Every species is unique. Humans are no different. In taxonomy, the derived traits that are differentiating a species from even its closest relatives are called autapomorphies. Humans appear to have various such autapomorphies (e.g., menopause, concealed ovulation, and, for what it is worth, our varied hair that often is ever-growing). Penn et al. put forward an interesting account that tries to explain what is unique about the human mind – what we hence might call human cognitive autapomorphies. Such proposals often attract scepticism from comparative psychologists, as there is a suspicion that they are designed to justify human moral superiority (and hence how we allow ourselves to treat nonhumans), rather than to advance science (e.g., Cartmill Reference Cartmill1990). On the other hand, Penn et al. charge that comparative cognitive psychologists have long been biased towards emphasising continuity. Whatever the past motives, there are compelling new reasons why comparative psychologists ought to care about identifying human cognitive autapomorphies, as well as identifying what precisely are the traits that humans and other species share. Establishing this information can seriously narrow down the search space for identifying the neural and genetic underpinnings of these cognitive traits. Chances are that humans' cognitive autapomorphies have something to do with our brain autapomorphies. Subsequently, this knowledge would also inform us about when animal models of human conditions are useful and when they are not (Suddendorf & Corballis Reference Suddendorf and Corballis2007b). I therefore believe that these questions about human cognitive autapomorphies are important to keep on the agenda.

Penn et al. may well be right when they claim that only humans can reason about “higher-order relations in a structurally systematic and inferentially productive fashion” (sect. 11, para. 10). They come to this conclusion after critically reviewing comparative data on a selection of what they regard as fundamental human abilities. There is merit in the general approach of identifying where and how other animals fail on tasks that humans master, and in seeking out what the limits have in common (cf. Premack Reference Premack2007). This approach is not new, of course, and Penn et al.'s proposal is not the only non-modular account to pursue it. In fact, recursion (Corballis Reference Corballis2007b), meta-representation (Suddendorf Reference Suddendorf, Corballis and Lea1999), and reasoning about ternary relations (Halford et al. Reference Halford, Wilson and Phillips1998a) have previously been argued to go a long way towards explaining what sets human cognition apart. There seem to be structural similarities in the limits of nonhuman competence across domains. But whether these, or the current version of these proposals, really reflect one trait, and to what extend nonhuman animals' performances can be explained by their lacking this trait, is yet to be established.

There is ongoing debate about animal competence in each of the domains discussed, and what Penn et al. single out as the best current evidence is by no means uncontroversial. For example, citing Dally et al. (Reference Dally, Emery and Clayton2006) as the best evidence for theory of mind in animals is a questionable choice, given that these authors themselves state that what they described need not require theory of mind (cf. Suddendorf Reference Suddendorf2006). There are also concerns about Povinelli's reinterpretation hypothesis (see Suddendorf & Whiten Reference Suddendorf, Whiten, Sterelny and Fitness2003 for a detailed discussion) that equally apply to the present incarnation. But I would like to take this opportunity here to comment on a missing piece to any explanation of the apparent discontinuity between human and nonhuman minds.

The authors assert that standard evolutionary mechanisms produced the human mind (e.g., Note 1 of the target article). So there is no special pleading to explain the human case. The extant uniquely human traits, however, are as much a function of whatever drives the evolution of the human capacities, as they are a result of what the minds of related species are like, and which of them happened to have survived. A gap is defined by both its sides. Only 40,000 years ago, that gap between the human and nonhuman mind would have been a lot smaller as we shared the planet with Homo neanderthalensis and Homo floresiensis. In an important sense, then, to explain today's apparent discontinuity, is to explain why these species went extinct and only ours survived. This is, of course, a fundamentally different kind of enterprise than the one Penn et al. pursue, but it goes to the heart of the mystery of human uniqueness. To be sure, it remains unclear why the other hominids perished and the gap between human and nonhuman mind widened, but given our historical violent tendencies, an active role of Homo sapiens sapiens in their demise cannot be ruled out.

Today, our closest living relatives, the apes, are also all at the verge of extinction (and humans no doubt have had a hand in that). To our great grandchildren the discontinuity between human and nonhuman minds may be even wider when their closest living relatives are monkeys, not apes. Penn et al. pay little attention to differences in competence between extant primate species. There is, however, strong evidence that great apes share with humans some sophisticated cognitive abilities that other primates do not (Suddendorf & Whiten Reference Suddendorf and Whiten2001). For example, great apes can entertain secondary representations as evident in their competence in numerous domains ranging from mirror self-recognition (e.g., Povinelli et al. Reference Povinelli, Gallup, Eddy, Bierschwale, Engstrom, Perilloux and Toxopeus1997) to understanding invisible displacement (e.g., Collier-Baker & Suddendorf Reference Collier-Baker and Suddendorf2006). This is evidence for continuity, and phylogenetic reconstruction allows us to trace the evolution of these capacities to our common great ape ancestor (Suddendorf & Whiten Reference Suddendorf and Whiten2001).

Systematically mapping what cognitive faculties humans share with which other primate relatives may be as important as identifying what traits are human cognitive autapomorphies. Penn et al. briefly acknowledge that there might be other discontinuities in the contexts of cooperation (Sterelny Reference Sterelny2003), numeracy (Deheane Reference Dehaene1997), and mental time travel (Suddendorf & Corballis Reference Suddendorf and Corballis2007a), but they hint that their proposal might explain what is unique about these too. Perhaps so. But a lot more work needs to be done to establish this, or its role in other potentially autapomorphies-harbouring contexts such as culture (Herrmann et al. Reference Herrmann, Call, Hernandez-Lloreda, Hare and Tomasello2007) and imagination (Whiten & Suddendorf, Reference Whiten and Suddendorf2007), or aesthetics, morals, and humour. A systematic and sober approach might eventually lead us to consensus. This could enable comparative psychology to live up to its potential of making a seriously enlightening contribution to identifying the neural and genetic underpinnings of cognition.