Abstract Discussions about the evolution of human social cognition usually portray the social environment of early hominins as highly hierarchical and violent. In this evolutionary narrative, our propensity for violence was overcome in our lineage by an increase in our intellectual capacities. However, I will argue in this article that we are at least equally justified in believing that our early hominin ancestors were less aggressive and hierarchical than is suggested in these models. This view is consistent with the available comparative and palaeoanthropological evidence. I will show that this alternative model not only does not support long-held views of human origins, but also has important consequences for debates about the evolution of our capacity for normative guidance. 1 Introduction 2 Philosophical Motivation 3 The Puzzle of Hominin Evolution 4 The Mosaic Hypothesis 5 Evidence for the Model 6 Palaeoanthropological Support 7 Philosophical Consequences 1 Introduction Reconstructions of the last common ancestor of chimpanzees (Pan troglodytes), bonobos (Pan paniscus), and humans (Homo sapiens) are important in understanding human origins. These discussions usually portray the Pan/Homo last common ancestor (Pan/Homo LCA, hereafter) as a chimpanzee-like hominid (Dart ; Lee and DeVore ; Wrangham and Peterson ; for a historical reconstruction of this debate, see Pickering ). This has long been the prevailing view in the philosophical and biological literature, and normative and moral cognition is no exception. Recent accounts of the evolution of the human capacity for normative guidance such as Kitcher () rely on this approach. I will argue that since the demonic male view, and the evolutionary models of normative thinking based on it, no longer stand up, we need an alternative explanation of this capacity that relies on a different view of human origins. The goal of this article is to articulate such a view. I will argue in this article that we are as justified in using an alternative model of early hominins, and perhaps even the Pan/Homo LCA, as we are in believing that early ancestors were chimpanzee-like. According to this model, early hominins were much more socially tolerant and less aggressive than usually assumed. I ground this claim in both the comparative evidence and the palaeoanthropological record. As a result, I will argue that this model does not fit well with views such as the demonic male view (Wrangham and Peterson ) or the killer ape hypothesis (Dart ).1 More important, I will show here that such a model has important consequences for philosophical debates about the origin of our capacity for normative guidance (Kitcher , , ). The article is organized as follows: In Section 1, I will explain the philosophical motivations behind this debate. In Section 2, I will discuss the problem of reconstructing the social behaviour of our early hominin ancestors. In Section 3, I shall explain the specific model of these ancestors I want to propose. In Sections 4 and 5, I will provide evidence in favour of this model. Finally, in Section 6, I will draw out the philosophical moral of this discussion for our understanding of our capacity for normative guidance. 2 Philosophical Motivation Philosophers have argued that the deep history of why we became moral agents is relevant to normative philosophy. For one way to understand human nature is to understand its genealogy. One primary example is the role that various origin stories of morality have played in moral philosophy (Hobbes ; Rousseau ; Nietzsche ; see also Korsgaard ). Another example is how the evolutionary genealogy of our moral faculties has become a way to vindicate (Kitcher , ) or debunk (Ruse and Wilson ; Ruse ; Joyce ) morality. As a result, genealogical projects in philosophy become highly sensivitive to different assumptions about our hominin baseline. Depending on these assumptions, for instance, some evolutionary narratives will become more vindicatory than others. According to the demonic male hypothesis (Wrangham and Peterson ) and the killer ape hypothesis (Dart ), we evolved from a chimpanzee-like hominin whose basic social nature was characterized by hostile intergroup relations. Human and chimpanzee males share a capacity for violence because our common ancestor also possessed a genetic predisposition for such a capacity. On this view, not only is this predisposition an important aspect of human psychology, it also substantially contributed to the evolution of our lineage by constraining the path and setting the pace of human social–cognitive adaptations. Human ancestors were distinctively aggressive, and this trait was preserved thanks to the role of war and interpersonal aggression in the evolution of our lineage. Emotional reactivity led to social groups controlled by aggressive alpha males, but the increased cognitive demands of cooperative hunting and tool-making helped us to control our aggressive tendencies. Put another way, from a cognitive point of view, human evolution can be seen as the story of the emergence of different forms of top-down control over our more disruptive and less reliable emotional nature.2 Humans are predisposed to violence and dominance, but we overcame these limitations through the steady increase of our intellectual capabilities (see also Pinker ). This picture radically changes, however, if a different ape species—for instance, the bonobo instead of the chimpanzee—turns out to be a comparatively better model of the social behaviour of our last common ancestor. If the social world of our forebears was more cooperative and peaceful than depicted by the chimpanzee referential model, neither the killer ape hypothesis nor the demonic male view of our social nature would be completely right. I will argue in this article, for instance, that to a large extent, emotional and affective processes played a central role in the evolution of peaceful and cooperative human societies, rather than being solely a matter of emerging top-down control mechanisms. This hypothesis has consequences for ongoing philosophical debates. For example, recently there has been a lot of interest in the connection between morality, sexual selection, and cooperation (Joyce ; Miller ; Kitcher ). But all these theories are built on the assumption that the social organization of early hominins closely resembled the social organization of the chimpanzee. If the sexual behaviour of these hominins was less characterized by high levels of intermale and intersex aggression than in chimpanzees, then the conditions for sexual selection would be radically different. Similarly, cooperation in a more socially tolerant ancestor would be different from the type of cooperation we find in highly hierarchical and aggressive primate social groups—the cognitive challenges are different and so are the mechanisms required to face them. I shall illustrate this point with Kitcher’s () hypothesis about the evolution of our capacity for normative guidance. According to Kitcher, the origins of the ethical project cannot be understood in terms of biological altruism nor in terms of behavioural altruism. The social life of our primate ancestors required a capacity for ‘psychological altruism’—roughly, a capacity to align one’s desires in response to the perceived desires of others, and not in expectation of some future benefit. In other words, Kitcher understands the emergence of human altruistic capacities as the gradual evolution of the cognitive and motivational psychological mechanisms underlying them (see also Sober and Wilson ). This presupposes a form of belief–desire psychology, for ‘altruists are intentional agents whose effective desires are other-directed’ (Sober and Wilson , p. 20) In this view, psychological altruism fostered complex forms of cooperation, and vice versa, that ultimately led to the appearance of norms and the beginning of ethical practice. Yet psychological altruism in chimpanzees is limited in scope, as it was also in early hominins. Kitcher argues that to overcome these limitations, soon after the split with our sister lineages ancestral hominin groups developed a capacity for normative guidance—that is, a capacity to understand and respond to commands. He then offers a genealogy of this capacity that ‘changed the preferences and intentions of some ancestral hominids, leading them to act in greater harmony with their fellows and thus creating a more smoothly cooperative society’ (Kitcher , p. 74; see also Kitcher , p. 172). Kitcher’s (, p. 409) genealogy of our capacity for normative guidance is vindicating because it leads to ethical progress, beginning with its ancestral role in remedying failures of altruism in our chimpanzee-like hominin ancestors: Tens of thousands of years ago, our remote ancestors began the ethical project. They introduced socially embedded normative guidance in response to the tensions and difficulties of life together in small groups. They were equipped with dispositions to psychological altruism that enabled them to live together, but the limits of those dispositions prevented them from living together smoothly and easily. Out of their normative ventures have emerged some precepts we are not likely ever to abandon, so long, at least, as we make ethical progress, the vague generalizations that embody ethical truths. On Kitcher’s view, the ethical project is a form of social technology that has played a central role in the gradual improvement of our hominin social life. This role is a vindicating one. Certainly, his strategy might seem unconventional since progress is usually explained in terms of truth. Instead, he thinks that his genealogy of moral cognition can make sense of ethical truth and ethical knowledge based on this notion of progress—the second part of his book is devoted to this issue. Progress is just functional efficiency. For moral practices have an original function, namely, to remedy the failures of altruism that lead to social conflict. This is what Kitcher calls ‘pragmatic naturalism’. As Kitcher (, p. 210) put it: ‘Pragmatic naturalism retains a notion of ethical truth for expository purposes, but it starts from the concept of ethical progress’. As with any other genealogical argument, Kitcher’s vindication of the ethical project is sensitive to issues about our hominin baseline. His account of the role of normative guidance only makes sense in the context of a demonic male view. Male aggression is not a marginal feature of Kitcher’s (, p. 59, Footnote 40) analysis, since he takes chimpanzees rather than bonobos as the model for our hominid past, and chimpanzee societies are male-dominated. In his view, the evolution of normative guidance was initially grounded in fear of punishment, and the beginnings of the ethical project are seen as a transition from a state of limited psychological altruism to one where commands are followed out of fear. This was so because the social life of our forebears was chimpanzee-like: Begin with chimpanzee societies in which a crude precursor of punishment is already present. Conflicts within these groups are often settled through the interventions of a dominant animal. Here rank or physical strength (or both as concomitants of each other) prevail, and a dispute is settled—not always, of course, through the infliction of pain or discomfort on the animal whose initial defection gave rise to the conflict. (, p. 87) In these social groups, the capacity to understand and obey commands was favoured by natural selection because it helped avoid the cost of being punished by the dominant individual. Thus, Kitcher’s view can be understood as a form of demonic male view. Kitcher’s account of our capacity for normative guidance is important and enlightening. But his evolutionary account relies too heavily on a version of the so-called chimpanzee referential doctrine (Sayers et al. ), and a version of the demonic male view—the idea that dominance and male aggression were the cardinal challenges in the evolution of human sociality. His vindicating genealogy thus follows the typical narrative of this family of views, in which high-level cognition plays the leading role in the expansion of the prosocial tendencies of our lineage. As we will see later, if the model of our early ancestors I propose here is correct, Kitcher’s account of the emergence of normative guidance would not be quite right. To the extent that his philosophical views (for example, his vindication of the ethical project) rely on his evolutionary genealogy, they need to be reassessed in light of the plausibility of different models of the social behaviour of early hominins. 3 The Puzzle of Hominin Evolution Evolutionary explanations of cognition require a historical and a comparative context in order to determine the hominin baseline of social–cognitive capacities. This baseline can be established through research in comparative psychology. Most of the supporting evidence for the proposed model I present here comes, in particular, from the comparative literature on chimpanzees and bonobos. Chimpanzees and bonobos are our closest living relatives. According to current estimates, the human lineage diverged from the Pan lineage about 6–4.5 million years ago (Prüfer et al. ), while chimpanzees and bonobos diverged from each other more recently, about 1–2 million years ago. As a result, chimpanzees and bonobos are very similar in many respects, but they are also significantly different in key social and sexual behaviours. The differences in social behaviour are particularly intriguing. Chimpanzees show a clear linear dominance hierarchy among males, with male dominance over females (Goldberg and Wrangham ). They also display relatively low levels of cooperation (Hirata and Fuwa ). In contrast, hierarchical relationships among bonobos are not always clearly defined (Kanō ). Female dominance is common, and it is based on female alliances against aggressive males (Vervaecke et al. ). Moreover, experimental evidence also suggests that bonobos are more similar to humans in the way they solve various cooperative problems (Hare et al. ). Sexual and play behaviours are different as well. In bonobos, sexual interactions occur in mixed- and same-sex pairings, and it is also used for conflict resolution (de Waal and Lanting ; de Waal ). Play behaviour is common in adult bonobos, especially among females (Palagi ). In contrast, chimpanzee sexual behaviour is less rich and diverse (Goodall ). Sexual interaction does not typically occur in same-sex pairings, and (as in other primates) high-ranking males monopolize oestrus females. Unlike bonobos, play behaviour is only frequent among chimpanzee infants, and no gender bias in terms of play behaviour has so far been found. These behavioural differences are important because apes can be used as referential models, that is, anatomical and behavioural proxies of our last common ancestor. In these models, the ethology, ecology, and cognitive skills of great apes are used to infer the traits that are most likely to have been the ancestral condition of modern humans. These traits may be homologies (traits inherited from a common ancestor), analogies (traits that have evolved independently due to similar selective pressures), or a combination of both. Moreover, although it is true that the recent split and stark differences between both species suggest that a wide range of social behaviours are quite plastic and evolutionarily labile, this could hardly be the whole explanation of these differences. As we will soon see, comparative studies in Pan show that neuro-anatomical differences may be responsible for these behaviours, which indicates that these traits are not just a consequence of immediate differential responses to highly idiosyncratic socio-ecological factors.3 Thus, given the behavioural differences between chimpanzees and bonobos, it is reasonable to assume that our early hominin ancestors were, in part, a mosaic of traits seen in both Pan species.4 This is a puzzle for hominin evolution since chimpanzees and bonobos constitute two very different models of our last common ancestor. The differences between these models have important consequences—for example, the demonic male hypothesis is only plausible if the last common ancestor was more chimpanzee-like than bonobo-like. In the next sections of this article, I argue that our best model of the social behaviour of early hominins is not only one that carries features of chimpanzees, bonobos, and probably other species, but also one that stresses the comparative similarities between bonobos and those early ancestors. This ‘mosaic model’, I claim, has important consequences for our understanding of the evolutionary trajectory of our distinctive prosocial tendencies. 4 The Mosaic Hypothesis On the view I want to defend here, early hominins were a mosaic of different traits seen not only in chimpanzees but also in other primate species. So, the key problem is to determine which particular aspects should be included in the mosaic on the basis of the available evidence. I focus, in particular, on one version of this hypothesis: that bonobos are to some degree a constitutive part of that mosaic. Of course, my concern here is not whether bonobos are closer to us than chimpanzees. Nor is it which species better resembles, say, the Pan/Homo LCA. My claim is a comparative one, namely, that bonobos are in some important respects a more suitable model of the social behaviour of early hominins and the Pan/Homo LCA with respect to our equally distant relative, the chimpanzee. The overall picture of this comparative model is one in which early hominin ancestors were characterized by a level of social tolerance and prosocial skills that went beyond the usual chimpanzee referential model. This is not a minor issue. For increased social tolerance and enhanced prosocial skills diminish the role of aggression and dominance in the evolution of our lineage. They make aggression and dominance less restrictive constraints on the evolutionary trajectory of the lineage when the selective pressures for increased cooperation escalated. Adaptations for tolerance and prosociality make the evolutionary trajectory toward seemingly distinctive human traits such as imitative learning (Galef , ; Tomasello ) or collective foraging (Tomasello et al. ) more accessible. The feasibility of the mosaic hypothesis and the version of this model I propose here are supported in the first place by genetic evidence. Recently, Prüfer and colleagues completed the sequencing of the bonobo genome and have compared it to the already sequenced genome of chimpanzees and humans. They showed that about 1.6% of the human genome is more closely related to (that is, more similar to homologues in) bonobos than chimpanzees, while 1.7% of the human genome is more closely related to the chimpanzee than to the bonobo genome (Prüfer et al. , pp. 2–3). Given the behavioural differences between chimpanzees and bonobos, they argue that the last common ancestor of these three species could have possessed traits seen in both Pan species, at least in principle (, p. 527). This genetic evidence not only gives prima facie motivation for the idea that the Pan/Homo LCA had some bonobo-like traits. It also suggests that bonobos can be useful referential models. The value of bonobos as models of early hominins is likely not only limited to common ancestry, though. It is also plausible that many features we see in this extant species resemble those we see in humans because both species underwent similar selective regimes (see Section 6). Either way, I argue that it is quite possible that our early hominin ancestors, and even perhaps the Pan/Homo LCA, were characterized by: group hunting behaviour; enhanced emotional control; increased aversion against aggression (specially intermale and intergroup aggression); enhanced brain connectivity for empathy (top-down and bottom-up control of aggressive impulses); increased mind reading skills; increased cooperative and sharing tendencies; a non-linear or ill-defined hierarchy; and non-exclusive male dominance. Traits (2)–(4) are comparative features, that is, they are traits of early hominins that are well above the hypothesized levels of a chimpanzee-like model of that ancestor. According to these features, the social life of our early ancestors was in these respects more bonobo- than chimpanzee-like. This does not rule out, of course, the possibility that other species could be better models for these features. But for the purpose of the present argument, these comparative claims suffice. Our evolutionary trajectory would be less constrained by aggressive and dominant tendencies, such that overcoming them would be displaced (so to speak) from the centre of our evolutionary narrative. In other words, if the present model is on the right track, there would be sufficient reasons to be sceptical about the killer ape hypothesis or the demonic male view of the social–cognitive capacities of our early hominin ancestors. Moreover, from a philosophical point of view, this model would lead us to reassess naturalistic arguments based on these assumptions, such as Kitcher’s evolutionary narrative of the emergence of our capacity for normative guidance. These arguments would instead be linked to a different picture of the trajectory of hominin social evolution and the timing of the appearance of more complex forms of social cognition. As I argue later, the fossil record supports the view that very early in our lineage, hominins were less aggressive and more tolerant than commonly assumed by chimpanzee referential models. 5 Evidence for the Model The features of the proposed model are closely linked to social behaviour. In behavioural phylogenetics, it is possible to reconstruct an ancestor’s behaviours if such behaviours are present in all of its living descendants. This argument relies on considerations of parsimony. To the extent that parsimony is a guide, group hunting would be characteristic of our last common ancestor. For recent evidence shows that this behaviour is also present in the bonobo (Surbeck and Hohmann ). The same goes for some aspects of physical cognition such as tool manufacture and use (Ingmanson ; Gruber et al. ). Since humans also possess those behavioural traits, it is possible to infer that the Pan/Homo LCA did (1) hunt in groups. It is true that given that traits such as tool manufacture and use are present in all great apes and also in other primate species, their presence in early hominins is a somewhat more conservative phylogenetic inference than group hunting. The set of data points is significantly smaller in the latter case. Nonetheless, there is evidence that by 3.4 million years ago, hominins were using stone tools to hunt large mammals (McPherron et al. ), which pushes the plausibility of ape-like hunting much deeper in the hominin lineage. Therefore, it is just as likely, if not more so, that group hunting was present in the Pan/Homo LCA as it is that it emerged very early in our lineage and then independently in Pan. In addition, the neural circuitry in humans that mediates anxiety, empathy, and the inhibition of aggression is better developed in bonobos than in chimpanzees. Bonobos and humans have a similarly organized orbitofrontal cortex and a relatively smaller area 13 (Semendeferi et al. ). Differences in the organization and size of these parts of the brain influence emotional reactions and social behaviour—for example, area 13 is known to be associated with changes in emotional states and disinhibition of emotional reactions. Bonobos and humans also possess a similar distribution of von Economo (VEN) neurons in the anterior cingulate and frontoinsular cortex (Nimchinsky et al. ). In humans, their hypothesized functions include self- and social awareness, self-control, and empathy (Allman et al. , ), which would be crucial for bonobo social organization and its typically weak dominance hierarchy. They are also thought to be an important part of the circuitry responsible for rapid, intuitive choice in complex social situations (Allman et al. ).5 Similarly, recent comparative studies have shown that two pathways—one connecting the amygdala and the anterior cingulate cortex, and the other connecting the amygdala and the ventromedial prefrontal cortex—are larger in bonobos than chimpanzees (Rilling et al. ). The former is implicated in emotion regulation in humans, while the latter enables the restraint of aggression via top-down suppression of aggressive impulses from the amygdala (Davidson et al. ; Pezawas et al. ; Meyer-Lindenberg et al. ). The same pathway may also be involved in controlling aggressive impulses via a bottom-up relay of perceived distress in others to the ventromedial prefrontal cortex that inhibits anti-social behaviour (Blair , ). Insofar as the above neurobiological traits are examples of fine-grained similarities, then parsimony suggests that the early hominins possessed (2) enhanced emotional control, (3) increased aversion against aggression, and (4) enhanced brain connectivity for empathy with respect to a hypothetical chimpanzee-like model of these ancestors. A broader look at the neurobiology of other empathic and tolerant primate species gives some additional support to this view. For callitrichid monkeys, for instance, are quite socially tolerant but their social behaviour relies on somewhat different neural circuitry. They possess small brains and their empathic behaviour is mediated by physiological responses that are especially geared to cooperative breeding (Fernandez-Duque et al. ). This indicates that empathy and emotion regulation are not necessarily related to an increase in grey and white matter connectivity as in bonobos and humans, which makes a hypothesis about convergent evolution less likely. Bonobos are also more socially tolerant than chimpanzees, especially when co-feeding (Hare et al. ). They show a stronger stress hormone response to feeding competition (Wobber et al. ). They have also been described as more nervous and shy than chimpanzees (de Waal and Lanting ). As in humans, these differences in temperament are associated with enhanced social–cognitive skills. Studies with young children, for instance, show a strong connection between shyness and mindreading skills (Wellman et al. ). Similarly, bonobos outperform chimpanzees in tasks related to mindreading, while chimpanzees are more skilled at tasks requiring the use of tools and an understanding of physical causality (Herrmann et al. ). Differences in mindreading skills, however, cannot be explained solely on the basis of social tolerance. These differences are products of a particular neural system for understanding the intentional states of others. The medial prefrontal cortex and the temporoparietal junction are known to be implicated in mindreading capabilities in humans (Gallagher and Frith ; Saxe and Kanwisher ). Thus, the fact that bonobos also have increased grey matter in the dorsomedial prefrontal cortex compared with chimpanzees seems to be telling. Mindreading skills in apes are typically linked to competitive contexts (Call and Tomasello ), but there is no reason to think that food and mating competition is stronger in bonobos than chimpanzees. Thus, explaining this increased capacity in bonobos through a convergent selective gradient seems problematic. Levels of tolerance also affect sharing behaviour in Pan. Chimpanzees share food with conspecifics only under some circumstances—for example, food transfer from mother to offspring (Ueno and Matsuzawa ) or when the food is not valuable and not monopolizable (Blurton-Jones ; Gilby ). However, peaceful food sharing in wild bonobos seems to contradict the usual sharing-under-pressure hypothesis (Yamamoto ). Under experimental conditions, active and voluntary food sharing also seems to be present in bonobos (Hare and Kwetuenda ), even among strangers and when food is easily monopolizable (Tan and Hare ). Moreover, recent studies suggest that selection on emotional reactivity critically shapes a species’ ability to solve social problems (Hare et al. ; Hare and Tomasello ). This hypothesis, for instance, predicts that bonobos will cooperate more successfully in food-retrieval tasks than chimpanzees because tolerance levels are higher in bonobos. So, although in experiments both species have been shown to be equally successful at cooperating when food is difficult to monopolize, tests with monopolizable food have shown that bonobos are much more prone to cooperation than chimpanzees (Hare et al. ). Given the differences in temperament between chimpanzees and bonobos, it is at least as plausible that early hominins possessed (5) increased mindreading skills and (6) increased cooperative and sharing tendencies with respect to a hypothetical chimpanzee-like model of the Pan/Homo LCA as it is to adopt the standard chimpanzee referential model. This is a non-negligible difference in social–cognitive abilities. The fact that these differences are correlated with particular neurobiological similarities between bonobos and humans also deserves attention. The chimpanzee’s mindreading and cooperative capacities cannot simply be taken to represent those of early hominins. Naturally, sexual behaviour in all the three species has important differences. But a crucial similarity between bonobos and humans is that both species use sexual behaviour in a social context. Unlike chimpanzees, female bonobos are continuously sexually active and attractive. So, in bonobos and humans, sexual intercourse can be initiated at any point, which in turn increases bonding between individuals. Bonobos with lower testosterone levels and attenuated testosterone responses engage more often in amicable relationships with unrelated females and have greater reproductive success (Surbeck et al. ). Therefore, bonobo males benefit from affiliative long-term association with females (Surbeck et al. ), which facilitates more egalitarian and peaceful social lives. Similarly, hypothalamus and amygdala size have been shown to predict social play frequency in non-human primates but not non-social play (Lewis and Barton ). Bonobos—females more than males—seem to use play to assess physical skills, the willingness of other individuals to invest in a relationship, and to strengthen already existing social bonds. Adult bonobos play much more frequently than chimpanzees. This asymmetry is important because it has been shown in experiments that both species use grooming and play as social currency (Schroepfer-Walker et al. ). Play is a valuable social interaction and can be used to establish social preferences depending on the amount of playful interactions between individuals. Thus, play behaviour could also have a crucial role in the bonobo social organization and its typically weak dominance hierarchy. To the extent that the above neurobiological similarities are correlated with the more egalitarian social structure of bonobos, they would suggest that early hominins lived in (7) less hierarchical and arguably (8) less male-dominated social groups with respect to a hypothetical chimpanzee-like model of the Pan/Homo LCA. Explanations of the evolution of the bonobo usually argue that reduced male aggression towards females was sexually selected (Wrangham and Peterson ). But it is at least equally likely that this trait was inherited from the common ancestor, especially in light of the fact that the traditional evolutionary scenario for the split between chimpanzees and bonobos is not supported by our current knowledge about the formation of the Congo River (Takemoto et al. ).6 Granted, this is not conclusive evidence for the mosaic hypothesis or the particular model I have offered in the previous section. However, even if the case for the model is not compelling enough, we have good reasons to think that the social behaviour of early hominins, including the Pan/Homo LCA, was in many respects not chimpanzee-like. The chimpanzee referential model can no longer be the default assumption. 6 Palaeoanthropological Support Although certainly thin, the above evidence suggests that the Pan/Homo LCA was in some respects more bonobo-like than chimpanzee-like. In this section, I will argue that even if the Pan/Homo LCA was not characterized by the features ascribed in the model, we still have reasons to think that they evolved very early in our lineage. For the palaeoanthropological evidence suggests that early hominins were much more socially tolerant than the chimpanzee referential doctrine suggests (Sayers et al. ). Fossil evidence is central to whatever model of our hominin ancestry we choose. Referential models are constrained by phylogenetic inferences—after all, phylogenetic analysis can be understood as a form of referential modelling (Duda and Zrzavý ). But fossil evidence particularly restricts the scope and shape of these models. Generally speaking, referential models are either based on homology through shared descent (McGrew ) or analogy through convergent evolution (DeVore and Washburn ; Fernandes ; Jolly ; Perry et al. ). The above model can be considered rather neutral regarding this issue. Palaeoanthropological evidence, however, suggests that even if some aspects of the proposed model are not homologies—that is, ancestral traits of the Pan/Homo LCA that have been retained by bonobos and humans—they might have evolved fairly early in our lineage. This view is supported by fossil evidence from Sahelanthropus, Orrorin, and Ardipithecus indicating that our lineage was less aggressive and less male-dominated than assumed by the traditional chimpanzee referential model (Pickford and Senut ; Brunet et al. ; Haile-Selassie et al. ; White et al. ). Early hominins and the Pan/Homo LCA could also have been very different from both Pan species. Fossil evidence from Ardipithecus ramidus, for instance, indicates that this early hominin was well-adapted to bipedality, although it retained arboreal capabilities (Lovejoy et al. ). This means a more human-like locomotion system, quite different from that seen in any extant ape. Another important difference is that A. ramidus appears to be neither a ripe fruit specialist like Pan, nor a folivorous browser like Gorilla, but rather a more generalized omnivore (Suwa et al. ). However, the same fossil evidence also suggests that the social behaviour of the Pan/Homo LCA was more bonobo-like than chimpanzee-like in many important respects and that this social behaviour is likely to be an ancestral condition. Evidence from A. ramidus is particularly telling. The fossil record of this ancestor is rich and the completeness of some remains makes sex assessment relatively reliable (White et al. , ). Dating estimates place this hominin at circa 4.4 million years ago, very close to the split between these two lineages, which makes this ancestor highly relevant for reconstructing the morphology and behaviour of the Pan/Homo LCA. A. ramidus remains reveal that this hominin was characterized by reduced canine teeth and low body size dimorphism. In basal dimensions, the canines of A. ramidus are approximately as large as those of female chimpanzees and male bonobos, although their crown heights are shorter; they are comparable to those of Australopithecus anamensis and Australopithecus afarensis (Suwa et al. ). They are also ‘feminized’ in shape. The size of the upper canine tooth is not only similar to that of females, but also less sharp than those of chimpanzees. Reduced canine teeth dimorphism is a common feature of the hominin clade: along with A. ramidus, this trait is seen in Sahelanthropus (Brunet et al. , p. 150), Orrorin (Senut et al. ), and Ardipithecus kadabba (Haile-Selassie ). Since the canine tooth is usually used as a weapon in intermale and intergroup conflicts, the less pronounced upper canine teeth suggest that early hominins, including A. ramidus, were characterized by relatively little intermale and intergroup aggression compared to chimpanzees. Similarly, A. ramidus is also expected to have shown little sexual dimorphism in body size—comparable to that of chimpanzees or humans, as opposed to orangutans or gorillas (White et al. ). In higher primates, body size dimorphism is usually coupled with strong canine dimorphism. Using dimorphism to infer behaviour in early hominids is usually problematic because their unique combination of minimal canine size dimorphism and intense body mass dimorphism (Plavcan and van Schaik ). But this is not the case in A. ramidus. As a consequence, lack of sexual dimorphism seems to indicate that males did not compete against each other for dominance. While intermale and intergroup aggression are frequent among chimpanzees, A. ramidus possessed low levels of agonistic male–male competition (Clark and Henneberg )—and even, perhaps, male–female codominance as in bonobos (Suwa et al. , p. 57). We cannot be sure about these aspects of the social behaviour of our early ancestors, but we can infer them indirectly. For early hominins do not seem to have any of the adaptations for agonistic male–male competition present in other living primates. In turn, reduced male sexual dimorphism does not seem to have an obvious survival advantage. Yet this trait could have led to a reproductive advantage through sexual selection—for example, because bipedalism facilitated provisioning, which would have been a more efficient mating strategy (Lovejoy ). Parallel evolution does not always seem to give us the most parsimonious reconstruction of these traits. Chimpanzees are more sexually dimorphic than bonobos and humans, and australopithecines were more sexually dimorphic than both extant Pan species (Gordon et al. ; Van Arsdale and Wolpoff )—which is true in terms of body size but not canine size (Leutenegger and Shell ; McHenry ; Plavcan and van Schaik ). Therefore, to the extent that australopithecines are direct ancestors of modern humans (and not a paraphyletic sister lineage, which they may be), this loss of sexual dimorphism must have not only occurred twice independently, in Pan and in Homo, but also in A. ramidus.7 Another option would be to suggest that low sexual dimorphism is, in fact, the ancestral condition, with a pattern of increasing dimorphism in australopithecines and chimpanzees. Australopithecines would be a paraphyletic sister lineage (an alternative pointed out to me by Kim Shaw-Williams, personal communication), or not as sexually dimorphic as it has often been claimed (Reno et al. , ). In this way, the evolutionary trajectory of the human lineage could be explained by postulating fewer evolutionary reversals or by invoking less drastic shifts. However, this would challenge the current picture of Australopithecus as a very aggressive, highly sexually dimorphic genus, and perhaps even its place as direct human ancestor. Nonetheless, this is a hypothesis worth exploring further (see Figure 1). Figure 1. View largeDownload slide Diagram of two evolutionary arrangements of five hominid species. According to one view (solid line), A. ramidus and A. afarensis are direct ancestors of humans (H. sapiens). But A. ramidus, bonobos (P. paniscus), and humans are characterized by low sexual dimorphism and low levels of intermale and intergroup aggression, while A. afarensis and chimpanzees (P. troglodytes) show increased levels of sexual dimorphism and aggression. On top of that, A. ramidus and humans are characterized by an omnivorous diet and a similar dentition, although A. afarensis and Pan have specialized masticatory apparatus. A more parsimonious reconstruction (dashed line) would then be to consider A. afarensis not as the direct ancestors of modern humans, but rather as part of a paraphyletic sister lineage. Figure 1. View largeDownload slide Diagram of two evolutionary arrangements of five hominid species. According to one view (solid line), A. ramidus and A. afarensis are direct ancestors of humans (H. sapiens). But A. ramidus, bonobos (P. paniscus), and humans are characterized by low sexual dimorphism and low levels of intermale and intergroup aggression, while A. afarensis and chimpanzees (P. troglodytes) show increased levels of sexual dimorphism and aggression. On top of that, A. ramidus and humans are characterized by an omnivorous diet and a similar dentition, although A. afarensis and Pan have specialized masticatory apparatus. A more parsimonious reconstruction (dashed line) would then be to consider A. afarensis not as the direct ancestors of modern humans, but rather as part of a paraphyletic sister lineage. It is not clear whether body size dimorphism in australopithecines is a consequence of male–male competition, since their canines have a variety of features inconsistent with their use as a weapon (Greenfield ). There are multiple reasons that could potentially explain the increase in body size dimorphism in australopithecines, such as reduction in female body size (Leigh and Shea ), predator defence (Clutton-Brock et al. ), and Rench’s rule (see Fairbairn ). Although there is no necessary link between the specific features ascribed by the model and the palaeoanthropological evidence, the common theme of reduced (or controlled) aggression in early hominins stands. Even if the Pan/Homo LCA was very different from the proposed model, a decrease in these aggressive tendencies seems to have occurred very early in our lineage. Notably, for instance, increased levels of social tolerance have been associated with early heterochronic changes in craniofacial growth. So, it might well be the case that in A. ramidus the energetic demands of craniofacial growth were redirected to provisioning (Clark and Henneberg ). 7 Philosophical Consequences The model I have defended in this article has important philosophical consequences for descriptive theories of ethics. For it gives us a different picture of the evolution and nature of our capacity for normative guidance—that is, our capacity to grasp norms and to make normative judgements (Kitcher , , ). In Section 1, I suggested that Kitcher’s account of the emergence of the capacity for normative guidance is a particular form of the demonic male view. Similar to this view, Kitcher’s evolutionary scenario relies on a chimpanzee-like social environment where dominance and aggression are the key driving forces behind human evolution. On Kitcher’s account, dominant alpha males punish anyone who disrupts the established social order, and this makes normative guidance, at least initially, psychologically grounded in fear. In addition, as in the demonic male view, the evolution of our capacity for normative guidance is partly the story of the gradual expansion of top-down mechanisms of control (in the form of some sensitivity to commands) over our less reliable emotional nature. If an agent is able to understand the normative structure of its chimpanzee-like social environment, that agent will be able to avoid the costs imposed by aggressive alpha males. The motivational force to obey these commands comes for free in this case, since they help the agent to avoid situations in which the anticipated consequences are feared or disliked. The above model, then, bears important consequences for Kitcher’s view of normative guidance and its function. For, according to him, normative guidance has to be more explicit, more a matter of offline cognition. But the model of early hominins I presented in Section 3 strongly suggests that neither the demonic male view nor Kitcher’s () account of our capacity for normative guidance are plausible. On the model I presented, the social world of our last common ancestor is not male-dominated (8), their social organization is less hierarchical (7), and aggression and fear of punishment play less of a role in regulating social cohesion. On the contrary, this ancestor is characterized by its enhanced emotional control (2), increased aversion against aggression (3), empathy and positive emotions (4), and enhanced perspective-taking capacities (5). If this is correct, normative guidance would not have been selected for avoiding punishment by very aggressive and authoritative alpha males. Kitcher’s vindicating genealogy becomes murky. The tendencies of some individuals to monopolize resources and to impose social order through aggression would have been largely regulated in our lineage through more bottom-up, affective processes. No sensitivity to commands is required. No norms are invoked. Another explanation is necessary. Kitcher’s evolutionary account of normative guidance is not the only available explanation. It is also not the best. I think a better explanation of the shift towards normative guidance can be framed in terms of shared intentionality (Tomasello and Carpenter ). Shared intentionality seems to account for much of the distinctive features of human psychology. It has been argued, for instance, that such a capacity is responsible for the appearance of joint attention, cooperative communication, imitative learning, and teaching, which are at the basis of cultural learning and the social norms and traditions we see in human culture (Tomasello ). Although joint activities and behavioural traditions are common among great apes, humans substantially differ from other apes in their underlying psychological mechanisms. Chimpanzees and bonobos can attribute some psychological states such as perceptions and goals to others (Tomasello et al. ), but they are neither intrinsically motivated to share those psychological states nor are they able to represent these mental states in a joint, collective fashion (Call ). Primates do form social expectations, but they lack the capacity to form normative ones (von Rohr et al. ). Normative expectations depend for their emergence and maintenance on shared acceptance and commitment. Joint goals, for instance, are normatively binding mental states of the form, ‘We intend to do x’. If someone unexpectedly abandons the joint activities that these states bring about, other group members may demand an explanation and censure that partner (Warneken et al. , , ). Thus, abandoning the joint activity naturally entails a risk of reprisal (Gilbert ). Similarly, it has been argued that shared intentionality also has straightforward consequences for moral cognition (Tomasello ) since, as Christine Korsgaard (, p. 25) has nicely put it: ‘The primal scene of morality […] is not one in which I do something to you or you do something to me, but one in which we do something together’. Given that much of the empirical work on this psychological phenomenon comes from the comparative literature, the theory of shared intentionality offers a helpful framework to put normative guidance within an evolutionary context. This capacity, for instance, is thought to be closely linked to the selective pressures resulting from cooperative activities such as cooperative breeding and collaborative foraging. The former is often considered a previous step for the full emergence of shared intentionality (Hawkes , ) because although cooperative breeding leads to greater prosocial skills, it does not, in itself, entail higher cognition (Burkart et al. ). For this reason, it has been argued that the selective pressures of collaborative foraging, which are more cognitively demanding in terms of coordination, would explain the emergence of the type of complex cognition underlying shared intentionality, starting with Homo erectus and continuing with Homo heidelbergensis (Tomasello et al. ). Since it is only with the emergence of collaborative foraging that we can fully explain the emergence of shared intentionality, it is only then that we would expect social norms to emerge, where we think of these norms as mutually understood expectations that bear social force and that are enforced by third parties. The fact that the increase in the gradient of human cooperation could be partially explained by the role of normative thinking in facilitating coordination explains why some (Sterelny ; Sterelny and Fraser [forthcoming]) see only a partial or incomplete vindication in this type of genealogy: many norms could have evolved to fix coordination problems in situations where multiple equilibria are possible. In sum, one idea for further exploration would be to think of our capacity for normative guidance as having been selected for when hominins became more interdependent foragers, to avoid disappointing a relationship partner’s expectations in a more tolerant social environment (Tomasello et al. ). Norms would be conceived as shared expectations about how individuals ought to behave in a given situation, represented as joint intentional states. These expectations would have been necessary for carrying out tasks that required complex coordination, such as collaborative foraging and even more so for building the kind of collective cultural institutions that are the distinctive feature of behaviourally modern humans. Acknowledgments Thanks to Kim Sterelny, Ben Fraser, Matteo Mameli, Michael Tomasello, Josep Call, Frans de Waal, and Rachael Brown for their constructive comments on earlier versions of this article. I also thank the two anonymous referees for this journal for further improvements. I am pleased to acknowledge the financial support for this work provided by the Australian National University, the Max Planck Institute for Evolutionary Anthropology, and the Konrad Lorenz Institute for Evolution and Cognition Research. Footnotes 1 Something similar can be said about the man-the-hunter hypothesis (Lee and DeVore ). For hunting and aggression are usually considered to be a package deal. However, the model of the Pan/Homo LCA I will propose in this article does not rule out the idea that hunting played an important role in the evolution of normative guidance. 2 Top-down control is understood here as the processing of sensory and affective information that is driven by cognitive processes such as goals or intentions. Bottom-up processing is the reverse of top-down processing, that is, the processing of sensory and affective information that depends more directly on features of the stimulus input (for a more detailed discussion, see Rauss and Pourtois ). 3 Evolutionary lability can lead to these neuro-anatomical differences. In plasticity-first hypotheses, phenotypic plasticity can produce developmental variants that might increase fitness (Levis and Pfennig ). Selection can then refine the trait from an initial suboptimal version through genetic accommodation or even genetically assimilate the trait when environmental sensitivity is not favoured (Moran ; Waddington ; West-Eberhard ). However, although the robust neuro-anatomical differences between chimpanzees and bonobos might be the result of some form of genetic accommodation or assimilation, they cannot be explained merely as an immediate response to environmental change or stress. 4 Of course, this does not rule out the possibility that early hominins and the Pan/Homo LCA would have been in some respects very different from both Pan species. Fossil evidence in A. ramidus, for instance, indicates that the Pan/Homo LCA could have possessed anatomical adaptations for bipedalism and omnivory. This evidence will be discussed in more detail in Section 5. 5 As pointed out by one of the reviewers of this article, the importance of VEN neurons can be more fully understood when taken with the hypothesis that developmental and degenerative diseases such as autism (Allman et al. ; Santos et al. ), frontotemporal dementia (Seeley et al. ; Santillo and Englund ), and schizophrenia (Brüne et al. ) may be connected with its recent evolutionary history. Since all these disorders affect the social brain, these findings seem to support the idea that these neurons have acquired a specific role in mammals living in large and complex social groups (Cauda et al. ). 6 According to this hypothesis, the formation of the Congo River isolated an ancestral population of the common ancestor of chimpanzees and bonobos around 2 million years ago (Wrangham and Peterson ; Wrangham ). This population did not have to compete with gorillas for resources, which allowed females to form coalitions and resist the advances of males. Since coercion was not an efficient mating strategy, sexual selection favoured less aggressive males. This led to the evolution of bonobos and their distinctively low levels of aggression. However, the current geological evidence contradicts this scenario because it indicates that the present Congo River was formed much earlier, around 34 million years ago. 7 A similar problem occurs with diet. A. ramidus and modern humans are omnivorous, but australopithecines were largely frugivorous, similar to extant Pan. They lack the particular dental adaptations that are characteristic of omnivores. This means that these adaptations would have disappeared in australopithecines to reappear later in the human lineage and then disappear again in Pan. References Allman J. M. , Tetreault N. A. , Hakeem A. Y. , Manaye K. F. , Semendeferi K. , Erwin J. 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