Language and Complexity: Evolution Inside Out

Here is a video of Terrence Deacon, someone who needs no introduction on this website, giving a talk at Irving K. Barber Learning Centre about his latest research into language evolution:

I personally think these ideas being circulated by Deacon are our best avenue for exploring how language evolved. At its core, his proposal is that the relaxation of selection pressures allows genetic control to be offloaded onto epigenetic processes. This in turn allows for a greater influence of social transmission due to development being open to experiential modification. Our capacity for language, then, is a story of how developmental and evolutionary dynamics interact. As a recent post over at Babel's Dawn notes, this is basically a three-phase scenario:

  1. Standard primate brain in which midbrain areas (older parts of the brain) control vocal emotional communications.
  2. 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).
  3. “Unmasked selection” fixes new functional coordination and drives the brain’s anatomical reorganization.

The first aspect we need to appreciate is how Darwinian-like processes operate at the developmental-level. Deacon cites many instances, such as the fine-tuning of axonal connection patterns in the developing nervous system, where developmental processes are achieved through selection-like operations. Importantly, though, the logic differs from natural selection in one respect: “selection of this sort is confined to differential preservation only, not differential reproduction. In this respect, it is like one generation of the operation of natural selection”. The point he's trying to get across is that these intraselection processes are taking place right across nature. Take, for instance, the genus Spalax (the blind mole rat): during development its thalamic visual nucleus is dominated by brainstem auditory and somatic projections. This is because the blind mole rat has vestigial eyes (hence the name), with projections from their small retinas being out-competed in favour of somatic and auditory functions. As Deacon notes in a recent paper (which I wrote about here):

Experimental manipulations in other species, in which projections from one sensory modality are reduced in early development, likewise exhibit analogous takeover effects, and manipulations of the sensory periphery likewise demonstrate that intraselection adapts neural functional topography with respect to functional experience.

Such developmental flexibility is crucial in that it provides a general mechanism for natural selection to recruit in brain evolution. And as such, it is almost certainly relevant to the evolution of our brain in relation to language. From this evodevo perspective, Deacon goes onto highlight how Darwinian processes characterising natural selection (replication, variation, and differential fitness) have analogous counterparts in intraorganismic processes (redundancy, degeneracy, and functional interdependencies):

First, they involve processes that produce functional integration and/or adaptation even though they are generated by mechanisms that are dissociated from this consequence. Second, they all involve the generation of redundant variant replicas of some prior form (gene, cell, connection, antibody, etc.) brought into interaction with each other and with an external context in a way that allows these differences to affect their subsequent distribution. And third, their preservation and expression are dependent on correlation with context.

According to Deacon, these parallels with evolutionary processes are generally distinguished through the level at which selection operates — and how these interactions generate functional redundancy. Specifically, he looks at three types of redundancy: (i) internal redundancy; (ii) external redundancy; and, (iii) global external redundancy. By appealing to the dedifferentiation and redistribution effects of relaxed selection, the tendency is to shift from an innate, localised function onto a more distributed array of systems.

That's it for now. I'm sure I'll come back to this at a later point.