Evolution of the human brain: when bigger is better

Abstract

Comparative studies of the brain in mammals suggest that there are general architectural principles governing its growth and evolutionary development. We are beginning to understand the geometric, biophysical and energy constraints that have governed the evolution and functional organization of the brain and its underlying neuronal network. The object of this review is to present current perspectives on primate brain evolution, especially in humans, and to examine some hypothetical organizing principles that underlie the brain's complex organization. Some of the design principles and operational modes that underlie the information processing capacity of the cerebral cortex in primates will be explored. It is shown that the development of the cortex coordinates folding with connectivity in a way that produces smaller and faster brains, then otherwise would have been possible. In view of the central importance placed on brain evolution in explaining the success of our own species, one may wonder whether there are physical limits that constrain its processing power and evolutionary potential. It will be argued that at a brain size of about 3500 cm3, corresponding to a brain volume two to three times that of modern man, the brain seems to reach its maximum processing capacity. The larger the brain grows beyond this critical size, the less efficient it will become, thus limiting any improvement in cognitive power.

Evolution of brain elaboration

“…Experiments in which cortical cell number is increased in mice demonstrate that gyrification occurs as cortical surface area increases [100,101]. Gyrification benefits the organism by allowing a larger cortex to fit within the skull, as well as facilitating the formation of local (smallworld) connectivity that is more rapid and energy efficient than long-distance connections [102][103][104]. Parcellation, the subdivision of a brain region into functional and structural subdivisions, would also enable the formation of small world circuitry, and is observed in both mushroom bodies and cortex as they increase in size [58,75,105].…”

Neuronal factors determining high intelligence

“…Thus, the highest number of cortical neurons (especially those in the frontal lobe), the most efficient connectivity pattern, and consequently, the highest IPC are found in humans. Yet, according to the model developed by Hofman [59], the human brain lies about 20-30% below the optimum, which would be a brain of about 3500 cm 3 . This would be roughly twice the present human brain volume.…”

Information in the Biosphere: Biological and Digital Worlds

“…While this number is impressive, even in 2007, humanity's general purpose computers were capable of performing well over 1 x 10 18 instructions per second [5]. Estimates suggest that the storage capacity of an individual human brain is about 10 12 Bytes [28,29]. On a per capita basis, this is matched by current digital storage (5 x 10 21 Bytes per 7.2 x 10 9 people).…”

“…It has been suggested that there are energy and infrastructural constraints that ultimately govern human brain size and activity [85], and that our brain size may be approaching the evolutionary limits of cognitive power [29]. Given that physical restrictions may prevent evolutionary improvements in cognition, the integration of biological with digital processing and information storage is one way forward.…”