Hermann et al. (2007) Humans evolved specialized skills of social cognition

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.

• Humans have many cognitive skills not possessed by their nearest primate relatives. The cultural intelligence hypothesis argues that this is mainly due to a species-specific set of social- cognitive skills, emerging early in ontogeny, for participating and exchanging knowledge in cultural groups. 
• Gave a comprehensive battery of cognitive tests to large numbers of chimpanzees and orangutans, as well as to 2.5-year-old human children before literacy and schooling. 
• Supporting the cultural intelligence hypothesis and contradicting the hypothesis that humans simply have more “general intelligence,” we found that the children and chimpanzees had very similar cognitive skills for dealing with the physical world but that the children had more sophisticated cognitive skills than either of the ape species for dealing with the social world.

Adult humans have lots of cognitive skills that apes don't, but this is largely due to children's early emerging, specialized skills for absorbing the accumulated skillful practices and knowledge of their social group. 

Humans' especially powerful skills of social-cultural cognition early in ontogeny thus serve as a kind of "bootstrap" for the distinctively complex development of human cognition in general." 

The hypothesis predicts that there should be an age in early human ontogeny at which humans' skills of physical cognition (concerning e.g., space, quantities, causality) are very similar to our nearest primate relatives, but at which their skills of social-cultural cognition (specifically those most directly involved in cultural creation and learning, e.g. social learning, communication, theory of mind) are already distinctively human. 

Using the Primate Cognition Test Battery, which both physical cognition and social cognition components, they showed that the 3 species did not differ on physical cognition but did on social cognition. 

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The authors note that it is possible that what is distinctively human may not be social-cultural cognition as a specialized domain, but rather the ability to understand unobserved causal forces in general, including, as a special case, the mental states of others. Even here, though, it is plausible that this ability first evolved to enable humans to understand the mental states of others, and this generalized only later to the physical domain.