The awakening of consciousness on this planet, and especially of self awareness, coincides with the development of bigger brains. Compared with other mammals, primates such as chimpanzees and orang-utans have huge brains. And compared with our primate ancestors, the human brain is larger still. In the two million years since the Homo genus emerged, our brain has tripled in size over that of our hominid ancestors, and doubled in size over that of the first Homo species, Homo habilis.
Brain size is relative to body size and doesn’t necessarily reflect the creature’s level of intelligence. Whales and elephants have brains many times the size of ours, but don’t come close to us in terms of cognitive ability. So scientists calculate instead what they call the "encephalization quotient" (or EQ). It is the ratio of actual brain weight to the expected brain weight of a typical animal of that size. An EQ of 1 means that the ratio of the animal’s brain weight to body weight is typical for animals of comparable size. Here are some examples:
Homo sapiens 7.44
Dolphins 5.31
Chimpanzees 2.49
Rhesus monkeys 2.09
Elephants 1.87
Whales 1.76
Dogs 1.17
Cats 1.00
Sheep 0.81
Mice 0.50
Rats & Rabbits 0.40
This means that our human brain is 7.44 times larger than the average for an animal of our size, while the brain of a mouse is just half the size of what would be the average for an animal of its size.
EQ is not the same as IQ, but it does seem to vary with the level of intelligence a species requires to survive in its particular niche. Leaf-eating creatures surrounded by food, for example, require less intelligence, and hence a relatively smaller brain, than insect eaters that need intelligent strategies to capture their prey. Predator species generally have higher EQs than prey species. Apparently, the more a species needs to think, the larger is its relative brain size.
Social animals, especially those who live in highly interactive groups, also tend to have higher EQs. A recent survey of 38 genera, including gorillas, chimpanzees and humans, showed that species living in larger groups have larger cerebral cortices. It seems that larger groups require greater cognitive capacity in order to maintain cohesion in their complex social networks.
The expansion of the brain’s relative size is due largely to the evolution of a cerebral cortex – that layer of grey matter that covers both hemispheres of the cerebellum. A distinguishing feature of mammals, it expanded by 7% every ten million years during the last 65 million years of the Cenozoic Era. Today, among living mammals, an average of 57% of the brain’s surface area is devoted to neo-cortex. The increase was most dramatic among primates. In living monkeys and living and fossil hominids, the ratio averages 75%, and in humans it rises to a staggering 80%.
The neo-cortex is the centralised processing system that integrates information supplied by a mammal’s senses to create a picture of its world. As more sophisticated sensory receptors and a more complex cerebral cortex evolved, so too did the brain’s power to symbolically represent the world. A more and more detailed mental picture of the world gradually emerged, until eventually there arose the very elaborate worldviews that frame our human perception of the world.
What triggered the invention of this neo-cortex? We don’t know for sure. But it evolved in tandem with endothermy – the ability of mammals to maintain a constant body temperature. While this had huge survival value, it was also very costly. A mammal requires 5-10 times more energy than a reptile of similar body size just to maintain its body temperature. And brains themselves are energy gluttons, consuming up to 20% of the available energy. Meeting these energy requirements demands much greater efficiency in food gathering, which in turn requires a more detailed mental picture of the environment. Enter the cerebral cortex with its talent for constructing mental maps! – the talent that has exploded virtually without limit in Homo sapiens.
Still another implication of large brains is the anatomical fact that the birth canal of females who walk upright is not large enough to deliver a baby with a full-grown brain. Although a wider birth canal had already developed in Homo erectus, there is still a limit to how large the head of a human foetus can be and still pass through the birth canal. The evolutionary solution was to delay the full development of the human brain until after birth. Only when a baby is a year old does its brain growth slow, and even then it doesn’t stop. The brains of teen-agers are still a work in progress. The frontal lobes – which contribute judgment, inhibition, and self-awareness – may not be fully connected until as late as age 25. No other primate has this pattern of brain growth and such a prolonged period of childhood dependency, requiring a massive investment of shared parental care.
All of this provides the seedbed of human culture. Large brains, a burgeoning neo-cortex, heightened awareness, elaborate mental maps of the world, and a prolonged childhood during which we learn and absorb a shared worldview – all these come together to make human culture possible. All of these are correlated. But can we ever say what causes what? Correlation is not causation! The physical height and reading abilities of 5 year-olds and 10-year-olds correlate extremely well, but one has nothing to do with the other. So did endothermy trigger the development of larger and more complex brains? Or was it our coming together into more complex social networks? Was our greatly expanded neo-cortex the cause of human consciousness? And is human comnsciousness the cause of human culture with its shared worldviews? Perhaps. Or perhaps all of this, including the Big Brain Experiment itself, is simply the manifestation of evolution’s playful meandering towards greater complexity and heightened awareness.
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