Darwin's insistence that there was “no fundamental difference between man and the higher mammals in their mental faculties” (Darwin Reference Darwin1871, p. 35) may be counted one of his few significant mistakes. Comparative psychology has been bedevilled for over a century by the need to reconcile the obvious cognitive gap between the human and nonhuman mind with Darwin's identity thesis. We commend Penn et al. for bringing this issue to the fore. We suggest that the adoption of models based on a mistaken view of evolution, combined with a tendency to anthropomorphise, has led to a widespread perpetuation of Darwin's mistake (cf. Wynne Reference Wynne2005). The form of evolutionary continuity between man and beast that many contemporary researchers seek is, in fact, a tacit perpetuation of the old scala naturae concept (Hodos & Campbell Reference Hodos and Campbell1969). This is seen in the search for cognitive homologies in our closest relatives. In contrast, others have suggested that there is convergence of cognitive capabilities, such that birds or dogs may be more similar to humans than apes. We will discuss the different evolutionary scenarios in turn.

As the target article makes clear, many researchers still seek to extend homology to cognition. One example not considered by Penn et al. is the evolution of morality. De Waal and collaborators claim to have identified cognitive capacities in nonhuman primates that they consider signs of proto-morality, or at least cognitive traits such as empathy and unfairness avoidance, that are prerequisites for moral behaviour (de Waal Reference de Waal2006). Concurrently, de Waal (Reference de Waal1999) has advocated the use of what he calls “animal-centered anthropomorphism.” Thus, chimpanzees were said to “console” each other after fights (de Waal & van Roosmalen Reference de Waal and van Roosmalen1979), and capuchin monkeys and chimpanzees were reported to be averse to unfairness (Brosnan & de Waal, Reference Brosnan and de Waal2003; Brosnan et al. Reference Brosnan, Schiff and de Waal2005). De Waal (Reference de Waal2006) has suggested that “if closely related species act the same, the underlying mental processes are probably the same too” (p. 62), and hence it is assumed, anthropomorphically, that when apes and monkeys show behaviours that resemble certain human behaviours, they are thinking what we think under similar circumstances. Subsequent research, however, has revealed that there is in fact no evidence for “consolation” in chimpanzees (Koski & Sterck Reference Koski and Sterck2007) or for inequity avoidance in capuchin monkeys (Dubreuil et al. Reference Dubreuil, Gentile and Visalberghi2006; Wynne Reference Wynne2004b) or chimpanzees (Bräuer et al. Reference Bräuer, Call and Tomasello2006).

In contrast to the search for cognitive homologies in human's closest relatives, other researchers have sought evidence for evolutionary convergence in species distant from humans, such as birds and dogs (e.g., Emery & Clayton Reference Emery and Clayton2004b; Hare & Tomasello Reference Hare and Tomasello2005). The principle behind convergence is that evolutionary remote taxa may have reached similar solutions to certain cognitive problems because they have experienced similar selection pressures. The trouble for a comparative analysis is that we do not know what these selection pressures were. All we can observe is current behaviour. We may make guesses as to what kind of selection pressures there may have been, but these are only guesses, not testable hypotheses. Only the current behaviours (and their underlying mechanisms) are available to experimental investigation. Bolhuis and Macphail (Reference Bolhuis and Macphail2001; Reference Bolhuis and Macphail2002) have argued that the functional similarities between species, and the possible similarities in evolutionary history, could provide clues to the cognitive and neural mechanisms involved. But these cannot be more than clues – evolution cannot explain the mechanisms.

Take vocal learning, for example. For a long time, comparative psychologists studied apes in their quest for animal models of human speech and language acquisition. This seemed reasonable from the point of view of evolutionary homology – apes are our closest relatives (Wynne Reference Wynne2007). It has subsequently become clear that songbirds and marine mammals are better models for human vocal learning (Bolhuis & Gahr Reference Bolhuis and Gahr2006; Fitch Reference Fitch2000; Hauser et al. Reference Hauser, Chomsky and Fitch2002a). There are behavioural and neural similarities between bird song learning and speech acquisition (Bolhuis & Gahr Reference Bolhuis and Gahr2006; Doupe & Kuhl Reference Doupe and Kuhl1999; T. Q. Gentner et al. Reference Gentner, Fenn, Margoliash and Nusbaum2006; Gobes & Bolhuis Reference Gobes and Bolhuis2007). But there is no evolutionary principle from which these similarities could be deduced: they had to be discovered by experiment. Nor can we predict how far the similarities will stretch: If we want to know the nature of the neural or cognitive mechanisms of birdsong or human speech, we need to study them directly. Evolutionary convergence may give us clues concerning these mechanisms, but we need to investigate them to see whether these clues prove useful (Bolhuis Reference Bolhuis2005). It may be that the ancestors of modern songbirds and humans experienced comparable selection pressures that led to the evolution of vocal learning mechanisms. Or the apparent similarities in these two communicative systems may turn out to be superficial and adventitious. Unfortunately, evolutionary theory offers little guidance on this point.

Penn et al. have performed a great service in so clearly illuminating the cognitive gap between humans and other animals – a gap that we have emphasized elsewhere (e.g., Macphail Reference Macphail1998; Macphail & Bolhuis Reference Macphail and Bolhuis2001; Wynne Reference Wynne2004a; Reference Wynne2004c; Reference Wynne2007). The target article attempts to delineate where human cognition differs from that found in other species. It does not, however, offer a program of research for comparative psychology. Such a program would need to take cultural evolution into account, which may have been far more important than genetic evolution in the emergence of human cognition (Laland & Brown Reference Laland and Brown2002). Increasing costs and a greatly increased regulatory burden have led to a reduction in the range of species studied by psychologists in recent decades (Wynne & McLean Reference Wynne and McLean1999). Given that evolutionary considerations are of limited value when it comes to understanding the mechanisms of cognition, we believe a systematic program of reinvigorated comparative psychology is essential to understand the uniqueness of the human condition.