At one time, students of human evolution viewed the large brain as the premier human characteristic and assumed that it had to have evolved first. Subsequent fossil finds, such as Australopithicus, which were bipedal with small brains, made it clear that bipedalism came first and that big brains evolved gradually over a fairly long period of time. It is, however, not just large brains that allow humans to think differently than other species.
With regard to brain size, Francisco Ayala writes in his chapter in The Epic of Evolution: Science and Religion in Dialogue:
“Brain size is generally proportional to body size; relative to body mass, humans have the largest (and most complex) brain.”
Human brain complexity involves bilateralism (the division of the brain into two hemispheres) and the large frontal lobes. In his book The Brain: A Very Short Introduction, Michael O’Shea writes:
“If we trace the evolution of the human brain, the greatest and most rapid growth has occurred in the frontal lobes of the cortex, which accounts for some 40 per cent of the structure.”
In comparing human brain size with that of our relatives, the chimpanzee, Matt Ridley, in his book Nature via Nurture: Genes, Experience, and What Makes Us Human, reports:
“Almost everything in the human brain is bigger than its counterpart in the chimpanzee brain. Human beings apparently do more seeing, more feeling, more moving, more balancing, more remembering, and even more smelling than chimps.”
The large size of human brains also allows for an expansion of thought. William Howells, in an essay reprinted in The Human Way:Readings in Anthropology, writes:
“For a purely animal existence, our large brains are a luxury rather than a necessity. While other creatures can retain sensory and motor nervous patterns—i.e. do a certain amount of remembering—the plenteousness of our own brains not only make them act as much vaster storehouses but also allows us to think about things we have never actually experienced.”
Humans have more of their brains devoted to emotional areas than do chimpanzees. In an article in Free Inquiry, Armin Geetz writes:
“…the expansion of the emotional areas through several million years gave the first humans a wider palette of emotions allowing for such emotions and feelings such as love, awe, and delight. But they also allowed the emergence of guilt and shame, two powerful societal mechanisms.”
Some researchers feel that the evolution of the large brain in humans may have been driven, at least in part, by symbolic competence which involves the ability to conceive of relations which are not directly associated with the physical world. Both language and religion are closely associated with symbolic competence. Furthermore, symbolic competence seems to have come about in connection with tool use which requires not only an increase in the capacity for working memory but also other cognitive procedures, such as planning.
Large brains require lots of calories. In chimpanzees, only about 8% of the energy budget goes to the brain. In australopithecines, our bipedal, small-brained ancestors, this increases to 11% and in early humans (Homo erectus) it increases to 17%. In modern humans, about 20% of our energy budget is devoted to the brain. J.M. Adovasio, Olga Soffer, and Jake Page, in their book The Invisible Sex: Uncovering the True Roles of Women in Prehistory, write:
“To put it another way, the human brain consumes 16 times as much energy as muscle for each unit of weight.”
At birth, about 60% of the newborn’s fuel intake is required to support the energy needs of the brain.
The energy requirements of the large brain comes from the need to grow the brain—most of human brain growth occurs after birth and it is estimated that it takes 25-30 years to fully develop—and from the energy needed to maintain it. With regard to the maintenance of the large brain, Clive Gamble, John Gowlett, and Robin Dunbar report:
“This mainly has to do with the costs of keeping the neurons in a state of readiness to fire—a process that involves clearing the debris of neuronal activity out of the system, producing new neurotransmitters to allow the neuron to fire again, and maintaining the electrical charge across the neural membrane through the so-called ‘sodium pump’. The cost of maintaining neuronal readiness is roughly ten times that required for muscles to work, and the costs of actually firing the neurons (i.e. using your brain!) is considerably higher. Such expense also means that brains work best when supplied with high-quality foods.”
In evolutionary terms, the increased size of the human brain required new food sources; food sources which were richer in nutrients and calories. This new food source appears to have been meat. J.M. Adovasio, Olga Soffer, and Jake Page write:
“And, the logic goes, the larger proportion of the overall diet that meat became, the more brains could grow; the more brains could grow, the more complex social organization became and the more effective people became at exploiting rich food resources.”
The archaeological record at the present time suggests that hominins began to consume meat by about 2.4 million years ago. Prior to this time, the diet was based on plant foods. Early consumption of meat was probably not from hunting but from scavenging dead animals killed by other predators. It is not just meat alone, however, that allowed the growth of the human brain, but also food processing in the form of cooking. Daniel Lieberman, in his book The Story of the Human Body: Evolution, Health, and Disease, writes:
“…by incorporating more meat in the diet, and relying more on food processing, early Homo was able to spend less energy on digesting food and thus could devote more energy toward growing and paying for a larger brain.”
As a result, today’s humans have large brains and small guts.
Lieberman sees a correlation between a hunting and gathering way of life and the increase in brain size:
“Then, once hunting and gathering became successful enough to make more energy available, this way of life permitted selection for the evolution of larger brains. It is not coincidental that major increases in brain size occurred after the origins of hunting and gathering.”
With regard to brains, and more importantly, to the process of thought that goes on within the brains, Thomas Suddendorf writes in his book The Gap: The Science of What Separates Us from Other Animals:
“As it stands, however, it is not clear what it is about our brains that causes our minds to be special. The study of the brains of different primates has not clarified what their minds are capable of and what their limits are.”
In comparing modern human brains with those of other primates, it would appear that it is not just the larger size that makes humans distinct, but it is perhaps the architecture of the brain that allows humans to think differently. In her comparison of apes and humans in the Handbook of Human Symbolic Evolution, Kathleen Gibson writes:
“The human brain, therefore, is a more cortically dominated brain than that of the ape, and the human neocortex is more heavily fissured and exhibits more complex interconnectivity.”
Kathleen Gibson also reports:
“When enumerated in detail, these changes seem to be of a major magnitude, and readily lead to the conclusion that the human brain is a very different brain indeed from that of the ape.”
In addition to the large brain, human brains are distinguished by laterality: the brain has two halves which perform different functions. Human handedness—the fact that most people are right-handed—is related to laterality. In their book Thinking Big: How the Evolution of Social Life Shaped the Human Mind, Clive Gamble, John Gowlett, and Robin Dunbar write:
“This handedness may have arisen from the brain’s need for concentration in complex tasks. Dividing them between two hands and two hemispheres may have been too high a demand for the millisecond timing and fast responses that are required.”
With regard to archaeology, and particularly paleoarchaeology, the laterality of the brain can be seen in the archaeological record in the way stone tools were made. Clive Gamble, John Gowlett, and Robin Dunbar report:
“The striking of the blows in stone working tends to reflect a right-handed or left-handed pattern of working, and shaped tools too may have the working edge facing a preferred direction. All the major studies carried out so far strongly suggest population right-handedness, whether in modern humans, Neanderthals, or earlier Homo.”
The large brain is also important with regard to the human capacity for language. Looking at the design features that distinguish language from animal communication systems, Katherine Pollard, in a chapter in Becoming Human: Our Past, Present and Future, writes:
“Most of what distinguishes human language from vocal communication in other species, however, comes not from physical means but cognitive ability, which is often correlated with brain size.”
However, brain size alone is not the key to the evolution of the capacity for language. In his book The Language Instinct: How the Mind Creates Language, Steven Pinker writes:
“All the evidence suggests that it is the precise wiring of the brain’s microcircuitry that makes language happen, not gross size, shape, or neuron packing.”
Another possible function of brain size is the hypothesis that larger brains make larger groups possible. In his book Human Evolution, Robin Dunbar writes:
“Brain size determines social group size as a response to environmental conditions, and group size and environmental conditions between them impose demands on time budgets that must be satisfied if a new evolutionary step change is to be possible.”
It is easy to assume that anatomically modern humans are more intelligent than their ancient ancestors, but to assume that intelligence is based on overall brain size does not appear to be correct. Rita Carter, in her book The Human Brain, reports:
“The size of a person’s brain bears little relation to his or her intelligence, and every brain, whatever its weight and volume, has roughly the same number of neurons and synapses.”
More Human Origins
Human Origins: The Human Hand
Human Origins: Bipedalism
Human Origins: Sexual Selection
Ancient Africa: Australopithecus
Ancient Africa: Homo Naledi
The Ancient World: An Overview of Homo Erectus
Ancient World: A Short Overview of Homo Heidelbergensis