Feb 6. Human brains have shrunk over the past 30,000 years . . . about 10 percent during that period -- from 1,500 to 1,359 cubic centimeters, the size of a tennis ball. Women's brains, which are smaller on average than those of men, have experienced an equivalent drop in size. These measurements were taken using skulls found in Europe, the Middle East and Asia.
"I'd called that a major downsizing in an evolutionary eye blink," John Hawks of the University of Michigan told Discover magazine.
The hypothesis that big brains imparted a selective advantage to help hominids cope with complex and new situations sounds reasonable, since as Alphonso Lingis notes, ‘Most species do not evolve bigger brains because brains put heavy demands on nutrients, oxygen, and energy.’ [i] But does size matter? Human brain size doesn’t correlate to any of the intelligences. Einstein’s brain, a fetishised object, was disappointingly smaller than average), however, when researchers recently looked at Einstein’s preserved brain they found unusually well developed parietal regions on both sides and also a high number of transmission cells known as glea cells It is now thought they are much more important than just assisting nerve cells. [ii]
Evidence suggests that brain-to-body ratio measures information-processing ability - an evolutionary trend. Hominid brain size tripled from ape brain cc from 2·5 million years ago, possibly in several bursts. Hominid brain size tripled from ape brain cc from 2·5 million years ago, possibly in several bursts. Harry Jerison’s data on the evolution of brain and intelligence compares the brain weight/volume to body weight/volume for living mammals, fossil mammalian ungulates, and for living and fossil reptiles. There is evidence of a progressive trend of brain enlargement across phylogenetic lines. The growth in brain size from Australopithecus africanus to Homo habilis to Homo erectus, and early Homo sapiens appears continuous. This suggests three million years of persistent brain expansion.[iii] The innovations of language and culture required more than large brains. Robert Logan reminds us that Neanderthals had a larger brain capacity yet inferior cognitive skills due to inferior information processing, which demonstrates just how important language is.[iv] However, overall increases in hominid brain size had stopped by 1.2m years ago, when a few African hominids had the same brain cc as modern humans. Other African ancestors stopped increasing brain cc perhaps 200,000 years ago, and brains have been shrinking slowly since. So the question is why, during the most rapid period of brain cc growth, cultural development (as measured by stone tools) hardly changed - then, over a million years later, in the cultural explosion of early humans, brains actually started getting smaller? The answer is that the brain’s internal structures are plastic and uses aids to help us think and act, through cultural bricolage.
Various other mechanisms have been suggested for the revolution in cognitive abilities, from the original hypothesis of tool use; from food source complexity (Katy Milton)[v]; social life (Ken Richardson),[vi] (Derek Bicketon[vii]), and (Robin Dunbar[viii]) to complexity of the natural environment (Paul Shepard[ix]). Lingis argues for the latter two, noting that, ‘Current empirical evidence has dissociated the links between bipedalism, tool production, and brain size.’[x] Lingis, following Shepard, suggests that the constant social interaction among primates,[xi] leads to a sense of the self. [xii]
Ken Richardson observes, “Complexity of social life and increase in brain size seem to have evolved rapidly together.’ ‘Humans appear to have evolved a still further method of ‘information acquisition and storage’ - an interpersonal as well as intrapersonal one - and it is the regulation of such a system, at least partly, that our huge brains are for.”[xiii] Robin Dunbar has demonstrated a positive correlation between neo-cortical ratio (the ratio of neo-cortical volume to whole brain volume) and social group size.[xiv] Social relations are complex but so too are rituals and artistic practices where unpredictability. Human cognition is not dependent purely on brain size. Evidence suggests that brain-to-body ratio measures information-processing ability, a progressive trend in evolution.
Katy Milton suggests that spider monkeys have larger brains for their body size than howler monkeys as a consequence of adaptation to food source complexity. Howler monkeys eat mainly leaves, an abundant and predictable food source. Spider monkeys eat ripe fruit, a concentrated but unpredictable food source. Spiders live in larger social groups, because they can exploit the large fruit patches more efficiently in large groups. They also have to recognize a greater diversity of food types and locations. Spider monkeys thus need a larger mental map of their territory. Spider monkeys have much smaller and shorter guts than howlers who have a large gut, to maximise low-quality food, and spend many hours, apparently resting, with guts are working overtime.[xv] Richard Dawkins comments, “It makes some sense to argue that an animal needs less computing power to find leaves, which are abundant all around, than to find fruit, which may have to be searched for, or to catch insects, which take active steps to get away.”[xvi] She hypothesises that big brains imparted a selective advantage to help us cope with complex and new situations, a reasonable hypothesis. The brain is designed to generate appropriate action in response to information from our environments – appropriate behaviour is a vital for social animals with more complex ways of negotiating their social environments, and itself even more complex. Richardson & Boyd argue that Cenozoic trends in cognitive complexity represent adaptations to an increasingly diverse environment. They detail a correlation between environmental deterioration and brain size in various mammalian lineages.[xvii] Experimental studies show rich environments put on cortical weight in rats.[xviii]
[i] Overall increases in hominid brain size had stopped by 1.2m years ago when a few African hominids had the same brain cc as modern humans. Other African ancestors stopped increasing brain cc perhaps 200,000 years ago, and brains have been shrinking slowly since. So the question is why during the most rapid period of brain cc growth, cultural development (as measured by stone tools) hardly changed then, then over a million years later in the cultural explosion of early humans brains were actually getting smaller? The answer is internal structures being plastic and the use of aids to help us think and act, through cultural bricolage. ‘Though on average the brain constitutes but two percent of body weight in modern humans, it consumes sixteen percent of the body's energy. Bigger brains affect prenatal and postnatal life for both mother and offspring. They require more time to mature and grow. They are complex and fragile and increase the possibilities of pathologies. They can adversely interfere with other neural and behavioral systems.’ Alphonso Lingis, 2002, p3.
[ii] The brain is just a part of the ecological organism, see Chap 1, and quote from Alan Dunning & Paul Woodrow, ‘Einstein’s Brain’, http://www.ucalgary.ca/~einbrain/EBessay.htm [DL 2.5.2003]. Glial cells play an active role in establishing and maintaining the fundamental patterns of neuron circuits. They produce growth and trophic factors, playing a key role in regeneration and plasticity. Some play an active role in the formation of myelin which speeds impulse conduction. Myelinated fibers conduct more rapidly than unmyelinated fibers. Some glial cells respond to rapid repair of myelin in demyelinating diseases such as MS and ALS. Glial cells play a crucial role in immunological responses to various infections and toxic agents. Glial cells increase in number when nerve cells grow with enrichment. Marian C. Diamond Why Einstein's Brain?’ 1999, http://www.newhorizons.org/neuro/diamond_einstein.htm
[iii] Harry Jerison, The Evolution of the Brain and Intelligence, Academic Press, 1972. In each geological era, predators (carnivores) have larger relative brain-to-body ratios than their prey (ungulates). Adaptation brought about by an arms race. Jerison provides further evidence for the arms race effect by examining the fauna of South America at a time when it was an island cut off from the rest of the world. Before more modern predators arrived via the Panama land bridge, the arms race between a separate line of predators (carnivorous marsupials) and their prey produced an identical pattern of progressive evolution as evidenced by increasing brain-to-body ratios.
[iv] Robert Logan writes, ‘Speech arose primarily as a way to control information and then was used as a tool for communication... . Language and conceptual thought are dynamically linked parts of a dynamic cognitive system, the extended mind which provides an environment for their mutual development.’ Robert K. Logan, ‘The Extended Mind: Understanding Language and Thought in Terms of Complexity and Chaos Theory’, http://www.upscale.utoronto.ca/GeneralInterest/Logan/Extended/Extended.html Extending the argument from Robert K. Logan, The Fifth Language: Learning a Living in the Computer Age, Stoddart, 1995.
[v] Katy Milton, ‘Diet and primate evolution’, Scientific American, August 269:2, p86-93, 1993.
[vi] Complexity of social life and increase in brain size seem to have evolved rapidly together.’ ‘Humans appear to have evolved a still further method of ‘information acquisition and storage’ - an interpersonal as well as intrapersonal one - and it is the regulation of such a system, at least partly, that our huge brains are for.’ Ken Richardson, 1998, p86-8.
[vii] See also Derek Bickerton, ‘I chat, thereby I groom’, Nature 380, 1996, p303. Derek Bickerton, Roots of Language, Karoma, 1981; Language and Species, U of Chicago P, 1990; Language and Human Behaviour, U of Washington P, 1995.
[viii] Recent work shows a positive correlation between neo-cortical ratio (the ratio of neo-cortical volume to whole brain volume) and social group size. Robin Dunbar, Grooming, Gossip and the Evolution of Language, Faber and Faber, 1996; & 'Coevolution of neocortical size, group size and language in humans,' Behavioural and Brain Sciences, 16, 1993, p681-735.
[ix] Paul Shepard, Thinking Animals, Ed. Max Oelschlaeger. Athens: U of Georgia P, 1998. Lingis talks of complexity of savannah to follow Roger Shepard’s thesis: ‘For Shepard then it is not first manipulation that makes a succession of discrete visual patches into the continuity of objects; it is not first manipulation that extends a dimension of time in the depth of the spatial field. It is not the eye-hand correlation that is crucial for the development of our kind of perception. Instead he sees as crucial the linkage of a recent dominance of vision in previously nocturnal mammals, integrated upon cortical storage and integrative processes developed for olfactory and auditory stimuli.’ Lingis, 2002, p6. ‘The primates that left the relative monotony of the forest to advance into the complexity of the savanna evolved minds capable of greater discernment. Present-day humans are capable of discriminating 26 items—six binary dimensions.’ p7. However rainforests are very diverse.
[x] ‘Digs in Laetoli in Tanzania and in Ethiopia revealed that the earliest-known tools were found in sites in Ethiopia and east of Lake Turkana in Kenya dated around 2.4 million years ago. There is no evidence of substantial increase in brain size until 700,000 years later.’ Alphonso Lingis, 2002, p4.
[xi] ‘Monkeys and apes are not only gregarious, like the other herbivores of the savanna; they are constantly involved in interactions with one another. Since herbaceous food is at hand, their consciousness is for the most part fixed on members of their own group. Taking from one another, yielding to another, establishing and maintaining rank, play, and learning comprise a very large part of their activity. Often a manic agitation breaks out among them.’ Lingis, 2002, p10.
[xii] ‘Self-assessment is a perpetual concern: in relation to parents, siblings, competitors, and mates. A tenuous, vibrant personal status is the nucleus at the heart of all primate experience and action.’ Paul Shepard, 1998, p19.
[xiii] Ken Richardson, ‘The Origins of Human Potential: Evolution Development and Psychology’, Routledge, 1998, p86-8.
[xiv] Robin Dunbar, Grooming, Gossip and the Evolution of Language, Faber and Faber, 1996 & 'Coevolution of neocortical size, group size and language in humans', Behavioural and Brain Sciences, 16, 1993, p681-735.
[xv] Katy Milton, ‘Diet and primate evolution’, Scientific American, August 269:2, p86-93, 1993.
[xvi] Richard Dawkins, The Blind Watchmaker, New York: Norton, 1987, p189-190.
[xvii] Peter Richardson & Robert Boyd, ‘Climate, culture, and the evolution of Cognition,’ in Celia Heyes & Ludwig Huber Eds., The Evolution of Cognition, MIT, 2000. We can only guess why brains enlarged, but we know more about development changes that produced a larger brain/body ratio. See Terrence W. Deacon, The Symbolic Species, Norton, 1997; and Philip V. Tobias, The Brain in Hominid Evolution, Columbia UP, 1971.
[xviii] ‘Rats were housed in either standard conditions, impoverished conditions with few stimuli, or enriched environments with several cage and with objects to explore... the cortex of animals in the enriched condition was found to be about five percent heavier than the cortex of environmentally impoverished rats and about two percent heavier than the cortex of rats reared under standard conditions... There is some evidence that these cortical gains are due to informal learning about the environment by the animals, not to some general sensory stimulation.’ Daniel P. Kimble, Biological Psychology, New York: Rinehart & Winston, Inc., 1988, p108.