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The size of the brain is a frequent topic of study within the fields of anatomy, biological anthropology, animal science and evolution. Measuring brain size and cranial capacity is relevant both to humans and other animals, and can be done by weight or volume via MRI scans, by skull volume, or by neuroimaging intelligence testing. The relationship between brain size and intelligence remains a controversial although frequently investigated question.
In humans, the right cerebral hemisphere is typically larger than the left, whereas the cerebellar hemispheres are typically closer in size. The adult human brain weighs on average about 1.5 kg (3.3 lb). [1] In men the average weight is about 1370 g and in women about 1200 g. [2] [ contradictory ] The volume is around 1260 cm3 in men and 1130 cm3 in women, although there is substantial individual variation. [3] Yet another study argued that adult human brain weight is 1300-1400 g for adult humans and 350-400 g for newborn humans. There is a range of volume and weights, and not just one number that one can definitively rely on, as with body mass. It is also important to note that variation between individuals is not as important as variation within species, as overall the differences are much smaller. The mechanisms of interspecific and intraspecific variation also differ.
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 Homininae), starting from about 600 cm3 in Homo habilis up to 1680 cm3 in Homo neanderthalensis, which was the hominid with the biggest brain size. [5] Some data suggest that the average brain size has decreased since then, [6] including a study concluding the decrease "was surprisingly recent, occurring in the last 3,000 years". [7] [8] However, a reanalysis of the same data [7] suggests that brain size has not decreased, and that the conclusion was made using datasets that are too dissimilar to support quantitative comparison. [9] [4]
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] |
Homo floresiensis is a hominin from the island of Flores in Indonesia with fossils dating from 60,000-100,000 years ago. [15] Despite its relatively derived position in the hominin phylogeny, CT imaging of its skull reveals that its brain volume was only 417 cm3, [14] less than that of even Homo habilis , which is believed to have gone extinct far earlier (around 1.65 million years ago. [16] ). The reason for this regression in brain size is believed to be island syndrome [17] in which the brains of insular species become smaller due to reduced predation risk. This is beneficial as it reduces the basal metabolic rate without significant increases in predation risk. [18]
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.
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]
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, [31] and after the age of 40 it begins declining at 5% per decade, speeding up around 70. [32] Average adult male brain weight is 1,345 grams (47.4 oz), while an adult female has an average brain weight of 1,222 grams (43.1 oz). [33] (This does not take into account neuron density nor brain-to-body mass ratio; men on average also have larger bodies than women.) Males have been found to have on average greater cerebral, cerebellar and cerebral cortical lobar volumes, except possibly left parietal. [34] The gender differences in size vary by more specific brain regions. Studies have tended to indicate that men have a relatively larger amygdala and hypothalamus, while women have a relatively larger caudate and hippocampi. When covaried for intracranial volume, height, and weight, Kelly (2007) indicates women have a higher percentage of gray matter, whereas men have a higher percentage of white matter and cerebrospinal fluid. There is high variability between individuals in these studies, however. [3]
However, Yaki (2011) found no statistically significant gender differences in the gray matter ratio for most ages (grouped by decade), except in the 3rd and 6th decades of life in the sample of 758 women and 702 men aged 20–69. [35] The average male in their third decade (ages 20–29) had a significantly higher gray matter ratio than the average female of the same age group. In contrast, among subjects in their sixth decade, the average woman had a significantly larger gray matter ratio, though no meaningful difference was found among those in their 7th decade of life.
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 frontal lobes, and temporal lobes, whereas in women there is increased volume loss in the hippocampi and parietal lobes. [3] 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. Overall white matter volume does not appear to decline with age, although there is variation between brain regions. [40]
Adult twin studies have indicated high heritability estimates for overall brain size in adulthood (between 66% and 97%). The effect varies regionally within the brain, however, with high heritabilities of frontal lobe volumes (90-95%), moderate estimates in the hippocampi (40-69%), and environmental factors influencing several medial brain areas. In addition, lateral ventricle volume appears to be mainly explained by environmental factors, suggesting such factors also play a role in the surrounding brain tissue. Genes may cause the association between brain structure and cognitive functions, or the latter may influence the former during life. A number of candidate genes have been identified or suggested, but they await replication. [41] [42]
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. [51] 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. Such changes (e.g. revising for medical exams) have been shown to last for at least 3 months without further practicing; other examples include learning novel speech sounds, musical ability, navigation skills and learning to read mirror-reflected words. [52] [53]
The largest brains are those of sperm whales, weighing about 8 kg (18 lb). An elephant's brain weighs just over 5 kg (11 lb), a bottlenose dolphin's 1.5 to 1.7 kg (3.3 to 3.7 lb), whereas a human brain is around 1.3 to 1.5 kg (2.9 to 3.3 lb). Brain size tends to vary according to body size. The relationship is not proportional, though: the brain-to-body mass ratio varies. The largest ratio found is in the shrew. [54] Averaging brain weight across all orders of mammals, it follows a power law, with an exponent of about 0.75. [55] There are good reasons to expect a power law: for example, the body-size to body-length relationship follows a power law with an exponent of 0.33, and the body-size to surface-area relationship follows a power law with an exponent of 0.67. 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. [56]
This power law formula applies to the "average" brain of mammals taken as a whole, but each family (cats, rodents, primates, etc.) departs from it to some degree, in a way that generally reflects the overall "sophistication" of behavior. [57] Primates, for a given body size, have brains 5 to 10 times as large as the formula predicts. Predators tend to have relatively larger brains than the animals they prey on; placental mammals (the great majority) have relatively larger brains than marsupials such as the opossum. A standard measure for assessing an animal's brain size compared to what would be expected from its body size is known as the encephalization quotient. The encephalization quotient for humans is between 7.4-7.8. [58]
When the mammalian brain increases in size, not all parts increase at the same rate. [59] In particular, the larger the brain of a species, the greater the fraction taken up by the cortex. 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. The evolution of Homo sapiens over the past two million years has been marked by a steady increase in brain size, but much of it can be accounted for by corresponding increases in body size. [60] There are, however, many departures from the trend that are difficult to explain in a systematic way: in particular, the appearance of modern man about 100,000 years ago was marked by a decrease in body size at the same time as an increase in brain size. Even so, it is noteworthy that Neanderthals, which became extinct about 40,000 years ago, had larger brains than modern Homo sapiens. [61]
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 is a measure of the volume of the interior of the skull of those vertebrates who have a brain. The most commonly used unit of measure is the cubic centimetre (cm3). The volume of the cranium is used as a rough indicator of the size of the brain, and this in turn is used as a rough indicator of the potential intelligence of the organism. Cranial capacity is often tested by filling the cranial cavity with glass beads and measuring their volume, or by CT scan imaging. [66] [67] A more accurate way of measuring cranial capacity, is to make an endocranial cast and measure the amount of water the cast displaces. In the past there have been dozens of studies done to estimate cranial capacity on skulls. Most of these studies have been done on dry skull using linear dimensions, packing methods or occasionally radiological methods.[ citation needed ]
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). [69] [70] Neurological functions are determined more by the organization of the brain rather than the volume. Individual variability is also important when considering cranial capacity, for example the average Neanderthal cranial capacity for females was 1300 cm3 and 1600 cm3 for males. [71] 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. When these areas were adjusted to match anatomically modern human proportions it was found Neanderthals had brains 15-22% smaller than in anatomically-modern humans. [72] When the neanderthal version of the NOVA1 gene is inserted into stem cells it creates neurons with fewer synapses than stem cells containing the human version. [73]
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
Human evolution is the evolutionary process within the history of primates that led to the emergence of Homo sapiens as a distinct species of the hominid family that includes all the great apes. This process involved the gradual development of traits such as human bipedalism, dexterity, and complex language, as well as interbreeding with other hominins, indicating that human evolution was not linear but weblike. The study of the origins of humans, also called anthropogeny, anthropogenesis, or anthropogony, involves several scientific disciplines, including physical and evolutionary anthropology, paleontology, and genetics.
Homininae, also called "African hominids" or "African apes", is a subfamily of Hominidae. It includes two tribes, with their extant as well as extinct species: 1) the tribe Hominini ―and 2) the tribe Gorillini (gorillas). Alternatively, the genus Pan is sometimes considered to belong to its own third tribe, Panini. Homininae comprises all hominids that arose after orangutans split from the line of great apes. The Homininae cladogram has three main branches, which lead to gorillas, and to humans and chimpanzees via the tribe Hominini and subtribes Hominina and Panina. There are two living species of Panina and two living species of gorillas, but only one extant human species. Traces of extinct Homo species, including Homo floresiensis have been found with dates as recent as 40,000 years ago. Organisms in this subfamily are described as hominine or hominines.
Early modern human (EMH), or anatomically modern human (AMH), are terms used to distinguish Homo sapiens that are anatomically consistent with the range of phenotypes seen in contemporary humans, from extinct archaic human species. This distinction is useful especially for times and regions where anatomically modern and archaic humans co-existed, for example, in Paleolithic Europe. Among the oldest known remains of Homo sapiens are those found at the Omo-Kibish I archaeological site in south-western Ethiopia, dating to about 233,000 to 196,000 years ago, the Florisbad site in South Africa, dating to about 259,000 years ago, and the Jebel Irhoud site in Morocco, dated about 315,000 years ago.
Solo Man is a subspecies of H. erectus that lived along the Solo River in Java, Indonesia, about 117,000 to 108,000 years ago in the Late Pleistocene. This population is the last known record of the species. It is known from 14 skullcaps, two tibiae, and a piece of the pelvis excavated near the village of Ngandong, and possibly three skulls from Sambungmacan and a skull from Ngawi depending on classification. The Ngandong site was first excavated from 1931 to 1933 under the direction of Willem Frederik Florus Oppenoorth, Carel ter Haar, and Gustav Heinrich Ralph von Koenigswald, but further study was set back by the Great Depression, World War II and the Indonesian War of Independence. In accordance with historical race concepts, Indonesian H. erectus subspecies were originally classified as the direct ancestors of Aboriginal Australians, but Solo Man is now thought to have no living descendants because the remains far predate modern human immigration into the area, which began roughly 55,000 to 50,000 years ago.
Craniometry is measurement of the cranium, usually the human cranium. It is a subset of cephalometry, measurement of the head, which in humans is a subset of anthropometry, measurement of the human body. It is distinct from phrenology, the pseudoscience that tried to link personality and character to head shape, and physiognomy, which tried the same for facial features. However, these fields have all claimed the ability to predict traits or intelligence.
Encephalization quotient (EQ), encephalization level (EL), or just encephalization is a relative brain size measure that is defined as the ratio between observed and predicted brain mass for an animal of a given size, based on nonlinear regression on a range of reference species. It has been used as a proxy for intelligence and thus as a possible way of comparing the intelligence levels of different species. For this purpose, it is a more refined measurement than the raw brain-to-body mass ratio, as it takes into account allometric effects. Expressed as a formula, the relationship has been developed for mammals and may not yield relevant results when applied outside this group.
Neuroscience and intelligence refers to the various neurological factors that are partly responsible for the variation of intelligence within species or between different species. A large amount of research in this area has been focused on the neural basis of human intelligence. Historic approaches to studying the neuroscience of intelligence consisted of correlating external head parameters, for example head circumference, to intelligence. Post-mortem measures of brain weight and brain volume have also been used. More recent methodologies focus on examining correlates of intelligence within the living brain using techniques such as magnetic resonance imaging (MRI), functional MRI (fMRI), electroencephalography (EEG), positron emission tomography and other non-invasive measures of brain structure and activity.
Homo rhodesiensis is the species name proposed by Arthur Smith Woodward (1921) to classify Kabwe 1, a Middle Stone Age fossil recovered from Broken Hill mine in Kabwe, Northern Rhodesia. In 2020, the skull was dated to 324,000 to 274,000 years ago. Other similar older specimens also exist.
An occipital bun, also called an occipital spur, occipital knob, chignon hook or inion hook, is a prominent bulge or projection of the occipital bone at the back of the skull. It is important in scientific descriptions of classic Neanderthal crania. It is found among archaic Homo species, as well as Upper Pleistocene Homo sapiens and present-day human populations.
The evolution of human intelligence is closely tied to the evolution of the human brain and to the origin of language. The timeline of human evolution spans approximately seven million years, from the separation of the genus Pan until the emergence of behavioral modernity by 50,000 years ago. The first three million years of this timeline concern Sahelanthropus, the following two million concern Australopithecus and the final two million span the history of the genus Homo in the Paleolithic era.
Archaic humans is a broad category denoting all species of the genus Homo that are not Homo sapiens. Among the earliest modern human remains are those from Jebel Irhoud in Morocco, Florisbad in South Africa (259 ka), and Omo-Kibish I in southern Ethiopia. Some examples of archaic humans include H. antecessor (1200–770 ka), H. bodoensis (1200–300 ka), H. heidelbergensis (600–200 ka), Neanderthals, H. rhodesiensis (300–125 ka) and Denisovans,
In physical anthropology, post-orbital constriction is the narrowing of the cranium (skull) just behind the eye sockets found in most non-human primates and early hominins. This constriction is very noticeable in non-human primates, slightly less so in Australopithecines, even less in Homo erectus and completely disappears in modern Homo sapiens. Post-orbital constriction index in non-human primates and hominin range in category from increased constriction, intermediate, reduced constriction and disappearance. The post-orbital constriction index is defined by either a ratio of minimum frontal breadth (MFB), behind the supraorbital torus, divided by the maximum upper facial breadth (BFM), bifrontomalare temporale, or as the maximum width behind the orbit of the skull.
There is much to be discovered about the evolution of the brain and the principles that govern it. While much has been discovered, not everything currently known is well understood. The evolution of the brain has appeared to exhibit diverging adaptations within taxonomic classes such as Mammalia and more vastly diverse adaptations across other taxonomic classes. Brain to body size scales allometrically. This means as body size changes, so do other physiological, anatomical, and biochemical constructs connecting the brain to the body. Small bodied mammals have relatively large brains compared to their bodies whereas large mammals have a smaller brain to body ratios. If brain weight is plotted against body weight for primates, the regression line of the sample points can indicate the brain power of a primate species. Lemurs for example fall below this line which means that for a primate of equivalent size, we would expect a larger brain size. Humans lie well above the line indicating that humans are more encephalized than lemurs. In fact, humans are more encephalized compared to all other primates. This means that human brains have exhibited a larger evolutionary increase in its complexity relative to its size. Some of these evolutionary changes have been found to be linked to multiple genetic factors, such as proteins and other organelles.
Homo erectus is an extinct species of archaic human from the Pleistocene, with its earliest occurrence about 2 million years ago. Its specimens are among the first recognizable members of the genus Homo.
Paleoneurobiology is the study of brain evolution by analysis of brain endocasts to determine endocranial traits and volumes. Considered a subdivision of neuroscience, paleoneurobiology combines techniques from other fields of study including paleontology and archaeology. It reveals specific insight concerning human evolution. The cranium is unique in that it grows in response to the growth of brain tissue rather than genetic guidance, as is the case with bones that support movement. Fossil skulls and their endocasts can be compared to each other, to the skulls and fossils of recently deceased individuals, and even compared to those of other species to make inferences about functional anatomy, physiology and phylogeny. Paleoneurobiology is in large part influenced by developments in neuroscience as a whole; without substantial knowledge about current functionality, it would be impossible to make inferences about the functionality of ancient brains.
Neanderthals are an extinct group of archaic humans who lived in Eurasia until about 40,000 years ago. The type specimen, Neanderthal 1, was found in 1856 in the Neander Valley in present-day Germany.
Neanderthal anatomy differed from modern humans in that they had a more robust build and distinctive morphological features, especially on the cranium, which gradually accumulated more derived aspects, particularly in certain isolated geographic regions. This robust build was an effective adaptation for Neanderthals, as they lived in the cold environments of Europe. In which they also had to operate in Europe's dense forest landscape that was extremely different from the environments of the African grassland plains that Homo sapiens adapted to with a different anatomical build.
The neuroscience of sex differences is the study of characteristics that separate brains of different sexes. Psychological sex differences are thought by some to reflect the interaction of genes, hormones, and social learning on brain development throughout the lifespan.
The history of anthropometry includes its use as an early tool of anthropology, use for identification, use for the purposes of understanding human physical variation in paleoanthropology and in various attempts to correlate physical with racial and psychological traits. At various points in history, certain anthropometrics have been cited by advocates of discrimination and eugenics often as part of novel social movements or based upon pseudoscience.
Homo naledi is an extinct species of archaic human discovered in 2013 in the Rising Star Cave system, Gauteng province, South Africa, dating to the Middle Pleistocene 335,000–236,000 years ago. The initial discovery comprises 1,550 specimens of bone, representing 737 different skeletal elements, and at least 15 different individuals. Despite this exceptionally high number of specimens, their classification with other Homo species remains unclear.
We find that the adult male human brain contains on average 86.1 ± 8.1 billion NeuN-positive cells ("neurons") and 84.6 ± 9.8 billion NeuN-negative ("nonneuronal") cells. [...] These findings challenge the common view that humans stand out from other primates in their brain composition and indicate that, with regard to numbers of neuronal and nonneuronal cells, the human brain is an isometrically scaled-up primate brain.
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