Science - June 11, 2004
To judge from some recent discussions about evolution, genes, and the mind, a mother could be concerned that her child, born with only about 30,000 genes, might have serious developmental problems or at least be unlikely to follow a typical human developmental pathway. Experts have made much of the claim that 30,000 genes aren't nearly enough to specify the vast number of connections in the brain (the "gene shortage") (1). Our genetic endowment might not even be sufficient to make us reliably human because, according to some, genes cannot specify particular developmental outcomes (2, 3). One could get the impression that all the genome can do, because of its limited information capacity, is to specify the basic properties that allow the brain to be an "organ of plasticity" (4). Humans must be human, on this view, because some parameter, like degree of plasticity or number of neurons, has been tweaked. Moreover, there has "not been enough time" for many new psychological capacities to have evolved via changes in the genome since the divergence of the human and chimpanzee lineages (5). Such arguments have been widely invoked to downplay the role of evolution in shaping the human mind as we observe it today. They have also been used to argue against the view, associated with the growing field of evolutionary psychology, that the many specialized psychological abilities of humans are due to natural selection specifically for those abilities.
For our hypothetical mother, this might be worrying news indeed. Her child might just as easily turn out to have the brain of a chimpanzee. The worried parent will find such fears assuaged by Gary Marcus's new book, The Birth of the Mind. With clarity and precision, Marcus, a developmental psychologist at New York University, lays to rest the rumors of a gene shortage and also rebuts the argument that minds are too complex to have been designed over evolutionary time by the process of natural selection. He shows instead that minds are built over the course of individual development by genetically regulated processes that have been molded by natural selection to build brains that are functionally organized in ways that promoted human survival and reproduction in the evolutionary past.
Marcus begins by observing that the brain is far from the only place in the body where our small genome gives rise to complex, functionally organized structures; the liver or the heart, he notes, might just as easily suffer from a gene shortage. He surveys the state of the art of our understanding of genes. Rather than static pictures or blueprints of phenotypes, genes are active "agents" that interact in precisely orchestrated ways to build organisms.
The author shows us how this view allows us to understand the fantastically complex, yet fantastically well-coordinated, generation of the mind. In cognitive science, it has long been customary to think of the brain as a computer. Marcus shows that the developmental system that builds the brain can also be thought of as an algorithmic system, one that operates through frequent interactions with its internal and external environments. He likens the genome to a compressed file, and the cellular machinery with which it interacts to a decompressor. However, this developmental system is full of ingenious devices not typically found in silicon-based computers, including gradients and switches that allow its operations to be context-sensitive, feedback loops, and self-generated "test patterns" that allow the system to tune itself. Such phenomena challenge our standard notions of flexibility and plasticity as being fundamentally at odds with genetic control. It is precisely because of genetically specified developmental procedures that the brain is able to achieve its astounding plasticity. Plasticity is not simply responsiveness to change (as when a basketball responds to being punctured) but responsiveness that produces the correct outcome in diverse circumstances. As Marcus makes clear, although we are vastly more complex than desktop computers and therefore have potentially many more ways of breaking, the fact that our developmental process is relatively far less prone to crashing while booting up from the zygote has everything to do with natural selection for specific developmental outcomes.
Perhaps most important, Marcus tackles a question fundamental to current debates about the mind: How could so few genes account for the large array of humans' specialized psychological skills? Here, arguments have focused on the idea of modularity, the notion that specific skills are handled by specific areas or circuits in the brain. Evolutionary psychologists have argued, following William James's insight over a century ago, that the flexibility and power of human intelligence result from natural selection having added, not removed, specialized machinery to our minds. However, many have intuited that there could not be very many modules because of the gene shortage, a lack of evolutionary time, or both. Clearly, the blueprint idea of a one-to-one-mapping between genes and modules (in which an entirely new suite of genes is required for every new module) seems to lead inevitably to a gene shortage. But Marcus shows that our knowledge of developmental genetics debunks this simplistic view. He discusses ways in which a complex regulatory system can build distinct units without an entirely new set of instructions for each. For example, an animal with 60 legs would not necessarily need 10 times as many genes as a six-legged animal, and although human arms and legs differ considerably, we do not require an entirely distinct set of genes for each type of limb. The same considerations apply to the components of the brain. Marcus points to many ways in which evolution can generate features of organisms' phenotypes that are modular in design without an equivalently modular genome. He also mentions interesting mechanisms for the generation of novel structures--such as duplication of genes within the genome, which allows modification of the copy without loss of function of the original--that could provide useful insights for those seeking to understand the origin of novel psychological capacities in humans.
The account Marcus offers will be refreshing to those who are tired of simple-minded debates about the role of genes and evolution in shaping the human mind. If there is a drawback to the book, it is that the author doesn't show us exactly how a tiny number of genes builds such a complex brain, only that they can. But he is hardly to blame for this, given that we have a long way to go before we have a complete understanding of brain development. The strengths of The Birth of the Mind lie in its sophisticated exposition of how genes guide development and its convincing argument that we need not hold out hope for some magical, as yet undiscovered, process to account for the brain's complexity. Plain old natural processes, about which we know much already, will do.
1. P. Ehrlich, Human Natures: Genes, Cultures, and the Human Prospect (Island, Washington, D.C., 2000).
2. D. Buller, V. Hardcastle, Brain Mind 1, 307 (2000).
3. R. Lickliter, H. Honeycutt, Psychol. Bull. 129, 819 (2003).
4. J. L. Elman et al., Rethinking Innateness: Connectionism in a Developmental Framework (MIT Press, Cambridge, MA, 1996).
5. M. Tomasello, The Cultural Origins of Human Cognition (Harvard Univ. Press, Cambridge, MA, 1999).
The reviewer is at the Center for Behavior, Evolution, and Culture and the Center for Culture, Brain, and Development, Department of Anthropology, University of California, Los Angeles, 341 Haines Hall, Box 951553, Los Angeles, CA, 90095-1553, USA. E-mail: email@example.com
Volume 304, Number 5677, Issue of 11 Jun 2004, pp. 1601-1602.
Copyright © 2004 by The American Association for the Advancement of Science. All rights reserved.