We always should have been suspicious of this picture, but now we have no excuse for continuing with it. In the past 30 years, research has explored the distinctive ways in which children as well as animals think, and the discoveries deal the coup de grâce to the ladder of nature. The primatologist Frans de Waal has been at the forefront of the animal research, and its most important public voice. In Are We Smart Enough to Know How Smart Animals Are?, he makes a passionate and convincing case for the sophistication of nonhuman minds.

De Waal outlines both the exciting new results and the troubled history of the field. The study of animal minds was long divided between what are sometimes called “scoffers” and “boosters.” Scoffers refused to acknowledge that animals could think at all: Behaviorism—the idea that scientists shouldn’t talk about minds, only about stimuli and responses—stuck around in animal research long after it had been discredited in the rest of psychology. Boosters often relied on anecdotes and anthropomorphism instead of experiments. De Waal notes that there isn’t even a good general name for the new field of research. Animal cognition ignores the fact that humans are animals too. De Waal argues for evolutionary cognition instead.

Psychologists often assume that there is a special cognitive ability—a psychological secret sauce—that makes humans different from other animals. The list of candidates is long: tool use, cultural transmission, the ability to imagine the future or to understand other minds, and so on. But every one of these abilities shows up in at least some other species in at least some form. De Waal points out various examples, and there are many more. New Caledonian crows make elaborate tools, shaping branches into pointed, barbed termite-extraction devices. A few Japanese macaques learned to wash sweet potatoes and even to dip them in the sea to make them more salty, and passed that technique on to subsequent generations. Western scrub jays “cache”—they hide food for later use—and studies have shown that they anticipate what they will need in the future, rather than acting on what they need now.

From an evolutionary perspective, it makes sense that these human abilities also appear in other species. After all, the whole point of natural selection is that small variations among existing organisms can eventually give rise to new species. Our hands and hips and those of our primate relatives gradually diverged from the hands and hips of common ancestors. It’s not that we miraculously grew hands and hips and other animals didn’t. So why would we alone possess some distinctive cognitive skill that no other species has in any form?

De Waal explicitly rejects the idea that there is some hierarchy of cognitive abilities. Nevertheless, an implicit tension in his book shows just how seductive the ladder-of-nature view remains. Simply saying that the “lower” creatures share abilities with creatures once considered more advanced still suggests something like a ladder—it’s just that chimps or crows or children are higher up than we thought. So the summary of the research ends up being: We used to think that only adult humans could use tools/participate in culture/imagine the future/understand other minds, but actually chimpanzees/crows/toddlers can too. Much of de Waal’s book has this flavor, though I can’t really blame him, since developmental psychologists like me have been guilty of the same rhetoric.