The strength of Deacon’s presentation was that it described a mechanism for the brain changes that support language. The old view that language functions are confined to a few regions like Broca’s area and Wernicke’s area, or even the left hemisphere can no longer stand. Language processing involves complex coordination between multiple systems. But the modern human brain is a relatively recent acquisition. How did all that complexity evolve and become coordinated?\
Deacon proposes a three-phase scenario:
- Standard primate brain in which midbrain areas (older parts of the brain) control vocal emotional communications.
- A duplication of a section of the genome leads to “relaxed selection” and extensive cross talk between many cerebral cortical systems (newer parts of the brain).
- “Unmasked selection” fixes new functional coordination and drives the brain’s anatomical reorganization.
Much of the presentation consisted of a discussion of gene duplication. From time to time a gene accidentally duplicates itself and is passed along twice to the next generation. Deacon gave examples of this happening in the case of the gene for hemoglobin in the blood, homeobox genes that govern body shape, and the rhodospin gene duplications that allow for color vision. It was an impressive list making the point that gene duplication can accomplish major changes. It is not like a point change in a gene that typically produces a small change that may become one of many bits of fine tuning. At the same time, it is not a “hopeful monster,” which is a polite way of invoking a miracle.
Duplicated genes allow for “relaxed selection.” At the start the duplicate gene merely repeats what the original gene did. Selection pressure is “relaxed.” The original gene does the work for which it evolved. The duplicate gene does not matter so much, so selection pressures on it are “relaxed.” They don’t go away entirely. If the gene drifts into something too harmful, natural selection gets rid of it. But within a range, the new duplicate of the old gene is able to drift making random changes. In the brain, these random changes would create “cross talk” between previously separate regions.
Deacon discussed the Bengalese Finch as an example of what happens when you remove natural selection from an organism’s natural history. For 250 years the Bengalese Finch has been bred for a white color. Freed from the pressures of natural and sexual selection it not only looks different from its wild version (the white rump munia); it has changed its song as well. The song is more complicated than the wild version, involves multiple brain areas, and is learned socially, taught from one generation to the next, rather than being innate.
I want to insert two thoughts here that come from me, not from Deacon, so take them for what they are worth:
An example of an important piece of cross talk in the human brain is the linkage between reflexive attention and deliberative attention. This cabling enables language to direct attention to a topic by using words to pilot reflexive attention and then stay focused on the topic through deliberative attention. (See: How the Brain Supports Conversation)
This business about duplicate genes does not mean that the human lineage has language because it was lucky enough to get the right duplicate gene. Every such change has to be, at a minimum, supportable by the body, and brain tissue is highly expensive. Normally, we would expect brains to be as big as they can be. A duplicate gene supporting cortex could only survive selection if there was enough energy to feed the enlarged brain. Two sources of increased energy in the human lineage are the addition of cooperation and the introduction of cooked food. Presumably cooperation predates the cooking. So the human lineage had already moved toward something new before the brain began to become a source of so much vanity.
Unmasked selection merely means that relaxed selection has ended and the force of natural selection returned. Eventually the weight of the individually tolerable changes produced by drift in the duplicate gene bring the drift to a halt. Natural selection returns.
At this point a more familiar idea from Deacon’s repertoire comes into play: co-evolution. This process was important in Deacon’s book The Symbolic Species and was also described in Christiansen’s talk yesterday. In the co-evolution of brain and language traits that change rapidly (e.g., cultural traits) face a selective pressure to be learnable and usable. Things that change slowly (e.g., the connections between different brain regions) change to handle language’s recurring learning and production demands.
Deacon said more than this, but that’s my takeaway. If you’re looking for a mnemonic to sum it up just remember the grand CRU vintage served at Utrecht: from Chimp to Relaxed to Unmasked selection.