Brain size
The size of the brain is a frequent topic of study within the fields of
Humans
In humans, the right
Variation and evolution
From early primates to hominids and finally to Homo sapiens, the brain is progressively larger, with exception of extinct Neanderthals whose brain size exceeded modern Homo sapiens. The volume of the human brain has increased as humans have evolved (see
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.[10] Similarly, sociocultural explanations draw attention to externalization of knowledge and group decision-making, partly via the advent of social systems of distributed cognition, social organization, division of labor and sharing of information as possible causes.[11][12][7]
Name | Brain size (cm3)[13] |
---|---|
Homo habilis | 550–687 |
Homo ergaster | 700–900 |
Homo erectus | 600–1250 |
Homo heidelbergensis | 1100–1400 |
Homo neanderthalensis
|
1200–1750 |
Homo sapiens
|
1400 |
Homo floresiensis | 417 [14] |
H. floresiensis' small brain
Hydrocephalus
Exceptional cases of hydrocephalus, such as what was reported by John Lorber in 1980 and by a study with rats,[19][20] suggest that relatively high levels of intelligence and relatively normal functioning are possible even with very small brains.[21][22] It is unclear what conclusions could be drawn from such reports – such as about brain capacities, redundancies, mechanics and size requirements.
Biogeographic variation
Efforts to find racial or ethnic variation in brain size are generally considered to be a pseudoscientific endeavor[23][24][25] and have traditionally been tied to scientific racism and attempts to demonstrate a racial intellectual hierarchy.[25][26][27][28]
The majority of efforts to demonstrate this have relied on indirect data that assessed skull measurements as opposed to direct brain observations. These are considered scientifically discredited.[26][29]
A large-scale 1984 survey of global variation in skulls has concluded that variation in skull and head sizes is unrelated to race, but rather climatic heat preservation, stating "We find little support for the use of brain size in taxonomic assessment (other than with paleontological extremes over time). Racial taxonomies which include cranial capacity, head shape, or any other trait influenced by climate confound ecotypic and phyletic causes. For Pleistocene hominids, we doubt that the volume of the braincase is any more taxonomically 'valuable' than any other trait."[30]
Sex
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. Brain volume peaks at the teenage years,
However, Yaki (2011) found no
Total cerebral and gray matter volumes peak during the ages from 10–20 years (earlier in girls than boys), whereas white matter and ventricular volumes increase. There is a general pattern in neural development of childhood peaks followed by adolescent declines (e.g. synaptic pruning). Consistent with adult findings, average cerebral volume is approximately 10% larger in boys than girls. 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.[36] Young girls have on average relative larger hippocampal volume, whereas the amygdalae are larger in boys.[3] However, multiple studies[37][38] have found a higher synaptic density in males: a 2008 study reported that men had a significantly higher average synaptic density of 12.9 × 108 per cubic millimeter, whereas in women it was 8.6 × 108 per cubic millimeter, a 33% difference. Other studies have found an average of 4 billion more neurons in the male brain,[39] corroborating this difference, as each neuron has on average 7,000 synaptic connections to other neurons.
Significant dynamic changes in brain structure take place through adulthood and aging, with substantial variation between individuals. In later decades, men show greater volume loss in whole brain volume and in the
Genetic contribution
Adult
Intelligence
Studies demonstrate a correlation between brain size and intelligence, larger brains predicting higher intelligence. It is however not clear if the correlation is causal.[43] The majority of MRI studies report moderate correlations around 0.3 to 0.4 between brain volume and intelligence.[44][45] The most consistent associations are observed within the frontal, temporal, and parietal lobes, the hippocampus, and the cerebellum, but only account for a relatively small amount of variance in IQ, which suggests that while brain size may be related to human intelligence, other factors also play a role.[45][46] In addition, brain volumes do not correlate strongly with other and more specific cognitive measures.[47] In men, IQ correlates more with gray matter volume in the frontal lobe and parietal lobe, which is roughly involved in sensory integration and attention, whereas in women it correlates with gray matter volume in the frontal lobe and Broca's area, which is involved in language.[3]
Research measuring brain volume, P300 auditory evoked potentials, and intelligence shows a dissociation, such that both brain volume and speed of P300 correlate with measured aspects of intelligence, but not with each other.[48][49] Evidence conflicts on the question of whether brain size variation also predicts intelligence between siblings, as some studies find moderate correlations and others find none.[43] A recent review by Nesbitt, Flynn et al. (2012) points out that crude brain size is unlikely to be a accurate measure of IQ. Brain size is known to differ between men and women, for example (men on average have larger bodies than women), but without well documented differences in IQ.[43]
A study in 2017 find that the density in grey matter actually increases in adolescence. This finding also show that while females have lower brain volume, proportionate to their smaller size, they have higher grey matter density than males, which could explain why their cognitive performance is comparable. Thus, while adolescents lose brain volume, and with females having lower brain volume than males, this is compensated for by an increase in density of grey matter.[50]
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.
Other animals
The largest brains are those of
This power law formula applies to the "average" brain of mammals taken as a whole, but each
When the mammalian brain increases in size, not all parts increase at the same rate.
Not all investigators are happy with the amount of attention that has been paid to brain size. 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.[62] Moreover, they point out that intelligence depends not just on the amount of brain tissue, but on the details of how it is structured. It is also well known that crows, ravens, and grey parrots are quite intelligent even though they have small brains.
While humans have the largest encephalization quotient of extant animals, it is not out of line for a primate.[63][64] Some other anatomical trends are correlated in the human evolutionary path with brain size: the basicranium becomes more flexed with increasing brain size relative to basicranial length.[65]
Cranial capacity
Cranial capacity is a measure of the volume of the interior of the
Knowledge of the volume of the cranial cavity can be important information for the study of different populations with various differences like geographical, racial, or ethnic origin. Other things can also affect cranial capacity such as nutrition.[68] It is also used to study correlating between cranial capacity with other cranial measurements and in comparing skulls from different beings. It is commonly used to study abnormalities of cranial size and shape or aspects of growth and development of the volume of the brain.[citation needed] Cranial capacity is an indirect approach to test the size of the brain. A few studies on cranial capacity have been done on living beings through linear dimensions.[citation needed]
However, larger cranial capacity is not always indicative of a more intelligent organism, since larger capacities are required for controlling a larger body, or in many cases are an adaptive feature for life in a colder environment. For instance, among modern Homo sapiens, northern populations have a 20% larger visual cortex than those in the southern latitude populations, and this potentially explains the population differences in human brain size (and roughly cranial capacity).
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. This is greater than the average of modern humans.[74][75]
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. It compares the size of the brain of the specimen to the expected brain size of animals with roughly the same weight.[76] This way a more objective judgement can be made on the cranial capacity of an individual animal. A large scientific collection of brain endocasts and measurements of cranial capacity has been compiled by Holloway.[77]
Examples of cranial capacity
- Orangutans: 275–500 cm3 (16.8–30.5 cu in)
- Chimpanzees: 275–500 cm3 (16.8–30.5 cu in)
- Gorillas: 340–752 cm3 (20.7–45.9 cu in)
- Anatomically-modern human: average 1473 cm3[72]
- Neanderthals: 1500–1740 cm3
- Homo erectus; 850–1100 cm3
- Australopithecus anamensis; 365–370 cm3 [78]
- Australopithecus afarensis; 438 cm3 [79]
- Australopithecus africanus 452 cm3 [80]
- Paranthropus boisei 521 cm3
- Paranthropus robustus 530 cm3
See also
- Brain-to-body mass ratio
- Encephalization quotient
- List of animals by number of neurons
- Craniometry — includes historical discussion
- Neuroscience and intelligence
- Human brain
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Further reading
- Jabr, Ferris (28 November 2015). "How Humans Ended Up With Freakishly Huge Brains". Wired. Retrieved 29 November 2015.