Assembling language from spare parts

[Gary Marcus]

Dear INFO-CHILDES readers,

I thought I’d start the new year by surprising regular readers of  
this forum by agreeing, at least in part, with a theoretical position  
championed by Liz Bates. As noted in this forum over the last couple  
weeks,  Liz advocated a sort of language-from-spare-parts view,  and  
in the last couple years (for my own reasons) I’ve been arguing for  
something quite similar (Marcus, 2004a; Marcus 2004b, esp. Chapter 7;  
Marcus 2006; and in a just-published review of the genetic basis of  
language, Fisher & Marcus 2006).  In words that I very much agree  
with, Bates argued that

  the cognitive machinery that makes us human can be viewed as a new  
machine constructed out of old parts (Bates, 1999, and related quotes  
mentioned recently on Info-Childes)

At the same time, Liz’s perspective on this idea, which I think of as  
an extension of Darwin’s notion of descent with modification, is  
different from my own. For her, descent-with-modification was all  
about quantitative change, and the poster-child of evolutionary  
novelty was the giraffe:

[C]onsider the metaphor of the giraffe’s neck. Giraffes have the same  
number of neckbones that you and I have, but these bones are  
elongated to solve the peculiar problems that giraffes are  
specialized for (i.e., eating leaves high up in the tree). As a  
result of this particular adaptation, other adaptations were  
necessary as well, including cardiovascular changes (to pump blood  
all the way up to the giraffe’s brain), shortening of the hindlegs  
relative to the forelegs (to ensure that the giraffe does not topple  
over), and so on… The giraffe’s neck is still a neck, built out of  
the same basic blueprint that is used over and over in vertebrates,  
but with some quantitative adjustments. It still does other kinds of  
“neck work”, just like the work that necks do in less specialized  
species, but it has some extra potential for reaching up high in the  
tree that other necks do not provide. If we insist that the neck is a  
leaf-reaching organ, then we have to include the rest of the giraffe  
in that category, including the cardiovascular changes, adjustments  
in leg length, and so on. In the same vein, our “language organ” can  
be viewed as the result of quantitative adjustments in neural  
mechanisms that exist in other mammals. (Bates, 1999)

In my view, Liz overemphasized the quantitative at the expense of the  
qualitative. Take color vision, for example. Much (though not  
everything) is known about how it arose, evolutionary, and while the  
process of color vision surely should be seen as the product of  
descent with modification, the capacity to see in color equally  
surely differs qualitatively from anything preceded it. In essence,  
the critical step involved an evolutionary process known as  
duplication-and-divergence. Around 400 million years ago,  gene for  
producing photoreceptor pigment was duplicated, and one copy of the  
gene was then free to diverge, yielding a second photoreceptor  
pigment with a slightly different spectral sensitivity. At that  
point, it became possible for ancestral retinas to distinguish not  
just the amount of light (monochromatic vision) impinging upon the  
eye but also the nature of that light (dichromatic vision).  In  
understanding the nature and evolution of color vision, it's  
important to recognize how color vision is continuous with ancestral  
systems for monochromatic vision, but missing the point to think of  
the dramatic reorganization that ensued as nothing more than  
quantitative change.  Liz’s conception of language through the  
quantitative change leads to a kind of language as cognition-writ- 
large perspective, whereas I think that process of evolutionary  
tinkering very much open the possibility of genuine novelty. Rather  
than trying to understand language as the product of (say) little  
more than an exceptionally large chimp brain, I suggest that we would  
be best off seeing language as the product of both qualitative and  
quantitative change.

I further depart from Liz’s company when the question of what kinds  
of genetic change might have taken place. In the 1999 paper quoted  
from above, Bates went on to suggest that

  developmental  neurobiologists have abandoned the idea that  
detailed aspects of synaptic connectivity are under direct genetic  
control, in favor of an activity-dependent account. There has to be  
something special about the human brain that makes language possible,  
but that “something” may involve highly distributed mechanisms that  
serve many other functions.

Here again, Bates's view seems half right; the notion that language  
relies on highly distributed mechanisms seems even more plausible  
several years later, and many (though perhaps not all) of the  
relevant neural regions play roles in other cognitive functions. But  
distribution, and even the important role of activity-dependence, do  
not diminish the importance of genetic contributions; in the same  
half decade since Bates wrote those words, it is has become amply  
clear that genes do in fact drive initial synaptic connectivity in  
very detailed ways. As I emphasized in The Birth of the Mind,  
molecular control of initial brain-wiring is both precise and  
powerful, even while it is flexible. Although activity-dependence  
(which I think of as rewiring) is crucial, endogeneous mechanisms are  
no less crucial; indeed, many neural hallmarks can develop in the  
absence of any activity at all (e.g, Crowley & Katz, 1999; Verhage,  
2000). A synthetic explanation of human cognition – and human  
uniqueness – must look for both qualititative and quantitative  
changes both in what is learned and what is contributed via biology.

– Gary Marcus

p.s. Brian MacWhinney alluded to a an excellent early statement of  
this view, in Kaplan & Werner (1963). For those interested, I tracked  
down the original wording:

Even in the emergence of novel functions and forms, however, there  
is, as a rule, an intertwining of continuous and discontinuous  
changes: though novel features come about by qualitative change,  
which necessarily implies discontinuity, the manner in which such  
features emerge may be gradual in various respects, e.g., there may  
be a gradual increase in the frequency of occurrence of the new over  
the old, or there may be employment of older forms for new functions  
before the new functions secure the formation of novel, function- 
specific means, etc

They in turn seem to have credited their idea to a reading of Mayr  
(1960), who of course traced his own work back to Darwin.

References

Bates, E. (1999). Plasticity, localization, and language development.  
In S. H. Broman & J. M. Fletcher (Eds.), The changing nervous system:  
Neurobiological consequences of early brain disorders. New York:  
Oxford University Press.

Crowley, J. C., & Katz, L. C. (1999). Development of ocular dominance  
columns in the absence of retinal input. Nature Neuroscience, 2(12),  
1125-1130.

Fisher, S. E., & Marcus, G. F. (2006). The eloquent ape: genes,  
brains and the evolution of language. Nature Reviews Genetics, 7(1),  
9-20.

Kaplan, B.  & Werner, H. (1963) Symbol Formation: An Organismic- 
Developmental Approach to Language and the Expression of Thought. Wiley.

Marcus, G. F. (2004a). Before the word. Nature, 431(7010), 745.

Marcus, G. F. (2004b). The Birth of the Mind: How A Tiny Number of  
Genes Creates the Complexities of Human Thought. New York: Basic Books.

Marcus, G. F. (2006). Cognitive Architecture and Descent with  
Modification. Cognition, in press.

Mayr, E., 1960. “The emergence of evolutionary novelties”. In Tax, S.  
(Ed.), Evolution after Darwin, Vol. I. Chicago: University of Chicago  
Press.

Verhage, M., et al. (2000). Synaptic assembly of the brain in the  
absence of neurotransmitter secretion. Science, 287(5454), 864-869.
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