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Wednesday, August 17, 2005

The Left Hand of the Dark Continent: Chimpanzees, Handedness, Brain Asymmetry, and Language

Note: I originally wrote this for, which is currently in flux. I'm archiving it here at telecommuniculturey, because there's nothing more cultural than language and because I found myself wanting to refer to it.

In their usual Today Is Opposite Day approach to clarity, the AP (and CNN, presumably by way of the AP) are reporting that a study published in this week’s PNAS that shows that most chimpanzees are left-handed (southpaws, ’cause it’s catchier [pitchier, I suppose—southpaws catch with their right hands {myself, I would have gone with “sinister,” because it’s just much cooler}]). For an AP science article, I suppose the errors are not so serious: The study is not in this week’s PNAS, but in this week’s early edition of the journal; one also needs to pad the sentence “Most chimps are southpaws” quite liberally with adjectival phrases, such as “in a subsample of the larger of the two known communities at Gombe” and “when termite fishing, but don’t ask about nutcracking.”

The leftyness, though gratifying to your wooly-headed, liberal correspondent, isn’t particularly relevant. I mean, it’s neat in an “ain’t that a hell of a thing?” kind of way, given that we estimate that 90% of individuals in (putatively) literate human populations are right-handed. But the key point is that they exhibit statistically signfiicant population-level handedness at all.

Previous studies (which are casually folded into this paper as well) have lacked the sample sizes and statistical power to demonstrate true handedness. Although this is still a wee sample (N = 17), it has the numbers behind it. By combining the samples from these studies, our intrepid authors were able to determine strong right-handed preference for nutcracking and weaker right-handed preference for “wadge-dipping” (I suppose in a world ravaged by Jerry Seinfeld, that sounds better than “sponging,” but not much). I anticipate many taking these supposed inconsistencies and saying “Ah ha! But that shows chimps aren’t handed at all! They use different hands for different things!” with a sigh of ill-founded relief.

“Why ill-founded?”, you are probably not asking yourself, because it is unlikely in the extreme that you are still reading. Well, it’s manifold ill-founded relief. The first problem with it is the failure to recognize that these are not the same chimps using different hands for different tasks. We don’t know what hand the lefty, termite-fishing Gombe chimps would use to crack nuts, because they don’t crack nuts. Not for lack of opportunity, mind you, there are plenty of nuts and plenty of appropriate hammerstones available in their environment. They just don’t. In fact, none of the wild chimp populations that have been studied engages in all three of these tool-using behaviors. Those of us who are not bending over backwards to ixnay on the ulturecay in nonhuman primates shrug and attribute the differences to a cultural preference. Those whose I M SPESHIAL t-shirts burst into indignant flames when the words “culture” and “chimpanzee” are uttered in the same sentence are still scouring the underbrush for an ecological explanation. But I digress.

The second problem is related, but more intertwined with the whole handedness issue. To be handed is to be cerebrally asymmetrical (see a recent Pharyngula entry for a cool piece with cooler pictures on how asymmetry, in general, is accomplished in superficialy symmetrical beasties like us). To be cerebrally asymmetrical, so many theories go, is to have all kinds of M@D 5K1LLZ. Researchers on schizophrenia have danced around implicating a lack of sufficient asymmtery in the disease, for example. Others have suggested that men and women be different, because lateralization is complex and affected by gonadal hormones, so it functions differently in women by virtue of their menstrual cycle. But the most persistent, most contentious shiny thing attached to asymmetry (and, by extension, to handedness) is the L word.

It is certainly true that language ability tends to hunker down in one side of the brain. For most people, whether they are sinister or dextrous, that’s the left side. Because language appears to be dependent on lateralization and handedness is dependent on lateralization, it was decided some time ago that they’re two great tastes that taste great together. As such, handedness became both a proxy for asymmetry (with a dash of linguistic ability strongly implied) and a necessary correlate of language (i.e., if handedness was not evident, language could not be present). It’s not, shall we say, the most robust relationship upon which to insist as proof, but it has certainly stuck.

This has important and direct implications for primatology and paleoanthropology/archaeology and no less important, but fuzzier implications for evolutionary biology in general. Language is a thorn in the side of all three fields. For evolution, it’s been declared “a disaster” by Chomsky, because there are supposedly no primate precursors evident.

For paleoanthropology and archaeology, the pressure is on to come up with a time, place, and species for the evolution of language. Part of the problem in reaching that goal is the criteria that have been used. To have language as it is used by modern humans, a species should have the anatomical stuff (fine control of breathing, fine control of muscles and structures used in phonation, and a low-positioned larynx [this is, of course, leaving aside the hairy question of gestural language produced manually and facially and processed visually, which is very likely to have preceeded a language produced vocally and processed aurally]), the neuroanatomical stuff (Broca’s area, Wernicke’s area, lateralization, and Ba'al knows what else that might be implicated in language), and a social environment conducive to language (high investment in child care, probably by multiple adults).

But we can’t agree on what markers would reveal whether or not these prerequisites are present. Basicranial flexure, which is argued to reveal the position of the larynx, has devolved into an “is not, is too” exercise. Homo erectus who has promisingly sophisticated behaviors, seems to have quite a small neural canal, indicating that his breath control was not all that. The same endocasts are used to argue for and against enlargement in Broca’s and Wernicke’s areas. Once a material culture record is in play (about 2.5 mya), so is handedness. We peer at tool industry after tool industry, we flint knap until our lily-white academic hands are raw and bleeding, and try to determine whether the magical right-hand bias is present, which would imply lateralization, which would imply the capacity for language, right? RIGHT?

Even if we could get two anatomists and/or two experimental archaeologists to interpret the same data in the same way, we are now told that the mere anatomical and neuroanatomical capacity for language won’t satisfy. There has to be evidence for symbolic behavior to show that a given species is making good use of the lop-sided brain and the Adam’s apple. This may prove to be the fatal mistake of the non-physical anthro types, as they’ve generated a rallying point that may draw primatologists and paleoanthropologists together.

A lot of the symbol debate centers, unsurprisingly, around Neanderthals. People get downright snippy about them. “Where’s your Cave Bear Cult now, Milford?” they say and “Ritual burial? Ha! The flowers didn’t pan out, did they?” (For the sake of not introducing yet another of my bioarchaeological pet peeves, I’ll forego the rant [ok, I’ll mostly forego it, because Look! It’s a parenthetical rant!] about how grave goods are not the only medium for symbolic behavior in a burial).

Bringing the symbol card into play with reference to Neandethals is very likely to be a serious misstep for those who really really really want language and culture to be the exlcusive property of anatomically modern humans (that’s you and me [well, it’s me, I assume it’s you, too {on the internet, no one knows you’re a dog}]). It carries with it the assumption that the physical structures that allow for spoken language evolved first, and the abilities inherent in language (e.g., the capacity for symbolism) came later. There’s a lot of baggage there about how language made the difference for us, why the Neanderthals didn’t make it, etc.

The problem with that is sneaking up behind them from primatology, though. The body of evidence supporting symbolic behaviors in nonhuman primates (who, most definitely, have few or none of the anatomical prerequisites for spoken/heard language) is growing. It’s looking more and more like the brain comes first and the body follows.

So where do handedness, lateralization, and this paper (remember the paper? I started out with it roughly 10 kya) fit in this? Maybe nowhere. The authors of the study conclude that handedness in chimps is present, runs in families, and varies either from task to task and/or from community to community. The most straightforward implication is that cerebral asymmetry is something that evolved more than 5 mya, when the human and Pan lineages were not yet distinct. Research on handedness in mountain gorillas tentatively moves that date back to at least 8 mya, when all the African apes (that, naturally, includes you and me [barring your secret identity as a dog]) were a single lineage, and what the heck, I’d argue that the date is probably going to get pushed even further back to something like 15 mya, when all the great apes were a twinkle in the eye of a creature like Pierolapithecus catalunicus, given the fact that we are pretty confident that orangutans are wild tool users, too.

The AP article ends with the author interpreting the early evolution of lateralization to mean that handedness and language have fuck all to do with one another. That certainly is the most parsimonious explanation, particularly given the evidence for some type of handedness in all kinds of vertebrates. However, it raises the intriguing possibility that a lot of the supersecretextradoublefudgey parts of our neuroanatomy have been accumulating over a pretty long stretch of time. All in all, somee pretty intriguing stuff out of a pretty simple paper.

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