The cause of the rapid evolutionary growth in hominid brain size remains a mystery and a major point of contention among anthropologists. (Hominids are humans and human-like primates). Our brains weigh roughly twice as much as those of our similarly-sized earliest human relative, Homo habilis, which lived two million years ago. Also, humans have extraordinarily large and complex brains compared with non-human modern primates. The human brain is several times larger than that of the macaque monkey – even after correcting for body size – and it is far more complicated in terms of structure. Although humans weigh about 20 percent more than chimpanzees, our closest living relative, the human brain weighs 250 percent more. And keep in mind that a huge brain is a serious investment – neural tissue guzzles a lot of energy.
Two main hypotheses have been proposed to explain why humans have evolved larger brains than their primate relatives:
- The general intelligence hypothesis suggests that bigger brains make humans better and faster at all kinds of cognitive skills, such as memorizing, learning, and planning ahead. In other words, humans differ from apes uniformly across physical and social cognitive tasks because they have greater general intelligence. Physical skills involve understanding concepts of space, quantities, and causality. Social skills involve understanding nonverbal communications, imitating another’s solution to a problem, and understanding that other individuals have their own beliefs and intentions. For example, biting and trying to break a plastic tube to retrieve the food inside demonstrates a physical skill, while following another’s example to pop open the tube to retrieve the food demonstrates a social skill.
- The cultural (or social) intelligence hypothesis says that bigger brains have enabled humans to develop, in particular, more complex social cognitive skills to interact in cultural groups.
One way to distinguish between these two hypotheses is to compare the cognitive abilities and skills of humans with other non-human primates. If the general intelligence hypothesis is true, then we expect to see a difference between humans and apes in both physical and social skills. If the cultural intelligence hypothesis is true, then we expect to see a difference primarily in social skills.
This experiment is exactly what was performed by Esther Herrmann and her colleagues of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and recently reported in the September 7 issue of the journal Science. They put 105 young German children (Homo sapiens), 106 chimpanzees (Pan troglodytes), and 32 of the more evolutionarily distant orangutans (Pongo pygmaeus) through a series of complex tests. The children were all about 2.5 years old, an age when they have about the same physical skill level of chimpanzees, and had been speaking for at least a year. The apes ranged in age from 3 to 21 and had all been made accustomed to humans. The researchers designed 16 different puzzles to tease out the differences in ability between humans and apes. The tests took between three and five hours and were spread between five and eight days over two weeks. The apes were tested in the sanctuaries where they live in Africa and Indonesia.
The results found that chimpanzees, human children, and orangutans were all equally successful in the physical skills tests. But the human children were significantly better at the social skills tests – scoring around 74 percent correct on the tests compared to scores of 33 percent from both groups of apes.
The findings support the cultural intelligence hypothesis but contradict the general intelligence hypothesis. Joan Silk, an anthropologist at the University of California, Los Angeles, who was not involved in the study, observed that “compared [with] baboons we waste an awful lot of time gossiping about one another.” Aside from gossiping, these increased social skills appear to carry strong evolutionary advantages, enabling humans to sustain relationships with others and help each other out in times of need. A growing body of evidence suggests that the quality of social relationships has measurable fitness consequences for individuals. However, this may have come with some hidden costs. Silk commented that “the human brain is a really complicated machine that goes wrong with some frequency. Mental illness may be the evolutionary cost of this complexity.”
- Herrmann, E., Call, J., Hernandez-Lloreda, M.V., Hare, B., Tomasello, M. (2007). Humans Have Evolved Specialized Skills of Social Cognition: The Cultural Intelligence Hypothesis. Science, 317(5843), 1360-1366. DOI: 10.1126/science.1146282
- Silk, J.B. (2007). Social Components of Fitness in Primate Groups. Science, 317(5843), 1347-1351. DOI: 10.1126/science.1140734