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Social Cognition and the Evolution of Language: Constructing Cognitive Phylogenies

2010; Cell Press; Volume: 65; Issue: 6 Linguagem: Inglês

10.1016/j.neuron.2010.03.011

1097-4199

W. Tecumseh Fitch, Ludwig Huber, Thomas Bugnyar,

Language, Metaphor, and Cognition

Human language and social cognition are closely linked: advanced social cognition is necessary for children to acquire language, and language allows forms of social understanding (and, more broadly, culture) that would otherwise be impossible. Both “language” and “social cognition” are complex constructs, involving many independent cognitive mechanisms, and the comparative approach provides a powerful route to understanding the evolution of such mechanisms. We provide a broad comparative review of mechanisms underlying social intelligence in vertebrates, with the goal of determining which human mechanisms are broadly shared, which have evolved in parallel in other clades, and which, potentially, are uniquely developed in our species. We emphasize the importance of convergent evolution for testing hypotheses about neural mechanisms and their evolution. Human language and social cognition are closely linked: advanced social cognition is necessary for children to acquire language, and language allows forms of social understanding (and, more broadly, culture) that would otherwise be impossible. Both “language” and “social cognition” are complex constructs, involving many independent cognitive mechanisms, and the comparative approach provides a powerful route to understanding the evolution of such mechanisms. We provide a broad comparative review of mechanisms underlying social intelligence in vertebrates, with the goal of determining which human mechanisms are broadly shared, which have evolved in parallel in other clades, and which, potentially, are uniquely developed in our species. We emphasize the importance of convergent evolution for testing hypotheses about neural mechanisms and their evolution. Social cognition encompasses a number of distinctive capacities, including social learning, imitation, gaze following, and theory of mind (TOM). Such mechanisms form core elements of animal social behavior and human imitative culture. Language can be defined as a bidirectional system that permits the expression of arbitrary thoughts as signals and the reverse interpretation of those signals as thoughts. Although most animals have communication systems that allow some biologically important concepts or emotions to be expressed vocally, visually, or otherwise, humans appear to be unique in possessing a system that allows any concept we can entertain to be expressed and understood. Yet although language itself is unique to our species, many of the mechanisms underlying it are shared with other species (Fitch, 2010Fitch W.T. The Evolution of Language. Cambridge University Press, Cambridge2010Crossref Google Scholar). Social cognition is closely linked to the evolution of language. Advanced social cognition is required for children to acquire language: sophisticated “mind-reading” abilities are necessary to deduce word meanings and communicate pragmatically (Clark, 1987Clark E.V. The principle of contrast: A constraint on language acquisition.in: MacWhinney B. Mechanisms of Language Acquisition. Erlbaum, Hillsdale, NJ1987: 1-33Google Scholar, Macnamara, 1972Macnamara J. Cognitive basis of language learning in infants.Psychol. Rev. 1972; 79: 1-13Crossref PubMed Scopus (101) Google Scholar). Second, once in place, language provides a powerful new tool for social cognition, one that is at the center of human culture. Our capacity to share thoughts socially allows human cultures to accumulate knowledge in a way that would be impossible without language and underpins the progressive accumulation of complexity seen in most aspects of culture, from science and technology to myth and religion. Together, social cognition and language probably formed an evolutionary cycle wherein advances in one fed advances in the other, and it is unclear what human cognition (social or otherwise) would be like without the powerful cultural augmentation that language provides. Research on nonhuman animals can play a central role in understanding the evolution of social cognition on its own, nonlinguistic, terms. Although language appears as a seamless whole, with phonology, syntax, semantics, and pragmatic processes working together, many dissociable mechanisms underlie linguistic competence. These mechanisms together make up the faculty of language in a broad sense, and most of them exist in some form in other animals. We can roughly classify these mechanisms by whether they involve signaling (e.g., perceptual and motor systems underlying speech and sign), semantics (central cognitive mechanisms supporting concept formation, expression, and interpretation), or syntax (structure-generating mechanisms that map between signals and concepts). Both signals and semantics have a strong social component. Signals used in linguistic communication, whether spoken, signed, or written, must be learned and shared among the members of a linguistic community, and this shared lexicon requires sophisticated imitation of complex signals. Semantic interpretation requires an ability to infer the intentions of a signaler based on rather indirect cues (such as gaze direction). When a child hears the word “rabbit” spoken, a huge number of possible meanings might be inferred (e.g., “cute,” “furry,” “hopping,” “dinner”). Despite this complexity (Macnamara, 1972Macnamara J. Cognitive basis of language learning in infants.Psychol. Rev. 1972; 79: 1-13Crossref PubMed Scopus (101) Google Scholar, Quine, 1970Quine W.V. On the reasons for the indeterminacy of translation.J. Philos. 1970; 67: 178-183Crossref Google Scholar), children typically hone in unerringly on the intended meaning of a speaker by relying on conceptual constraints on possible word meanings (Clark, 1987Clark E.V. The principle of contrast: A constraint on language acquisition.in: MacWhinney B. Mechanisms of Language Acquisition. Erlbaum, Hillsdale, NJ1987: 1-33Google Scholar, Markman, 1990Markman E.M. Constraints children place on word meanings.Cogn. Sci. 1990; 14: 57-77Crossref Google Scholar). Many of these constraints are shared with other species, suggesting that a rich set of conceptual building blocks was already in place before language evolution began (Cheney and Seyfarth, 2007Cheney D.L. Seyfarth R.M. Baboon Metaphysics: The Evolution of a Social Mind. University of Chicago Press, Chicago, IL2007Crossref Google Scholar, Kaminski et al., 2004Kaminski J. Call J. Fischer J. Word learning in a domestic dog: evidence for “fast mapping”.Science. 2004; 304: 1682-1683Crossref PubMed Scopus (192) Google Scholar, Seyfarth and Cheney, 2005Seyfarth R.M. Cheney D. Constraints and preadaptations in the earliest stages of language evolution.Linguist. Rev. 2005; 22: 135-159Google Scholar). Finally, human language rests upon a rich pragmatic basis (Grice, 1975Grice H.P. Logic and Conversation.in: Davidson D. Harman G. The Logic of Grammar. Dickenson, Encino, CA1975: 64-153Google Scholar), including a strong motivation to share novel information with others. This drive to share meaning seems so natural to us that it has taken many years to realize that it is very unusual among animals, with the closest parallel perhaps being the honeybee dance “language” (Hockett, 1960Hockett C.F. The origin of speech.Sci. Am. 1960; 203: 89-96Crossref PubMed Google Scholar, Lindauer, 1971Lindauer M. Communication among Social Bees. Harvard University Press, Cambridge, MA1971Crossref Google Scholar). But the drive to share novel information requires a signaler to know what the intended recipient does and does not know (TOM). Nonhuman primates generally fail to take receiver's knowledge into account when signaling (e.g., Cheney and Seyfarth, 1980Cheney D.L. Seyfarth R.M. Vocal recognition in free-ranging vervet monkeys.Anim. Behav. 1980; 28: 362-367Crossref Scopus (154) Google Scholar, Rendall et al., 2000Rendall D. Cheney D.L. Seyfarth R.M. Proximate factors mediating “contact” calls in adult female baboons (Papio cynocephalus ursinus) and their infants.J. Comp. Psychol. 2000; 114: 36-46Crossref PubMed Google Scholar), suggesting that TOM, to the extent that it is present at all, is not employed pragmatically in communication. In summary, social mechanisms needed for language acquisition include a capacity for imitation for the signaling component, and mind-reading and TOM for the semantic and pragmatic components. Numerous studies in animal cognition provide insight into the evolution of these mechanisms. Researchers in comparative cognition study multiple species, seeking to uncover similarities and differences in each of these cognitive mechanisms, studied at multiple levels of description, including the genetic, neural, and behavioral levels. Such similarities allow us to generate and test hypotheses about the evolution of cognition. Two broad kinds of similarities need to be distinguished, termed “homology” and “analogy,” both of which play important roles in cognitive phylogenetics. Homologous mechanisms (homologs) are shared by descent from a common ancestor that possessed the mechanism. For example, the differences in imitation abilities between apes and monkeys have been used to infer that the last common ancestor (LCA) of humans and great apes had well-developed imitation capacities, while the LCA of apes and monkeys did not. Similarly, the existence of trichromatic color vision in Old World monkeys, apes, and humans indicates that trichromacy evolved in the LCA of all catarrhines (Jacobs, 1996Jacobs G.H. Primate photopigments and primate color vision.Proc. Natl. Acad. Sci. USA. 1996; 93: 577-581Crossref PubMed Scopus (119) Google Scholar). Nonhuman primates have traditionally been the focus of comparative research on social cognition, typically by researchers seeking homologs of human mechanisms in order to infer the capabilities of our extinct ancestors. Recently, comparative research on social cognition has broadened considerably to include nonprimate mammals (dogs, rats, goats), many bird species (especially among corvids: jays, crows, ravens, and their relatives), reptiles, fish, and social insects (Table 1). Results of this work have often seemed surprising, revealing cognitive abilities in dogs or ravens that are lacking in our closer primate relatives. But surprise at such results is unwarranted, reflecting an outmoded “scala naturae” view of evolution in which cognitive capacities increase with a species' relatedness to humans (Striedter, 2004Striedter G.F. Principles of Brain Evolution. Sinauer, Sunderland, Massachusetts2004Google Scholar). From a modern Darwinian viewpoint, we instead expect a species' cognitive abilities to evolve to fit its “cognitive niche.” For example, we expect species relying on complex navigation to evolve excellent spatial memory, and species living in complex social environments to exhibit superior social cognition. This perspective leads us to expect convergent evolution of analogous cognitive mechanisms (analogs) in widely separated species that face similar cognitive problems. This table provides taxonomic information regarding the species discussed in this review. Only the common name is used in the main text. The major and minor clades help to contextualize the phylogenetic position of these species utilizing traditional Linnaean classifications, even when (as for class “Reptilia”) this traditional grouping is polyphyletic. The “social intelligence hypothesis” is a leading contemporary hypothesis that attempts to explain the evolution of intelligence, in general, as a result of selection for social intelligence in particular (Byrne, 1997Byrne R. Machiavellian intelligence.Evol. Anthropol. 1997; 5: 172-180Crossref Google Scholar, Dunbar, 2003Dunbar R.I.M. The origin and subsequent evolution of language.in: Christiansen M. Kirby S. Language Evolution. Oxford Unviersity Press, Oxford2003: 219-234Crossref Google Scholar, Humphrey, 1976Humphrey N.K. The social function of intellect.in: Bateson P.P.G. Hinde R.A. Growing points in ethology. Cambridge University Press, Cambridge1976: 303-317Google Scholar, Jolly, 1966Jolly A. Lemur social behavior and primate intelligence.Science. 1966; 153: 501-506Crossref PubMed Google Scholar). It follows from the simple fact that the most cognitively challenging entities most organisms must cope with are other animals, often conspecifics. This hypothesis contrasts with the older “physical intelligence hypothesis” that supposes that intelligence, particularly human intelligence, is the result of intense selection for the use of tools and other manipulations of the environment. Crucially, such contrasting hypotheses can be tested using convergent evolution. Because analogs reflect independent evolutionary events, they constitute statistically independent samples that can support rigorous testing of evolutionary hypotheses. In contrast, homologous mechanisms by definition evolved once, and their presence in multiple descendent species constitutes only a single data point. The 4000 or so passerine birds with vocal learning represent but a single evolutionary event. It is important to recognize, however, that convergent evolution can occur in homologous substrates. For example, hippocampal enlargement has apparently evolved repeatedly in different species of food-caching birds. Although the hippocampus itself is a homolog in these species, the episodes of enlargement are convergent and represent independent events. Furthermore, capabilities that are convergent at one level (e.g., behavioral) may employ mechanisms that are homologous at another level (e.g., genetic). The use of the same genes in the specification of convergently evolved traits appears to be surprisingly common in development, and we can expect many examples in the cognitive realm (Fitch, 2009bFitch W.T. The Biology & Evolution of Language: “Deep Homology” and the Evolution of Innovation.in: Gazzaniga M.S. The Cognitive Neurosciences IV. MIT Press, Cambridge, MA2009: 873-883Google Scholar). Thus, whether a given cognitive mechanism is homologous or convergent in a phylogenetic analysis depends on the hypothesis being tested and the level of analysis. In this paper, we review comparative research on social cognition, aiming to build tentative cognitive phylogenies of the mechanisms underlying social intelligence, and to test evolutionary hypotheses concerning such mechanisms. This broad comparative approach, which we call “cognitive phylogenetics,” has substantial promise to fuel our understanding of the evolution and neural basis of both human language and culture, and social cognition more generally. Although current data remain too incomplete to support definitive conclusions, they point to gaps in our present knowledge, and allow us to reject some long-standing assumptions about animal social cognition. Finally, we discuss the implications of empirical data from animals for hypotheses about language evolution. Social cognition involves a set of interacting but separable mechanisms, and the recent literature has led to an extensive dissection of social cognition and a correspondingly daunting profusion of terms. In this section, we discuss two sets of mechanisms: the use of gaze direction to infer another's focus of attention, and of TOM, in which one organism represents what another one does or doesn't know. For humans, monitoring others' head and eye orientation (gaze) is a central feature of social life and communication (Brooks and Meltzoff, 2002Brooks R. Meltzoff A.N. The importance of eyes: how infants interpret adult looking behavior.Dev. Psychol. 2002; 38: 958-966Crossref PubMed Google Scholar), even influencing eye anatomy (Kobayashi and Kohshima, 2001Kobayashi H. Kohshima S. Unique morphology of the human eye and its adaptive meaning: comparative studies on external morphology of the primate eye.J. Hum. Evol. 2001; 40: 418-435Crossref Scopus (121) Google Scholar). Newborn humans are already responsive to their mothers' visual orientation (Farroni et al., 2002Farroni T. Csibra G. Simion F. Johnson M.H. Eye contact detection in humans from birth.Proc. Natl. Acad. Sci. USA. 2002; 99: 9602-9605Crossref PubMed Scopus (404) Google Scholar), and coordination with others' head and eye orientation to look in the same direction (gaze following) or at a specific target (joint visual attention) develops during early ontogeny (Butterworth and Jarrett, 1991Butterworth G. Jarrett N. What minds have in common is space: spatial mechanisms serving joint visual attention in infancy.Br. J. Dev. Psychol. 1991; 9: 55-72Crossref Google Scholar, Johnson et al., 1998Johnson S. Slaughter V. Carey S. Whose gaze would infants follow? The elicitation of gaze following in 12-month-olds.Dev. Sci. 1998; 1: 233-238Crossref Google Scholar, Moll and Tomasello, 2004Moll H. Tomasello M. 12- and 18-month-old infants follow gaze to spaces behind barriers.Dev. Sci. 2004; 7: F1-F9Crossref PubMed Scopus (123) Google Scholar). These capacities undergird word learning via joint attention, and are considered a crucial step toward an understanding of mental states like attention and intention (Baron-Cohen, 1995Baron-Cohen S. Mindblindness: An Essay on Autism and Theory of Mind. MIT Press, Cambridge, MA1995Crossref Google Scholar, Tomasello et al., 2005Tomasello M. Carpenter M. Call J. Behne T. Moll H. Understanding and sharing intentions: the origins of cultural cognition.Behav. Brain Sci. 2005; 28: 675-691, discussion 691–735Crossref PubMed Scopus (0) Google Scholar). Gaze processing is a central aspect of human social intelligence. Unlike pointing (which has received much attention in the primate-centered literature), directed gaze is possible for virtually any vertebrate. Long underestimated, the importance of gaze for nonhuman animals is receiving increased interest (reviewed in Gómez, 2005Gómez J.-C. Species comparative studies and cognitive development.Trends Cogn. Sci. 2005; 9: 118-125Abstract Full Text Full Text PDF PubMed Scopus (57) Google Scholar). Different levels of gaze responsiveness may be distinguished in animals (Figure 1, cf. Povinelli and Eddy, 1996Povinelli D.J. Eddy T.J. What young chimpanzees know about seeing.Monogr. Soc. Res. Child Dev. 1996; 61: 1-152Crossref Google Scholar, Schloegl et al., 2007Schloegl C. Kotrschal K. Bugnyar T. Gaze following in common ravens (Corvus corax): ontogeny and habituation.Anim. Behav. 2007; 74: 769-778Crossref Scopus (34) Google Scholar). The most basic level concerns simple detection of others' gaze direction, particularly the awareness that one is being looked at. Gaze detection seems to be based on relatively simple mechanisms (Baron-Cohen, 1995Baron-Cohen S. Mindblindness: An Essay on Autism and Theory of Mind. MIT Press, Cambridge, MA1995Crossref Google Scholar, Povinelli et al., 1999Povinelli D.J. Bierschwale D.T. Cech C.G. Comprehension of seeing as a referential act in young children, but not juvenile chimpanzees.Br. J. Dev. Psychol. 1999; 17: 37-60Crossref Scopus (89) Google Scholar) that are phylogenetically widespread (reviewed in Emery, 2000Emery N.J. The eyes have it: the neuroethology, function and evolution of social gaze.Neurosci. Biobehav. Rev. 2000; 24: 581-604Crossref PubMed Scopus (561) Google Scholar), presumably because of their relevance to social or antipredator behavior. (A) A macaque monkey is aware that a human experimenter looks in its direction and thus refrains from taking the food. (B) A raven follows the gaze direction of a human experimenter above its head, i.e. it looks up. (C) A raven also follows the gaze of a human experimenter behind a visual barrier by relocating its position. (D) A dog uses the gaze direction of a human experimenter to find food hidden under one of two inverted cups. Dotted arrows indicate gaze direction. Full arrows indicate movement of test subjects. A second level of gaze responsiveness concerns the following of others' gaze direction. Originally described in primates (Povinelli and Eddy, 1996Povinelli D.J. Eddy T.J. What young chimpanzees know about seeing.Monogr. Soc. Res. Child Dev. 1996; 61: 1-152Crossref Google Scholara; Tomasello et al., 1998Tomasello M. Call J. Hare B. Five primate species follow the visual gaze of conspecifics.Anim. Behav. 1998; 55: 1063-1069Crossref PubMed Scopus (210) Google Scholar), gaze following has now been demonstrated in distantly related mammals (dogs, Miklósi et al., 1998Miklósi Á. Polgárdi R. Topál J. Csányi V. Use of experimenter-given cues in dogs.Anim. Cogn. 1998; 1: 113-122Crossref PubMed Google Scholar; goats, Kaminski et al., 2005Kaminski J. Riedel J. Call J. Tomasello M. Domestic goats, Capra hircus, follow gaze direction and use social cues in an object choice task.Anim. Behav. 2005; 69: 11-18Crossref Scopus (120) Google Scholar) and birds (ravens, Bugnyar et al., 2004Bugnyar T. Stöwe M. Heinrich B. Ravens, Corvus corax, follow gaze direction of humans around obstacles.Proc. R. Soc. Lond. B. Biol. Sci. 2004; 271: 1331-1336Crossref Scopus (88) Google Scholar; rooks, Schloegl et al., 2008aSchloegl C. Schmidt J. Scheid C. Kotrschal K. Bugnyar T. Gaze following in non-human animals: the corvid example.in: Columbus F. Animal Behaviour: New Research. Nova Science Publishers, New York2008: 73-92Google Scholar; and bald ibises, Loretto et al., 2010Loretto M.C. Schloegl C. Bugnyar T. Northern bald ibises follow others' gaze into distant space but not behind barriers.Biol. Lett. 2010; 6: 14-17Crossref PubMed Scopus (12) Google Scholar). Like gaze detection, gaze following may be based on a relatively simple mechanism (Povinelli and Eddy, 1996Povinelli D.J. Eddy T.J. What young chimpanzees know about seeing.Monogr. Soc. Res. Child Dev. 1996; 61: 1-152Crossref Google Scholara): a socially triggered orientation response may result in subjects aligning their view with that of another individual gazing toward something, allowing them to search for something of interest themselves. While this explanation may account for following gaze into distant space, it does not explain instances in which subjects track others' gaze direction geometrically behind visual barriers (geometrical gaze following; Tomasello et al., 1999Tomasello M. Hare B. Agnetta B. Chimpanzees, Pan troglodytes, follow gaze direction geometrically.Anim. Behav. 1999; 58: 769-777Crossref PubMed Scopus (137) Google Scholar). Simply looking for something of interest in the direction of the others' gaze would result in subjects searching in front of the barrier, but if they reposition themselves to look behind a barrier, it suggests they appreciate the difference between their own and another's line of sight (Povinelli and Eddy, 1996aPovinelli D.J. Eddy T.J. Chimpanzees: joint visual attention.Psychol. Sci. 1996; 7: 129-135Crossref Google Scholar). This ability has only been demonstrated in great apes (Bräuer et al., 2005Bräuer J. Call J. Tomasello M. All great ape species follow gaze to distant locations and around barriers.J. Comp. Psychol. 2005; 119: 145-154Crossref PubMed Scopus (96) Google Scholar) and two corvid species (Schloegl et al., 2008aSchloegl C. Schmidt J. Scheid C. Kotrschal K. Bugnyar T. Gaze following in non-human animals: the corvid example.in: Columbus F. Animal Behaviour: New Research. Nova Science Publishers, New York2008: 73-92Google Scholar). Geometrical gaze following is thought to rest on a cognitively more sophisticated mechanism; developmental data from ravens indicate that geometrical gaze following develops later and shows a different habituation pattern than gaze following into space (Schloegl et al., 2007Schloegl C. Kotrschal K. Bugnyar T. Gaze following in common ravens (Corvus corax): ontogeny and habituation.Anim. Behav. 2007; 74: 769-778Crossref Scopus (34) Google Scholar). A third level of gaze responsiveness is the ability to identify the others' target of attention, i.e., what others are looking at. Most nonhuman species, including apes, monkeys, and ravens, find it surprisingly difficult to use the gaze direction of a human experimenter, or a conspecific, as a cue to find hidden food (Anderson et al., 1996Anderson J.R. Montant M. Schmitt D. Rhesus monkeys fail to use gaze direction as an experimenter-given cue in an object-choice task.Behav. Processes. 1996; 37: 47-55Crossref PubMed Scopus (75) Google Scholar, Call et al., 2000Call J. Agnetta B. Tomasello M. Cues that chimpanzees do and do not use to find hidden food.Anim. Cogn. 2000; 3: 23-24Crossref Scopus (90) Google Scholar, Schloegl et al., 2008bSchloegl C. Kotrschal K. Bugnyar T. Do common ravens (Corvus corax) rely on human or conspecific gaze cues to detect hidden food?.Anim. Cogn. 2008; 11: 231-241Crossref PubMed Scopus (26) Google Scholar). Methodological changes (e.g., combination of gaze with other cues) and experience with human communicative gestures can improve performance in various species (chimpanzees, Barth et al., 2005Barth J. Reaux J.E. Povinelli D.J. Chimpanzees' (Pan troglodytes) use of gaze cues in object-choice tasks: different methods yield different results.Anim. Cogn. 2005; 8: 84-92Crossref PubMed Scopus (41) Google Scholar; capuchins, Vick and Anderson, 2000Vick S.J. Anderson J.R. Learning and limits of use of eye gaze by capuchin monkeys (Cebus apella) in an object-choice task.J. Comp. Psychol. 2000; 114: 200-207Crossref PubMed Google Scholar; ravens, Schloegl et al., 2008cSchloegl C. Kotrschal K. Bugnyar T. Modifying the object-choice task: is the way you look important for ravens?.Behav. Processes. 2008; 77: 61-65Crossref PubMed Scopus (17) Google Scholar; dolphins, Pack and Herman, 2004Pack A.A. Herman L.M. Bottlenosed dolphins (Tursiops truncatus) comprehend the referent of both static and dynamic human gazing and pointing in an object-choice task.J. Comp. Psychol. 2004; 118: 160-171Crossref PubMed Scopus (35) Google Scholar; and fur seals, Scheumann and Call, 2004Scheumann M. Call J. The use of experimenter-given cues by South African fur seals (Arctocephalus pusillus).Anim. Cogn. 2004; 7: 224-230Crossref PubMed Scopus (47) Google Scholar). Dogs, however, are outstanding in solving these tasks instantly and reliably across a large number of variations (Agnetta et al., 2000Agnetta B. Hare B. Tomasello M. Cues to food location that domestic dogs (Canis familiaris) of different ages do and do not use.Anim. Cogn. 2000; 3: 107-112Crossref Scopus (112) Google Scholar, Miklósi et al., 1998Miklósi Á. Polgárdi R. Topál J. Csányi V. Use of experimenter-given cues in dogs.Anim. Cogn. 1998; 1: 113-122Crossref PubMed Google Scholar, Miklósi et al., 2004Miklósi Á. Topál J. Csányi V. Comparative social cognition: what can dogs teach us?.Anim. Behav. 2004; 67: 995-1004Crossref Scopus (109) Google Scholar), and although they have not been tested formally for geometrical gaze following, they seem to understand how barriers impair others' perception (Bräuer et al., 2006Bräuer J. Kaminski J. Riedel J. Call J. Tomasello M. Making inferences about the location of hidden food: social dog, causal ape.J. Comp. Psychol. 2006; 120: 38-47Crossref PubMed Scopus (143) Google Scholar). Why do dogs outperform primates in such tasks? One explanation may be that, during domestication, dogs have been specifically selected to attend to human communicative cues (Hare et al., 2002Hare B. Brown M. Williamson C. Tomasello M. The domestication of social cognition in dogs.Science. 2002; 298: 1634-1636Crossref PubMed Scopus (355) Google Scholar, Miklósi et al., 2003Miklósi Á. Kubinyi E. Topál J. Gácsi M. Virányi Z. Csányi V. A simple reason for a big difference: wolves do not look back at humans, but dogs do.Curr. Biol. 2003; 13: 763-766Abstract Full Text Full Text PDF PubMed Scopus (227) Google Scholar). Most other species seem to have problems in understanding the cooperative, communicative nature of the task, or they may be biased by competitive motives (Hare and Tomasello, 2004Hare B. Tomasello M. Chimpanzees are more skillful in competitive than in cooperative tasks.Anim. Behav. 2004; 68: 571-581Crossref Scopus (138) Google Scholar). Competitive species like chimpanzees and ravens may thus find it difficult to develop certain gaze following skills, without this indicating a lack of mentalistic understanding (Gómez, 2005Gómez J.-C. Species comparative studies and cognitive development.Trends Cogn. Sci. 2005; 9: 118-125Abstract Full Text Full Text PDF PubMed Scopus (57) Google Scholar). Taken together, comparative evidence from human children, nonhuman primates, other mammals, birds, reptiles, and fish suggests that gaze responsiveness is widespread among vertebrates. In contrast, gaze following requires active use of others' gaze cues, and to date only five groups of mammals and three groups of birds are known to follow gaze. Simple mechanisms may account for tracking others' gaze into distant space, whereas more sophisticated mechanisms are required for geometrical gaze tracking, which has only been demonstrated in a handful of primate and corvid species. Most nonhuman species have problems in identifying the target of others' gaze. Surprisingly, dogs provide the best-attested exception, perhaps due to their high level of cooperativeness. How much ape or corvid failures depend on cognitive limitations, or cooperative versus competitive motivations, remains an open question. These data demonstrate the separability of gaze processing into multiple distinct mechanisms, perfect for building a cognitive phylogeny (see Discussion subsection). TOM is a core human capacity, underlying many pragmatic aspects of adult language use and closely tied to child language acquisition (de Villiers and Pyers, 2002de Villiers J.G. Pyers J.E. Complements to cognition: a longitudinal study of the relationship between complex syntax and false-belief-understanding.Cogn. Dev. 2002; 17: 1037-1060Crossref Scopus (145) Google Scholar). Since Premack and W