Fitting curves of brain growth in size against body size indicates no association between brain size growth and social complexity of various mammals. Two patterns of brain size relative to body size were found. The model A occurs in mammals whose body size increases continuously during adult life, e.g. in marsupials35, Model B characterises mammals whose body size stabilizes after reaching the adulthood. In both models, brain size remains in clear relationship to body size.
This study has shown that the human brain is a mammalian organ that, concerning its growth in size, in no particular way is exceptional. This is evident when comparing the pattern of brain growth in other mammals. Human brain anatomy is very similar to that of other primate brains36. Consequently, the results of our study challenge the long accepted notion that the human brain became especially large during hominin evolution, thus indicating the anatomical basis for Homo’s unusual abilities.
The findings of our study also confirm Passingham’s argument that not only is human brain growth rate within an expected variation range, but also, that mammalian brain growth rates are more similar than body growth rates. It was Ramon y Cajal who noted that mammalian brains have conserved similar anatomical features in relation to connectivity37. Furthermore, Kuhlenbek38 argues that mammalian brains share a common basal organisational blueprint which has been conserved during mammalian evolution39. This finding also concurs with the statement that the human brain’s cellular architecture shows high similarity to other mammals40. Recently, a study by Halley41 verified that brain growth rate minimally differed in fetal neurogenesis in eutherians; this was not correlated to variations in whole body or visceral organ growth rates. Thus, during prenatal development, brain growth rate of eutherians is noticeably conserved41.
The volume of the hominin braincase has tripled in the last, 3 million years (from about 450 ml to current 1350 ml.42. However, evolutionary hominin brain size increase matches increase in hominin body size43,44,45,46. Body size is measured either as the linear height, or weight, that in humans scales approximately to the second power of height47, a fact generally recognized by the construction of the Body Mass index as a ratio of weight to height squared. When the size of human brain is expressed as a linear dimension (a cube root of volume), its increases over the last 3 million years are comparable to those of height and weight44. The size of the human brain is proportional to the size of musculoskeletal system mass48; scaling of human brain size to body size is isometric, in contrast to other vertebrates and mammals, where brain size increases allometrically at a fraction of body size49,50. The often stressed unusual, among mammals, size of the human brain is only true when absolute values are considered. When the growth of human brain size is related to the body size growth, the exceptional place of humans among mammals disappears. It has been shown by Saniotis and Henneberg that among all hominins, past and present, the size of brain is proportional to body size41.
This human isometry occurred due to changes in body structure related to high quality diet and extraoral food processing decreasing size of the gastrointestinal tract and erect bipedalism together reducing body size43. Physiological regulation of the human brain by neurohormonal and neurotransmitter secretions follows the same principles as that of all other mammals, but the quantities of specific active substances may differ51. On this note it has been argued that neurohormonal regulation is a better indicator of human intelligence than brain size46.
Interestingly, human brain volume during the Holocene period has decreased by approximately 10% (100-150ml or one standard deviation)52,53. This recent decrease in brain volume has occurred during the advent of complex societies and accompanying technological and scientific developments54. It was simply a result of body size reduction52. The supposed anatomical uniqueness of the human brain is not confirmed by paleoanthropological evidence nor by comparative mammalian brain studies. In Model A, there seems to be continuing enlargement of the brain during adult life, concomitant with continued enlargement of body size. This can be a result of gliogenesis, neurogenesis or both. Neurogenesis in this case may be related to continuous growth of muscle mass15. This pattern of growth is shared by some marsupials (possums, koalas, wallabies and kangaroos) and some eutherians (rabbits, guinea pigs and pigs). The nature of the increase in adult brain size requires further investigation, especially in the areas of neuronal connectivity and structure which reveal differences between eutherian mammalian species. For instance, it has been noted that while human frontal lobes show greater connectivity in the gPFC than in the gPFC of other eutherians, the human frontal lobes are smaller than predicted in relation to non-human primates55.
Both models of brain size growth fit a number of mammals with different evolutionary histories, positions in trophic chains, geographic locations, environmental settings and behavioural characteristics. It seems, therefore, that specific intellectual abilities of particular mammals are a result of their brain physiologies and neurohormonal regulation rather than anatomical characteristics of their central nervous systems. This conclusion applies also to humans whose brain size growth is not indicative of their mental capacities.