Abstract
The evolution of human cognition has been inferred from anthropological discoveries and estimates of brain size from fossil skulls. A more direct measure of cognition would be cerebral metabolic rate, which is proportional to cerebral blood flow rate (perfusion). The hominin cerebrum is supplied almost exclusively by the internal carotid arteries. The sizes of the foramina that transmitted these vessels in life can be measured in hominin fossil skulls and used to calculate cerebral perfusion rate. Perfusion in 11 species of hominin ancestors, from Australopithecus to archaic Homo sapiens, increases disproportionately when scaled against brain volume (the allometric exponent is 1.41). The high exponent indicates an increase in the metabolic intensity of cerebral tissue in later Homo species, rather than remaining constant (1.0) as expected by a linear increase in neuron number, or decreasing according to Kleiber's Law (0.75). During 3 Myr of hominin evolution, cerebral tissue perfusion increased 1.7-fold, which, when multiplied by a 3.5-fold increase in brain size, indicates a 6.0-fold increase in total cerebral blood flow rate. This is probably associated with increased interneuron connectivity, synaptic activity and cognitive function, which all ultimately depend on cerebral metabolic rate.
Highlights
The most distinctive feature of modern Homo sapiens is the relatively large size of the brain and its high metabolic rate
Carotid canal dimensions were measured from the skulls of 35 specimens of 12 species of hominin including Australopithecus africanus (N = 8 individuals), A. afarensis (N = 3), A. boisei (N = 1), Homo habilis (N = 1), H. naledi (N = 1), H. rudolfensis (N = 1), H. georgicus (N = 1), Early and Late H. erectus (N = 5), H. heidelbergensis (N = 2), H. neanderthalensis (N = 5), H. floresiensis (N = 1) and archaic H. sapiens (N = 5)
The broad confidence interval is partly related to H. sapiens, which is a clear and statistically significant outlier, possessing a much larger brain relative to body mass compared with other hominin species
Summary
The most distinctive feature of modern Homo sapiens is the relatively large size of the brain and its high metabolic rate. To explain 2 the drivers for hominin brain evolution, an emphasis has been placed on understanding hominin cerebral metabolic evolution. The human body allocates 20–25% of total resting metabolic rate to brain function, compared with 8–10% for non-human primates and 3–5% for most non-primate mammals [5,6,7]. Hominin cerebral metabolic evolution has been proposed to relate to changes in neuronal function and the establishment of specialized communication and metabolic energy pathways [8]
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