Consciousness Blog 12 January 2014

Research carried out by two Harvard neuroscientists, Randy L. Buckner and Fenna M. Krienen, leads them to propose that the striking differences between human cognitive capabilities and those of our nearest relatives, the apes, may result from different neuronal behaviour in the embryos of our larger brains.

Comparison of FMRi scans of layered sections of human and ape brains show that, while ape brains display mostly short neuronal paths between sensory and motor cortices, in human brains there are many more long-distance neuronal paths, particularly marked in newly-evolved areas of the larger human brain which have more complex cognitive functions, termed 'association' areas.

The researchers describe a mechanism in the smaller, ape embryo brain by which developing neurons are attracted by chemical signals to link to the sensory and motor cortices; they speculate that in the larger human brain the chemical signalling is much less intense, either because tissue distances are greater, or because the signals have become muted, leading to more freedom for the neurons to make long-distance connections.

As is well known, the rapid expansion of the human brain to be three times the size of a chimpanzee brain took place alongside the emergence of advanced tool use, the development of language and the formation of social structures, between one and three million years ago, although there is no agreement about causation.

"Given the quick pace of change observed in the hominin lineage," say the researchers, "we are left with a puzzle: how did brain networks that underlie extraordinary human capabilities evolve so rapidly? A large part of the explanation must lie in the brain expansion that separates us from our ape cousins. . . . How might a large brain enable complex cognitive functions? One possibility is that the human brain possesses more computational capacity because it has a large number of neurons – estimated at 86 billion neurons using modern cell-counting techniques. . . . However, what has captured our interest is a peculiar feature of brain scaling that might prove critical. The feature concerns how brain scaling shifts the predominant circuit organization from one primarily linked to sensory–motor hierarchies to a noncanonical form vital to human thought. The emergent circuit organization may be a side effect, perhaps even to be considered a spandrel, of developmental rules and an organization inherited from our simpler mammalian ancestors but now expressed in a massively scaled cerebral cortex. The rapid expansion of the cortical mantle may have untethered large portions of the cortex from strong constraints of molecular gradients and early activity cascades that lead to local sensory hierarchies. What fill the gaps between these hierarchies are distributed, interconnected association networks that widely span the cortex, develop late, and are preferentially more dependent on protracted activity-dependent influences."

By the way, it is not the case that the sensory and motor cortices grew larger during late human evolution; they remain more or less the same size. It is the rest of the brain that has grown.
Say the authors: "Although prefrontal cortex is markedly expanded, so too are the temporal and parietal association regions, suggesting a coordinated increase in distributed cortical territories. Theories of cortical evolution should look for a parsimonious explanation for how multiple, distributed regions of association cortex might concurrently expand disproportionately relative to sensory regions."

The researchers do not settle on any one mechanism to explain the evolution of advanced human capabilities; but it is quite parsimonious, if rather simplistic, to propose that a simple increase in size might have been all that was needed to open up the field for different neuronal behaviours. They foresee many research possibilities to elucidate the matter further.

There is a particularly strong association between greater human brain capacity and the size of the human social group. Therefore, it is tempting to speculate that the availability of more 'free' neuronal capacity to allow the identification and remembering of a greater number of conspecifics and their characteristics may have been a crucial element permitting human social advancement.

Read more in Chapter Three of Agent Human by Michael Bell, The Evolution of Social Consciousness in Humans.


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