The Evolution Of Symbolic Language
Most organisms communicate, but humans are unique in communicating via symbolic language. This entails relationships between signifiers (e.g. words) and what's signified (e.g. objects or ideas), where what's special is the construction of a system of relationships among the signifiers themselves, generating a seemingly unlimited web of associations, organized by semantic regularities and constraints, retrieved in narrative form, and enabled by complex memory systems.
Humans are thus a symbolic species: symbols have literally changed the kind of biological organism we are. We think and behave in ways that are quite odd compared to other species because of the way that language has defined us. Symbolic language has become the dominant feature of the cultural environment to which we must adapt in order to flourish; the demands imposed by this niche have favored mental capacities and biases that guarantee successful access to this essential resource.
There are two perspectives on how language evolved.
Some propose that language appeared recently, and suddenly, due to some marvelous mutations that transformed "dumb brutes" into articulate speakers. If language is a recent feature of human social interaction -- arising, say, 100,000 years ago as an evolutionary afterthought -- then it would have had little opportunity to impose selection pressures; hence language abilities would be expected to have been inserted unsystematically into an otherwise typical (if enlarged) ape brain. If so, they should be poorly integrated with other cognitive functions, relatively fragile if faced with impoverished learning contexts, and susceptible to catastrophic breakdown as a result of genetic defects.
None of this seems to be the case.
If, instead, language has been around for a good deal of our evolutionary past, say a million years or so, the demands of language would have had time to affect brain evolution more broadly. A large network of subtle gene changes and neurological adjustments would be involved, resulting in a well-integrated and robust neurological function. Indeed, there is ample evidence to suggest that language is remarkably well-integrated into almost every aspect of our cognitive and social lives, that it utilizes a significant fraction of the forebrain, and is acquired robustly under even quite difficult social circumstances and neurological impairment. It is far from fragile.
If language-like communication has been a long-time feature of hominid evolution, then languages themselves must also have a long history. Since the language one learns must be passed from generation to generation, the more learnable its structures, and the better its fit to human limitations, the more effective its reproduction in each generation. Hence languages and brains are expected to have evolved in tandem. That said, brain evolution is a ponderously slow and unyielding process compared with the more facile evolution of languages, so we should expect that languages are more modified for brains than are brains for languages.
The world of symbols is an artificial niche, its ecology radically different from the biological niche we also occupy. In the same way that beaver dam-building has created an aquatic niche to which beaver bodies and behavior have adapted over their evolutionary history, our cognitive capacities have adapted to our self-constructed symbolic niche.
The intense and unusual demands of this niche are reflected in the ways that human cognition diverges from the patterns of other species. It has long been popular to think of human distinctiveness in terms of general intelligence, but this may have blinded us to a constellation of more subtle differences in social cognition (e.g. the ability to anticipate another's intended actions), in how we learn (e.g. a comparative ease at mimicking) and in motor capacities (e.g. unprecedented vocal control). All these adaptations contribute to our language abilities.
Language is in effect an emergent function, not some prior function that just required fine-tuning. Our inherited ("instinctive") vocalizations, such as laughter, shrieks of fright, and cries of anguish, are under localized, mostly subcortical, neurological control, as are analogous instinctive vocalizations in other animals. By contrast, language depends on a widely dispersed constellation of cortical systems. Each system is also found in other primate brains, where they engage in other functions; their collective recruitment for language was apparently driven by the fact that their previously evolved functions overlapped with particular processing demands necessitated by language. Old structures came to perform unprecedented new tricks.
Language evolution poses intriguing questions. For example, language is dependent on information maintained by culture. How did such a large fraction of our communicative capacity wind up offloaded onto social transmission? Moreover, the synergy of language systems requires the cooperative functioning of component brain systems, but this synergy would presumably need to have already been in place before selection could hone it for language. How is this paradox resolved?
Recent investigations of birdsong offer some clues in thinking about language evolution.
As expanded in an earlier blog, a comparative study of a recently domesticated bird and its feral cousin revealed that the domesticated lineage is a far more facile song-learner, with a much more complex and flexible song, despite the fact that the domesticated bird was bred for plumage coloration, not singing.
That this behavioral and neural complexity arose spontaneously was surprising given the common assumption that song complexity evolves under the influence of intense sexual selection, which was not operant under the breeding regime. One intriguing interpretation is that the relaxation of natural and sexual selection on singing was in fact responsible for its complexification. With song becoming irrelevant to species identification, territorial defense, mate attraction, predator avoidance, and so on, degrading mutations and existing deleterious alleles affecting the specification of the stereotypic song would not have been weeded out, the result being a reduction in the innate biases controlling song production. With specification of song structure no longer strictly controlled by the primary forebrain motor center, auditory experience, social context, learning biases, and attentional factors could all begin to influence singing, the result being that the domestic song became more variable, more complicated, and more influenced by social experience.
This story is relevant to the human because a number of features of human language adaptation also appear to involve a relaxation of innate constraints. Probably the clearest evidence for this is infant babbling, an unprecedented tendency to freely play with vocal sound production, with minimal innate constraint on what sound can follow what (save physical constraints on vocal sound generation). Babbling occurs in contexts of low arousal, whereas laughter, sobbing, and shrieking are each produced in high-arousal states with specific contextual associations. This reduction of emotional and contextual constraint on sound production opens the door for numerous other influences to play a role, allowing many more brain systems to participate in vocal behavior, including socially acquired auditory experience. In fact, such freedom from constraint is an essential precondition for being able to correlate learned vocal behaviors with the wide diversity of objects, events, properties, and relationships that language is capable of referring to. Hence an evolutionary de-differentiation process, while clearly not the whole story, may be a part of the story for symbolic language evolution.
Relaxation of selection may have contributed to other distinctively human traits as well. Perhaps the most striking feature of humans is their flexibility and cultural variety. Consider the incredible diversity of marital and kinship organizations. Most species have fairly predictable patterns of sexual association, kin association, and offspring care. By contrast, human mating and reproduction are largely controlled by symbolically mediated social negotiations. That one of the most fundamental biological functions has been off-loaded onto social-symbolic mechanisms is a signature feature of being a symbolic species.
Thus, because of symbols and with the aid of symbols, Homo sapiens has constructed and adapted to a niche unlike any other that ever has existed. We have been made in the image of the word.
Terrence Deacon is Professor of Anthropology and Neuroscience at the
University of California Berkeley. His most recent book, Mind from Matter: The Emergent Dynamics of Life, will be published by W.W. Norton in Fall 2010.