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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 evolutionary history of the human brain shows primarily a gradually bigger brain relative to body size during the evolutionary path from early primates to hominins and finally to Homo sapiens. This trend that has led to the present day human brain size indicates that there has been a 2-3 factor increase in size over the past 3 million years ...
For 28 modern human specimens from 190 to 25 thousand years ago, average brain volume was estimated to have been about 1,478 cc (90.2 cu in), and for 13 Cro-Magnons about 1,514 cc (92.4 cu in). In comparison, present-day humans average 1,350 cc (82 cu in), which is notably smaller.
The sequence of human evolution from Australopithecus (four million years ago) to Homo sapiens (modern humans) was marked by a steady increase in brain size. [264] [265] As brain size increased, this altered the size and shape of the skull, [266] from about 600 cm 3 in Homo habilis to an average of about 1520 cm 3 in Homo neanderthalensis. [267]
The brains of most species of Australopithecus were roughly 35% of the size of a modern human brain [39] with an endocranial volume average of 466 cc (28.4 cu in). [13]
For example, the average cranial capacity for Australopithecines is 440 cc, and the post-orbital constriction index is 0.66. [1] [5] However, with the evolutionary change in brain size in Australopithecines to the Homo genus, the average cranial capacity for Homo Habilis is 640 cc, and the post-orbital constriction index is 0.72.
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While decreased brain size has strong correlation with lower intelligence in humans, some modern humans have brain sizes as small as with Homo erectus but normal intelligence (based on IQ tests) for modern humans. Increased brain size in humans may allow for greater capacity for specialized expertise. [113]