

  3. No deer antlers wouldn't do that, unless they had obsidian tooth picks stuck in the end of them. Personally, I think the authors have really missed an opportunity to argue for Neanderthal dental hygienists.

  4. KB, one of the authors, David Frayer, did once suggest that Neanderthals practiced self-dentistry. Classic.

  5. Many of those scratches appear to be at or even below the gum line. May be that they had inflamed gums and were picking at them to relieve the pain/itch. But it would have to be a pretty small and sharp 'tool' to make those scratches.

  6. Ed!
    I agree about the gum line. 'Course, if they had periodontitis, perhaps their gums had receded so far that the lower portion of the crown was exposed. It still doesn't take into account the potential soft-tissue damage that would have been the case as the ?stone tool? made the pass along the tooth crown. Roll that around in your head for a minute. Oucchhhh.

  7. I have to disagree with you here on a couple of salient points. The evidence for dental wear and handedness goes back to experimental work by Bermudez de Castro et al (1988) and has been improved upon since then. Lozano et al.(2009)have shown the differences between taphoomic and general dietary microwear. These inferences have also been supported by ethnographic comparisons, where the contact with the teeth during cutting is accidental, despite the external action. Furthermore, the statistical frequency of these scratches on the the labial surface match those that we would expect for species level handedness in Neanderthals (~90%) with well over 50 individuals to date. The importance of the paper you are arguing against is that this is the first time it has been joined with right dominance in bone remodelling, which has been shown in vivo by Shaw et al (2011) to be extremely robust. As for your criticism of the role in language, that is totally valid. You do have to take a leap to conclude that Neanderthals had our language capability. Although the 'why' of this configuration is questionable, what you do not have to question is that language and motor cortex dominance results in cerebral asymmetry that is apparent in humans by 29-31 weeks of gestation (Zadi, 2011). Being that similar functional and physical asymmetries are documented in non-human primates (Cantalupo et al., 2008, Meguerditchian et al 2012, Sherwood et al., 2007), it is probable that these are also active in our nearest extinct relative.

    BERMÚDEZ DE CASTRO, J., BROMAGE, T. G. & JALVO, Y. F. 1988. Buccal striations on fossil human anterior teeth: evidence of handedness in the middle and early Upper Pleistocene. Journal of Human Evolution, 17, ϰϬϯ-412.

    CANTALUPO, C., RODES W., FREEMAN, H.& HOPKINS, W. 2008. Handedness for Tool Use Correlates With Cerebellar Asymmetries in Chimpanzees (Pan troglodytes). Behav. Neurosci. Behavioral Neuroscience, 122, 191-198.

    LOZANO, M., MOSQUERA, M., DE CASTRO, J. M. B., ARSUAGA, J. L. & CARBONELL, E. 2009. Right handedness of Homo heidelbergensis from Sima de los Huesos (Atapuerca,
    ^ƉĂŝŶͿ ϱϬϬ͕ϬϬϬ LJĞĂƌƐ ĂŐŽ͘ Evolution and Human Behavior, 30, 369-376.

    SHERWOOD, C.C., WAHL, E., ERWN, J.M., HOF, P.R. & HOPKINS, W.D. 2007. Histological asymmetries of primary motor cortex predict handedness in chimpanzees (Pan
    troglodytes). The Journal of Comparative Neurology, 525-537.

    ZAIDI, Z.F. Body asymmetries: Incidence, etiology and clincal implications Aust. J. Basic Appl. Sci. Australian Journal of Basic and Applied Sciences, 5, 2157-2191.

  8. @ N. B. S.
    Thanks for jumping in. The readers will appreciate learning that handedness is a trait shared with others of the great apes. I'm glad, too, that we can agree on what for me is the fundamental point--that the author's make an ill-considered leap in their paper. As for being the first to pair the microwear study with skeletal remodeling [in Neanderthals I presume], I wonder if the significance of that contribution isn't a bit overblown, given that there's nothing new about handedness in humans and great apes. I s'pose I could be missing a subtle point of yours. If so, forgive me, and consider the source! Again, thanks for taking the time to shine some light on the scope of these matters.


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