OPINION NATURE|Vol 458|16 April 2009 ESSAY Can evolution explain how minds work? Biologists have tended to assume that closely related species will have similar cognitive abilities. Johan J. Bolhuis and Clive D. L. Wynne put this evolutionarily inspired idea through its paces.

Darwin’s theory of evolution by natural selec- interpreted a monkey’s refusal to eat the cucum- to imitate sounds that are created by someone tion is broadly accepted among biologists, but ber as evidence of ‘inequity aversion’ prompted else. Our primate cousins show no inclination its implications for the study of cognition are by seeing another monkey being more gener- to do this. Yet many parrots and songbirds are far from clear. Few within the scientific pale ously rewarded. Yet, closer analysis1 has revealed striking vocal mimics. Furthermore, the way would argue against the proposition that life on that a monkey will still refuse cucumber when that they learn to sing is not unlike how human Earth has evolved and that this general princi- a grape is placed in a nearby empty cage. This infants learn to speak. Both children and the ple can be extended to the process of thought. suggests that the monkeys simply reject lesser chicks of parrots and songbirds learn many of But in taking an evolutionary approach, biolo- rewards when better ones are available. their vocalizations during a sensitive period gists have tended to assume that species with Such findings have cast doubt on the early in life. They also undergo a transitional shared ancestry will have similar cognitive straightforward application of Darwinism to period during which their attempts to speak abilities, and that the evolutionary history of cognition. Some have even called Darwin’s idea or sing increasingly come to resemble those of traits can be used to reveal how we and other of continuity of mind a mistake2. adults. Recent studies even suggest that starlings animals perform certain mental tasks. A closer can identify certain syntactic features of sound analysis suggests things aren’t so simple. One solution fits many patterns that non-human primates miss. In The Descent of Man, Darwin proposed Laboratory studies of a number of species The appearance of similar abilities in that there is “no fundamental difference performing a wide range of tasks indicate that distantly related species, but not necessarily in between man and the higher mammals in their different species may have arrived at similar closely related ones, illustrates that cognitive mental faculties” on the basis of his belief that solutions to cognitive problems because they traits cannot be neatly arranged on an evolu- all living species were descended from a com- have experienced similar selection pressures, tionary scale of relatedness. mon ancestor. He also suggested that “there is a not because they are closely related. In other much wider interval in mental power between words, evolutionary convergence may be more The wrong question one of the lowest fishes … and one of the higher important than common descent in account- The difficulty of not knowing whether shared apes, than between an ape and man”. To sup- ing for similar cognitive outcomes in different ancestry or convergence accounts for similar port his argument, he outlined cases in which animal groups. cognitive outcomes in different species is not forerunners of human intelligence could be For example, we now know that birds are the only problem with applying an evolution- found in “higher mammals”, including “simi- capable of feats that match or even exceed ary approach to cognition. Another major lar passions, affections, and emotions, … [such those reported in monkeys and apes. Rooks, stumbling block is that it is extremely difficult, as] … jealousy, suspicion, emulation, gratitude, for example, rub their bills together after one if not impossible, to identify the factors that and magnanimity”. of them has been involved in a confrontation originally drove the emergence of contempo- Darwin’s reports of “a sense of humour … with another bird. Equivalent stroking and rary animal and human traits. wonder and curiosity” or “the association of embracing in chimpanzees would be labelled According to Leda Cosmides and John ideas, and reason” in animals may seem far- ‘consolation’. The self-directed pecking that Tooby 4, protagonists of evolutionary psychol- fetched, but many contemporary researchers magpies show when they are put ogy, “Our modern skulls house do not shy away from using similar anthropo- in front of a mirror after a mark “Some have even a Stone Age mind”. Indeed, this morphic language in their interpretation of has been placed on their body is called Darwin’s assumption — that the human animal behaviour. Over the past two decades, similar to the reactions seen in mind evolved as a result of selec- researchers have reported that chimpanzees apes given the same treatment. In idea of continuity tion pressures faced by our Stone can empathize with other members of their magpies, this behaviour has been of mind a mistake.” Age ancestors — underpins the species, and that they reconcile and even con- interpreted as evidence for some field. Thus, the tendency of sole each other after conflicts. Monkeys and degree of self-recognition. But in apes, the same modern humans to spontaneously fear spiders apes have been credited with a sense of fair- behaviour has been thought to indicate a deeper rather than cars, which are far more danger- ness and aversion to inequity and, in the case level of self-consciousness. Caledonian crows ous, is thought to stem from the prevalence of of apes, an awareness of the mental states of outperform monkeys in their ability to retrieve poisonous arachnids, rather than dangerous others — in other words, a theory of mind. food from a trap tube — from which food can driving, during the Pleistocene. A closer look at many of these studies reveals, be accessed only at one end. The crows can also This approach overlooks the importance however, that appropriate control conditions work out how to use one tool to obtain a second of culture in shaping the human mind. It also have often been lacking, and simpler explana- with which they can retrieve food, a skill that assumes that all traits evolve as a result of natural tions overlooked in a flurry of anthropomor- monkeys and apes struggle to master. selection, whereas they may be inconsequential, phic overinterpretation. For instance, capuchin Researchers have tried for decades to teach or by-products of selection acting on some other monkeys were thought to have a sense of fair- apes some form of language, be it by using vis- trait. However, the most serious problem with ness because they reject a slice of cucumber ual symbols or gestures. But linguists generally this perspective is that cognitive traits of past if they see another monkey in an adjacent agree that the resulting efforts made by chimps generations leave little trace in the fossil record. cage, performing the same task, rewarded and bonobos don’t qualify as language3. One Without being able to reconstruct the mind of with a more-sought-after grape. Researchers of the prerequisites for language is being able our hunter-gatherer predecessors, we can only

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guess at the selection pressures they faced5. cally for storing food is not supported by the their application. When reconstructing the 6

J. FIELD J. Last but not least, evolutionary analyses have evidence . Comparative studies of storing and evolutionary history of cognitive traits, there is been used to tackle questions for which they non-storing bird species have failed to reveal no a priori reason to assume that convergence are ill suited. Nobel laureate Niko Tinbergen a consistent relationship between the size of will be more important than common descent famously considered evolution to be one of the hippocampus and food-storing capabil- or vice versa. In addition, evolutionary theory the four great problems in behavioural biol- ity. Moreover, food-storing species in general may suggest hypotheses about the mechanisms ogy. He pointed out that questions concerning do not perform any better in spatial-memory of cognition, but it cannot be used to actually the development, function and mechanistic laboratory tasks than non-storers6. study these mechanisms. causes of behaviour deserve equal weight6. Had it been discovered, a relationship As long as researchers focus on identifying Many of the questions about cognition asked between hippocampus volume and food stor- human-like behaviour in other animals, the job by psychologists and behavioural biologists ing would have hinted that a larger hippo- of classifying the cognition of different species concern the underlying causal mechanisms. campus underpins a superior spatial memory. will be forever tied up in thickets of arbitrary How does the dog open the gate? How does Even then, a causal analysis would be needed nomenclature that will not advance our under- the parrot imitate human vocal sounds? Evo- to test this hypothesis. It could be, for exam- standing of the mechanisms of cognition. For lutionary analyses, however, because they are ple, that the hippocampus affects another trait comparative psychology to progress, we must analyses of history, cannot uncover how an that differs between storers and non-storers. study animal and human minds empirically, animal achieves a particular feat. Questions about the causal underpinnings of without naive evolutionary presuppositions. ■ Take, for example, food storing in closely behavioural differences can be elucidated only Johan J. Bolhuis is in the Department of Biology related bird species. The marsh tit stores seeds in with a causal analysis, not through reconstruct- and Helmholtz Institute, Utrecht University, tree bark or in the ground, and is able to retrieve ing evolutionary history. the Netherlands. Clive D. L. Wynne is in the them several days later. Its close relative the great Department of Psychology, University of Florida, tit, on the other hand, doesn’t store at all. Such Theory and practice Gainesville, USA. differences between species in their reliance on Clearly, functional and evolutionary questions e-mails: [email protected]; [email protected] food stores have led researchers to suggest that are intertwined, as are questions of causation the ability to remember the location of buried and development. It is unclear, however, what 1. Wynne, C. D. L. Nature 428, 140 (2004). 2. Penn, D. C., Holyoak, K. J. & Povinelli, D. J. Behav. Brain Sci. food involves an adaptively specialized spatial an analysis of the evolutionary history of cogni- 31, 109–178 (2008). memory and brain structure, the hippocampus. tive behaviours could add to our understanding 3. Pinker, S. The Language Instinct: How the Mind Creates To verify this idea, researchers have attempted of how they work, even if such an analysis were Language (W. Morrow, 1994). 4. Cosmides, L. & Tooby, J. Evolutionary Psychology: A Primer to show that spatial memory and the size of the possible. At most, an evolutionary interpretation (1997); available at www.psych.ucsb.edu/research/cep/ hippocampus vary between species depending could provide clues to the underlying mecha- primer.html. on the degree to which they store food. nisms responsible — but such clues would have 5 Richardson, R. C. Evolutionary Psychology as Maladapted This is an interesting evolutionary hypoth- to be verified using controlled experiments. Psychology (MIT Press, 2007). 6. Bolhuis, J. J. & Verhulst, S. Tinbergen’s Legacy. Function and esis. But the suggestion that a specialized We are not suggesting an abandonment of Mechanism in Behavioral Biology (Cambridge Univ. Press, memory and hippocampus are needed specifi- Darwin’s insights. Rather, we call for care in 2009).

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