Natural selection, he argues, shapes cognitive abilities in the same way as it shapes traits such as wing length. As animals' challenges and habitats differ, so do their cognitive abilities. This idea, which he calls evolutionary cognition, has gained traction in psychology and biology in the past few decades.
For de Waal, evolutionary cognition has two key consequences. First, it is inconsistent with the concept of a 'great chain of being' in which organisms can be ordered from primitive to advanced, simple to complex, stupid to smart. Name a 'unique' human trait, and biologists will find another organism with a similar one. Humans make and use tools; so do wild New Caledonian crows (Corvus moneduloides). Humans develop cultures; so do humpback whales (Megaptera novaeangliae), which socially transmit foraging techniques. We can mentally 'time travel', remembering past events and planning for the future; so can western scrub jays (Aphelocoma californica), which can recall what they had for breakfast on one day, anticipate whether they will be given breakfast the next and selectively cache food when breakfast won't be delivered.
Furthermore, humans do not necessarily outdo other animals in all cognitive domains. Black-capped chickadees (Poecile atricapillus) store seeds in hundreds of locations each day, and can remember what they stored and where, as well as whether items in each location have been eaten, or stolen. Natural selection has favoured those prodigious feats of memory because they spell the difference between surviving winter and starving before spring. Human memory doesn't need to be as good: primates evolved in the tropics. “In the utilitarian view of biology,” de Waal argues, “animals have the brains they need — nothing more, nothing less.”
The second consequence of de Waal's view is that there is continuity across taxa. One source of continuity is based on evolutionary history: natural selection modifies traits to create new ones, producing commonalities among species with a common history. He points out that tool use is found not just in humans and chimpanzees, but also in other apes and monkeys, implying that relevant cognitive building blocks are shared across all primates. Continuity is also generated by convergent evolution, which produces similar traits in distantly related organisms such as New Caledonian crows and capuchin monkeys. De Waal opines that continuity “ought to be the default position for at least all mammals, and perhaps also birds and other vertebrates”.
...researchers are eager to understand what is distinctly human; some are driven by curiosity about how humans came to dominate the planet..Our success presumably has something to do with the emergence of a unique suite of cognitive traits...De Waal recognizes only one such trait: our rich and flexible system of symbolic communication, and our ability to exchange information about past and future. His commitment to the principle of continuity forces him to discount the importance of language for human cognition because of evidence of thinking by non-linguistic creatures. And he ignores compelling findings from linguists and developmental psychologists such as Elizabeth Spelke on the formative role of language in cognition.
A more satisfying book would leave readers with a clearer understanding of why, a few million years after our lineage diverged from the lineage of chimpanzees, we are the ones reading this book, and not them.