The Evolution of The Brain From The Primate Line
Created | Updated Jan 21, 2005
The major orders of mammals developed from primitive mammal-like reptiles during the Triassic period and further differentiated via adaptive radiation during the Tertiary period.
The primate order, to which humans belong, includes prosimians (lemurs, lorises and tarsiers) and simians. The latter further separates into new world and old world species.
Old World species fall into two categories; old world monkeys and hominoids (great apes, gibbons and humans!), and show several major features, namely a grasping hand, nails replacing claws, stereoscopic vision and larger brains.
Two million years ago, five recognised genera of hominid species evolved in response to climate changes associated with the beginning of the Ice Age. These included Ardipithecus, Australopithics, Kenyanthropus, Paranthropus, and Homo. Two of these groups predominated; the Australopithecines (smaller brained) and the line that led to the genus Homo (larger brained).
In the last 3-4 million years, the human brain has increased in size nearly two-fold, from 400mL to 1400mL, without any apparent change in body size. Humans are at the very top of the encephalisation index (relative size of the brain when compared to the body) and there are large differences between the encephalisation of modern humans and all present-day non-human primates (including the great apes, our nearest ancestors). Possible reasons for this difference in index include the fact that the human brain’s metabolic budget is significantly different from that of apes. Anthropoid primates use ~8% of resting metabolism for the brain whereas humans use an impressive 25% of resting metabolism for the brain.
The shape of the human skull changes dramatically throughout history to accommodate a forebrain etc. By looking at fossil endocasts, a comparison of each of the four lobes (frontal, temporal, parietal and occipital) will reveal ape-like or human-like organisation. A brain in which the parietal and temporal lobes predominate is human-like, whereas in ape-like brains these structures are similar. The frontal lobes of human brains are also considerably more convoluted than ape brains. It is interesting to note that brain enlargement is actually a disproportional process, with by far the most progressive structure being the neocortex.
Another noteworthy and interesting fact is that the neandertals actually had bigger brains than us, but the organisation of their brain may have been different to ours.
With regard to brain re-organisation, left-right cerebral hemispheric asymmetries are evident in the australopithecines, but neither the pattern nor direction is as strongly developed as in modern Homo. The degree of asymmetry appears to increase in later hominids. The appearance of a more human-like third inferior frontal convolution also provides another line of evidence for evolutionary reorganisation of the brain.
Larger brains required other physical adaptations, the most notable being a large pelvic opening in females to accommodate the increased brain size at birth.
The most logical explanation for an increased cranial capacity is that additional neurons are needed in order to deal with an increased amount of body mass; meaning the largest animals must have the largest brains. It is almost certain that to account for this evolutionary change, human ancestors must have found themselves in a stable environment with a high-energy food supply and minimum predation pressure. Selection pressures which drove the evolution of the brain are most likely to be accounted for by the type of behaviour a bigger and better brain made possible. It is not the bigger brain that was selected but the behaviour patterns made possible by this larger brain.
The primate order, to which humans belong, includes prosimians (lemurs, lorises and tarsiers) and simians. The latter further separates into new world and old world species.
Old World species fall into two categories; old world monkeys and hominoids (great apes, gibbons and humans!), and show several major features, namely a grasping hand, nails replacing claws, stereoscopic vision and larger brains.
Two million years ago, five recognised genera of hominid species evolved in response to climate changes associated with the beginning of the Ice Age. These included Ardipithecus, Australopithics, Kenyanthropus, Paranthropus, and Homo. Two of these groups predominated; the Australopithecines (smaller brained) and the line that led to the genus Homo (larger brained).
In the last 3-4 million years, the human brain has increased in size nearly two-fold, from 400mL to 1400mL, without any apparent change in body size. Humans are at the very top of the encephalisation index (relative size of the brain when compared to the body) and there are large differences between the encephalisation of modern humans and all present-day non-human primates (including the great apes, our nearest ancestors). Possible reasons for this difference in index include the fact that the human brain’s metabolic budget is significantly different from that of apes. Anthropoid primates use ~8% of resting metabolism for the brain whereas humans use an impressive 25% of resting metabolism for the brain.
The shape of the human skull changes dramatically throughout history to accommodate a forebrain etc. By looking at fossil endocasts, a comparison of each of the four lobes (frontal, temporal, parietal and occipital) will reveal ape-like or human-like organisation. A brain in which the parietal and temporal lobes predominate is human-like, whereas in ape-like brains these structures are similar. The frontal lobes of human brains are also considerably more convoluted than ape brains. It is interesting to note that brain enlargement is actually a disproportional process, with by far the most progressive structure being the neocortex.
Another noteworthy and interesting fact is that the neandertals actually had bigger brains than us, but the organisation of their brain may have been different to ours.
With regard to brain re-organisation, left-right cerebral hemispheric asymmetries are evident in the australopithecines, but neither the pattern nor direction is as strongly developed as in modern Homo. The degree of asymmetry appears to increase in later hominids. The appearance of a more human-like third inferior frontal convolution also provides another line of evidence for evolutionary reorganisation of the brain.
Larger brains required other physical adaptations, the most notable being a large pelvic opening in females to accommodate the increased brain size at birth.
The most logical explanation for an increased cranial capacity is that additional neurons are needed in order to deal with an increased amount of body mass; meaning the largest animals must have the largest brains. It is almost certain that to account for this evolutionary change, human ancestors must have found themselves in a stable environment with a high-energy food supply and minimum predation pressure. Selection pressures which drove the evolution of the brain are most likely to be accounted for by the type of behaviour a bigger and better brain made possible. It is not the bigger brain that was selected but the behaviour patterns made possible by this larger brain.
