Brain size
The size of the brain is a frequent topic of study within the fields of anatomy, biological anthropology, animal science and evolution.
[1] As Kamran Safi, researcher at the Max Planck Institute of Animal Behavior and the study’s senior author writes: “Sometimes, relatively big brains can be the end result of a gradual decrease in body size to suit a new habitat or way of moving—in other words, nothing to do with intelligence at all.”[2] In humans, the right cerebral hemisphere is typically larger than the left, whereas the cerebellar hemispheres are typically closer in size.
[9][10] However, a reanalysis of the same data[9] suggests that brain size has not decreased, and that the conclusion was made using datasets that are too dissimilar to support quantitative comparison.
[12] Proponents of recent changes in brain size draw attention to the gene mutation that causes microcephaly, a neural developmental disorder that affects cerebral cortical volume.
[18] Despite its relatively derived position in the hominin phylogeny, CT imaging of its skull reveals that its brain volume was only 417 cm3,[17] less than that of even Homo habilis, which is believed to have gone extinct far earlier (around 1.65 million years ago.[19]).
A smaller brain is beneficial as it reduces the basal metabolic rate without significant increases in predation risk.
[28][29][30][31] The majority of efforts to demonstrate this have relied on indirect data that assessed skull measurements as opposed to direct brain observations.
Racial taxonomies which include cranial capacity, head shape, or any other trait influenced by climate confound ecotypic and phyletic causes.
"[33] A human baby's brain at birth averages 369 cm3 and increases, during the first year of life, to about 961 cm3, after which the growth rate declines.
[36] (This does not take into account neuron density nor brain-to-body mass ratio; men on average also have larger bodies than women.)
However, such differences should not be interpreted as imparting any sort of functional advantage or disadvantage; gross structural measures may not reflect functionally relevant factors such as neuronal connectivity and receptor density, and of note is the high variability of brain size even in narrowly defined groups, for example children at the same age may have as much as a 50% differences in total brain volume.
Significant dynamic changes in brain structure take place through adulthood and aging, with substantial variation between individuals.
[5] Men show a steeper decline in global gray matter volume, although in both sexes it varies by region with some areas exhibiting little or no age effect.
[46] A recent review by Nesbitt, Flynn et al. (2012) points out that crude brain size is unlikely to be a accurate measure of IQ.
[46] A 2017 study found that the brains of women have a higher density of grey matter, which could compensate for the loss of volume.
[53] A discovery in recent years is that the structure of the adult human brain changes when a new cognitive or motor skill, including vocabulary, is learned.
[54] Structural neuroplasticity (increased gray matter volume) has been demonstrated in adults after three months of training in a visual-motor skill, as the qualitative change (i.e. learning of a new task) appear more critical for the brain to change its structure than continued training of an already-learned task.
The explanation for an exponent of 0.75 is not obvious; however, it is worth noting that several physiological variables appear to be related to body size by approximately the same exponent—for example, the basal metabolic rate.
[59] This power law formula applies to the "average" brain of mammals taken as a whole, but each family (cats, rodents, primates, etc.)
Thus, in the species with the largest brains, most of their volume is filled with cortex: this applies not only to humans, but also to animals such as dolphins, whales or elephants.
Roth and Dicke, for example, have argued that factors other than size are more highly correlated with intelligence, such as the number of cortical neurons and the speed of their connections.
It is commonly used to study abnormalities of cranial size and shape or aspects of growth and development of the volume of the brain.
[74] Neanderthals had larger eyes and bodies relative to their height, thus a disproportionately large area of their brain was dedicated to somatic and visual processing, functions not normally associated with intelligence.
[76] Parts of a cranium found in China in the 1970s show that the young man had a cranial capacity of around 1700 cm3 at least 160,000 years ago.
[77][78] In an attempt to use cranial capacity as an objective indicator of brain size, the encephalization quotient (EQ) was developed in 1973 by Harry Jerison.
