Developing Intelligence

Although most humans are right-handed, other animals don’t seem to show a similar motoric asymmetry. As Corballis mentions in his 2003 BBS article, even the great apes – our closest relatives in the animal kingdom – tend not to show a right-hand preference unless raised in captivity, suggesting handedness is learned through imitation of caregivers. So why should humans be the only species to show clear manual asymmetry, 9:1 in favor of righties?

While manual asymmetry may be unique to humans, cerebral asymmetry is not. Corballis reviews how animals as diverse as frogs, birds, mice, rats, gerbils and marmosets tend to show more reliance on their left hemispheres in producing and recognizing their species’ vocalizations. Even in humans, right-handed adults are more likely to show greater activity in their left cerebral hemispheres on language tasks. And the only situation in which chimpanzees show clear handedness is during pointing – when chimpanzees point, they tend to do so (around 2/3 of the time) with their right hands (some chimps have even been observed to point spontaneously in the wild, suggesting this behavior is not due merely to imitating humans.)

To explain handedness, Corballis constructs an elaborate argument about the evolution of language. According to his theory, communicative (as opposed to merely expressive) behavior began with bipedalism, freeing the hands for use in communication. Corballis argues that this communicative behavior may have slowly shifted towards facial movements, and that the final stages in the evolution of spoken language involved the addition of voicing to those “facial gestures.” Handedness emerged as vocalization was incorporated into manual gesture, extending the left-hemispheric dominance for communication into manual control.

Corballis argues that lateralization of speech may have emerged simply because the benefits of symmetrical neuronal organization are minimal when the output of the neural system is patterned in time rather than space – a hypothesis which relates to other theories of hemispheric lateralization, suggesting that the left hemisphere may be specialized for more rapid processing of temporally-patterned stimuli.

If you’re just interested in why animals don’t show handedness, there’s no need to read further – the rest of this post is dedicated to examining Corballis’s argument about the evolution of language, which I found surprising. Corballis uses six points to support his claims:


1) Chimps do not have control over their vocal systems (and presumably, nor did early humans)

Observation of monkeys in the wild suggests that their vocalization are more “emotionally driven” and “undirected” as opposed to being clearly communicative. Consistent with this claim, destruction of the monkey anterior cingulate – a limbic region associated with emotion – completely disrupts their ability to vocalize. In contrast, destruction of a monkey’s Broca’s area (err, it’s homologue) does nothing to affect their vocalization, even though this region has been famously associated with speech production in humans.

2) Chimps have much more control over their hands (and presumably, so did early humans)

Although chimps have not been successfully trained to speak, they have shown surprising ability to communicate by pointing to abstract signs and symbols. Even more fascinating is the observation that several populations of monkeys (both wild and captive) have been observed to invent gestures and use them to communicate with other monkeys!

3) Broca’s area contains mirror neurons

Mirror neurons are a class of cells which seem to be involved not only in the production of certain manual gestures, but also in the perception of those same manual gestures being produced by others (thus they “mirror” gestures, whether produced or perceived). The region in which these cells reside is critically involved in human language (Broca’s area) and similarly involved in monkey’s manual gestures.

4) Bipedalism may enable language

Corballis argues that language is gestural in origin, and that bipedalism allows for the hands to be used more freely for communicative purposes.

5) Anatomy for vocalized speech may have evolved later than gestural communication

Only homo sapiens (and perhaps also Neanderthals) show an enlargement of the hypoglossal nerve, which controls the muscles in the tongue necessary for speech. It therefore appears that speech emerged rather late, according to Corballis, suggesting a potential role for gesture in the origins of speech.

6) Gesture is integral to modern language

Corballis reviews evidence that isolated deaf children have invented their own sign languages, complete with their own forms of grammar, suggesting that there is a clear pathway from manual gesture to language. Furthermore, babies can learn to sign long before they learn to speak, and people themselves will adopt grammatical structure in their own gestures when asked to communicate through gesture alone. This evidence suggests a tight linkage between gesture and language, in some ways more close than that between speech and language.

As reviewed briefly above, Corballis proposes the following timeline for the evolution of language:

1) Bipedalism emerges 6 million years ago, and some primitive manual communication may have been enabled

2) The emergence of stone tools 2.5 million years ago, and the migration to open savanna from forested environments occurred 2 million years ago. These developments imply a shift away from manual communication (since the arms would be involved in carrying things during migration or using tools) and towards facial communication. This potentially explains why humans are the sole species with white sclera surrounding the iris: direction of gaze may have been enhanced as a communicative tool by this adaptation.

3) The third step was to add voicing to facial gestures, reflected in the lowering of the larynx to cause both swallowing and breathing to share the same passage (in which we are unique among primates, suggesting there must have been a large advantage to this change to offset the obvious cost of choking). Note that some believe this change occurred as recently as 170,000 years ago.

4) Corballis argues the final step was a cultural rather than evolutionary invention – early homo sapiens may have learned to uncouple speech from facial gesture so that speech was communicative on its own. Corballis notes that facial gestures still assume dominance over vocalizations among modern humans (see the McGurk effect), again suggesting our cultural heritage from gestural communication.

In summary, Corballis claims that handedness emerged only after speech, which was itself lateralized due to preexisting dominance of the left-hemisphere for communicative behaviors.

Comments

  1. #1 AK
    October 16, 2007

    Bipedalism emerges 6 million years ago,

    Filler proposes that it occurred more like 20 MYA, with the split between gibbons and the great ape lineages, which would have interesting consequences for the age depth of verbal communication. Kanziby now has learned 348 lexigrams, and understands over 3000 spoken English words“. It seems plausible (IMO) that the common ancestor of Chimps, Gorillas, and Homo sap may have used a level of manu-verbal communication equivalent to a modern pidgin.

  2. #2 Drugmonkey
    October 16, 2007

    You have a flawed premise here, although I will acknowledge right up front the very likely possibility that humans exhibit the most consistent and strongest laterality. Run a Pubmed on “hand preference macaque” for example. GC Westergaard has some semi-nat field work amongst a bunch of lab-based manual tests.

    In your presentation, the fact that chimps exhibit hand preference for some tasks is a clue. The real consideration is whether the types of manual motor behavior that chimps, gorillas or monkeys perform in a given observational environment require or benefit from precision and accuracy.

    MW Andrews 99, is another good example in that it shows that when trained to perform repetitive laboratory tasks that benefit from precision and accuracy, bonnet macaques exhibit strong hand bias. of course, anyone who’s trained laboratory primates to perform skilled motor behaviors could tell you this as well…

    my take is that given the laterality in the brain function and structure of nonhuman primates and the behavioral evidence, there is little support for Corballis’ categorical position as you present it. Now, it could be the case that it is the degree of lateralization that is important…

  3. #3 Derek James
    October 16, 2007

    You mean The Straight Dope led me astray?

    http://www.straightdope.com/classics/a4_085.html

    “Paw/claw/whatever preference is actually pretty common in the animal world, having turned up in most species tested, including parrots (mostly lefties) as well as rats, monkeys, and chimpanzees (50-50 right versus left). Why should there be a preference? One plausible guess is that it helps the animals learn faster.”

    I usually consider Cecil pretty reliable, but he only cites one study here, with cats, and doesn’t give any references. Now my curiosity is piqued.

  4. #4 John McKay
    October 16, 2007

    I’m not sure if this hold true for modern elephants, but the wear on mammoth and mastodon tusks shows that individuals showed a clear preference for one side or the other as a tool. That is, the individuals exhibited “handedness” (tuskedness?)

  5. #5 Renee
    October 16, 2007

    Animals DO show handedness. Horses for instance have a preference or favorite direction to Gallop in. They also have a dominant lead. Barrel Racing in particular displays this. There are by far fewer horses that will start their clover leaf pattern with the left barrel. This means the if they start on the left they will have 2 left turns and 1 right turn. and vise versa if you start on right.2rights and 1 left turn. so Barrel racers will start with what ever is their horses dominant lead, strongest/favorite direction to turn. Matter in fact it takes some time to teach a young horse to deliberately change leads. They will sometimes stubbornly refuse to get on the correct lead for the direction they are galloping in. Any how i dissagree with your article just food for thought. My moms dog i noticed also has a favorite, prefered lead!

  6. #6 Derek James
    October 16, 2007

    I was a little confused by your opening paragraph, since I’d read that handedness was fairly ubiquitous in non-human animals. Turns out it’s maybe not a settled matter. Here’s a recent paper providing evidence for handedness in wild chimps:

    http://www.pnas.org/cgi/reprint/102/35/12634?ck=nck

  7. #7 shannon
    October 17, 2007

    this is interesting…food for thought: elephants show “tuskedness” in that they prefer either their right-or-left tusk. elephants communicate through auditory signals and through low-frequency vibrations, and also through gestures of their trunks. not sure if the tusks are used in gesturing, and not sure if elephant’s brains process language in the same way that humans’ brains do (i.e. right/left hemisphere). would be interested in any insight anyone has.

  8. #8 Mr. G
    October 17, 2007

    Very interesting article, especially when taken in conjunction with today’s NYT article by John Tierney about gossip vs. facts. That article touches on the evolution of gossip as fulfilling the “social grooming” needs of group culture. Link:http://www.nytimes.com/2007/10/16/science/16tier.html?ex=1350187200&en=cb07fa6189474841&ei=5124&partner=permalink&exprod=permalink

  9. #9 Laelaps
    October 17, 2007

    I have to look up the reference, but I have seen some research on Capuchin Monkeys that seemed to show some preference for handedness, although (as you note) I’ll have to check to make sure that they were wild monkeys and not raised in captivity for this to be of any possible significance.

    Also, when you’re listing the first set of bullets you write “Chimps do not have control over their vocal systems” but the discussion of the bullet only mentions monkeys. Maybe it’s nitpicking, but perhaps a better distinction should be made between monkeys and apes in the categories as to divert confusion.

  10. #10 CHCH
    October 17, 2007

    Sorry for the delay in moderating your comments, especially since they’re so interesting.

    Corballis claims that the reports of hand preference in chimps is controversial, limited in scope, and often limited to animal populations living in captivity who could have learned handedness through imitation. So in comparison to handedness in humans – which is strong (9:1) and wide in scope (not limited to things like pointing) – animals do not show consistent handedness preferences.

    Corballis does discuss horse galloping, but do those same horses also show the same preference for other activities? The notable aspect of handedness in humans is that it’s a cross-activity preference.

  11. #11 Alex Morgan
    October 18, 2007

    What’s striking about humans is not that individual humans exhibit limb preferences; as many have pointed out, individuals from several different species exhibit limb preferences. Rather, what’s striking about humans is the extent to which we exhibit limb preferences at the population level. Even though individual parrots (rats, chimps, etc.) might prefer a particular limb, it’s a distinct question whether the species as a whole shows a tendency to prefer a particular limb. Around 90% of humans prefer the right hand. This is the fact that Corballis is attempting to explain.

The site is currently under maintenance and will be back shortly. New comments have been disabled during this time, please check back soon.