Buckner and Krienen put forward the fascinating idea that our advanced human capabilities may be a spandrel (i.e. a byproduct of the evolution of some other characteristic, rather than a direct product of adaptive selection). They note that the striking increase of hominid brain size over the past three million years (from ~ 400cc in chimps to ~600-800cc in H. habilis to 1,500-1,800 in H. sapiens) has gone with the enlargement not of the size of primary sensory and motor regions of the brain, but instead with the association regions between them, as if the primary regions had become untethered from each other. These association regions might be to form new circuits as they mature later in development in a more plastic and adaptive way than the primary regions. Now, instead of an automatic and tightly coupled linkage between sensory areas and motor areas driving behavior, the association cortices can insert the computations required for making more complex decisions, retrieving memories, and reflecting.
Phylogeny of the cortical mantle.
Schematic depictions of the cortex of placental mammals are shown with the size and positions of several conserved areas. Two organizational features are apparent in the phylogenic tree. Across all species, the relative positions of the areas are preserved, suggesting they arise from an ancient developmental template, or Bauplan, that is conserved. Second, as the brain is enlarged in primates a greater percentage of the cortical mantle falls between the primary and secondary sensory systems. The insets at the top represent hypothetical estimates of the mammalian common ancestor and the primate common ancestor. Dark blue, primary visual area (V1); light blue, secondary visual area (V2); green, middle temporal (MT) visual area; yellow, primary auditory area (A1); red, primary somatosensory area (S1); orange, secondary somatosensory area (S2).
The Tethering Hypothesis:
Bottom: The developing cortical mantle of the estimated mammalian common ancestor is schematically displayed as a thick line with two representative signaling gradients, labeled Signal A (red) and Signal B (blue). These gradients are heuristic presentations of the signal gradients present in the embryonic telencephalon (Figure 6). In the ancestral mammal, the signaling gradients and extrinsic activity from the sensory systems placed strong constraints on most of the developing cortex. Intermediate zones existed, colored in white, but represented a small portion of the cortical mantle. The resulting cortical organization included multiple sensory–motor hierarchies that occupied most of the mantle and formed canonical networks. Top: Following massive evolutionary expansion of the cortical mantle, in the presence of the same core signaling gradients, most of the cortical mantle emerges that is distant from the combined constraints of signaling gradients and extrinsic sensory activity. This emergent zone is illustrated as the large white area in the expanded cortical mantle. Untethered from sensory hierarchies, these distributed in-between zones are hypothesized to wire to one another and emerge as association cortex. The tethering hypothesis, which at this point should be considered a speculation, offers one framework to explain how association networks evolved their prominence and came to possess circuit properties vital to human cognition. The tethering hypothesis awaits further support or falsification.
