We diverged from our last common ancestor with chimpanzees and bonobos about 6 to 8 million years ago. For around 60 million preceding years, since our divergence from the rest of the mammalian line around the time of the extinction of the dinosaurs, our lineage had lived as tropical primates largely similar to those alive today. Humans are markedly different from the rest of our primate relatives, however. In a very short time, we were able to colonize virtually every habitat on the earth amenable to life. We were able to do this due to our unique ability to develop and transmit cultural ideas which allowed us to rapidly accumulate the technological abilities needed to behaviorally adapt to radically new environments. Cultural adaptation allowed us to shortcut the millions of years that would otherwise have been required for natural selection to adapt our body and behavior to the radically different environments that we came to inhabit.
Since our last common ancestor with the chimpanzee and bonobo line (who themselves diverged only about a million years ago), our brain has undergone a drastic growth. Although our brains seem to contain all of the same stuff as the brains of other primates, it is now about 3x the expected size for our body weight based on brain/body ratios of living primates. We have also developed technologies that far exceed those of of the chimpanzees and bonobos, who’s tool-kits are impressive when compared against other animal species but scarcely compare our own.
We might expect then, as many have, that our larger brains have led to greater intelligence across-the-board. This seems to be supported by our technological history when compared to that of chimps and bonobos. However, cognitive evolution should be expected to adapt a species to be able to perform the kinds of tasks that are important in evolutionary terms. In the time in which our brains were developing into the unusual form that they now display, we were living in a habitat similar to those of the chimpanzee/bonobo line. We both had to forage effectively, avoid predation, and navigate our social structures. Most of our cognitive resources did not need to exceed those of chimpanzees and bonobos, who are already highly intelligent.
Our intelligence rests on a large cultural endowment. None of us could ever have invented all of our technology in a lifetime, or even the limited technology of a small group of hunter-gatherers. Most of our technological advancement relies on an even more fundamental “cognitive technology,” such as language, numerical and measurement systems, and navigational concepts such as the cardinal directions. Without having been born into societies with these cognitive technologies, developing complex physical technologies would be next to impossible. This has led many researchers to argue that our superior general intelligence is an off-shoot of our advanced cultural cognition, rather than that our cultural intelligence is an off-shoot of a superior general intelligence. Without our cultural endowments, we would be, in most ways, cognitively identical to the apes.
Our large brain aside, what does differ about human cognition? What certainly seems to be different are the mechanisms by which we absorb our culture: our social cognition. Humans, chimpanzees, and bonobos all live in large social groups. Humans, however, are ultra-social. We live in complex societies on a scale that chimpanzees and bonobos would be unable to navigate. We are also highly adept at inferring the mental content of other humans.
There is some interesting recent research that suggests that chimpanzees have some knowledge of the mental processes of others. It seems, for example, that chimpanzees are aware of eachothers perceptual knowledge, such as whether another individual is able to see a certain item of food. Our social cognition exceeds that which we observe in any ape, however. For example, we can know what others know about what we know that they know. We are also highly skilled imitators. The common knowledge is “monkey see, monkey do,” but this is not justified empirically. Monkeys and apes are good at emulating behaviors by figuring out the intention of another individual as they perform a task and then inventing their own way of achieving the same result. They are very poor, however, at directly copying another’s specific actions. In the 1960’s and ’70’s, several families thought that if they raised infant chimpanzees in their homes alongside their own children, the chimpanzees would imitate human behaviors. What they found instead was that their children came to imitate the chimps.
This is known as the cultural intelligence hypothesis, presented by Herrmann et al. (2007). They propose that humans exceed apes in their social cognition, but are otherwise similar. To test this, they compared the performance of humans, chimpanzees, and orangutans on the Primate Cognition Test Battery (PCTB). The PCTB uses 16 simple cognitive experiments to assess cognitive abilities in both the physical domain – space, quantity, and causality – and in the social domain – social learning, communication, and theory of mind. They argued that if the human intelligence is generally higher, then we should expect to see humans out-perform the apes in most tasks. However, if the cultural intelligence hypothesis is true, we should expect that humans will be similar in the physical domains and superior in the social domains.
One difficulty that they had to face, however, was that humans have the cultural inheritance described above, which could easily give them an edge in many of the tasks. To get around this, they used 2.5 year old children, in comparison to a range of ages of chimps and orangutans. This has led to some controversy, as some did not find pitting 2.5 year olds against adult apes was a fair comparison. The tasks, however, were fairly straightforward and seem to me to be well within the expected abilities of toddlers. Herrmann et al. (2007) found that the data supported the cultural intelligence hypothesis. All three species performed comparably in the physical domains, but human children vastly outperformed the apes in the social domain.
(Fig. 1, for each domain. [Herrmann et al. 2007])
(Fig. 2, Summarized to physical and social domains. [Herrmann et al. 2007])
In another recent study, Schmitt et al. (2012) applied the PCTB to olive baboons and long-tailed macaques and found that their results were both similar to those of chimpanzees and orangutans in the previous study.
(Fig. 3, Comparisons of all four species. [Schmitt et al. 2012])
The hypothesis obviously needs more experimental support. However, the results so far at hand suggest a radically different idea of primate cognitive evolution than has been generally held before. Our species tends to hubristically self-congratulate on the basis of our superior intelligence. If that is proven to be a self-delusion, if our superior intelligence is only the accumulated minor insights and creations of the millions that came before us, then we will be forced to rethink what it means to be human, and more importantly, what it means for other species to not be human.
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Hermann, 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, 1360-1366. (Link)
Schmitt, V., Pankau, B., & Fischer, J. (2012). Old World Monkeys Compare to Apes in the Primate Cognition Test Battery. PloS ONE 7, e32024. (Link)