On Social Tolerance and the Evolution of Human Normative Guidance

Abstract

Discussions about the evolution of human social cognition usually portray the social environment of early hominins as highly hierarchical and dominant. In this evolutionary narrative, our propensity for violence was overcome in our lineage by a key increase of our intellectual capacities. However, I will argue in this paper that we are at least equally justified in believing that our early hominin ancestors were less aggressive and hierarchical than what is claimed by those models. This view is consistent with the available comparative and paleoanthropological 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.

Full text

On Social Tolerance and the
Evolution of Human Normative
Guidance
Ivan Gonzalez-Cabrera
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 hierarch-
ical than is suggested in these models. This view is consistent with the available compara-
tive 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.
1Introduction
2Philosophical Motivation
3The Puzzle of Hominin Evolution
4The Mosaic Hypothesis
5Evidence for the Model
6Palaeoanthropological Support
7Philosophical Consequences
1 Introduction
Reconstructions of the last common ancestor of chimpanzees (Pan troglo-
dytes), 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 [1953];Lee and DeVore [1969];Wrangham and Peterson
[1996]; for a historical reconstruction of this debate, see Pickering [2013]). This
has long been the prevailing view in the philosophical and biological literature,
Brit. J. Phil. Sci. 0(2017), 1–27
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and normative and moral cognition is no exception. Recent accounts of the
evolution of the human capacity for normative guidance such as Kitcher
([2011]) 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 ar-
ticulate 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 [1996]) or the killer ape hypothesis (Dart [1953]).
1
More important,
I will show here that such a model has important consequences for philosoph-
ical debates about the origin of our capacity for normative guidance (Kitcher
[1998],[2006],[2011]).
The article is organized as follows: In Section 1, I will explain the philo-
sophical motivations behind this debate. In Section 2, I will discuss the prob-
lem 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 pro-
pose. 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 [1994];Rousseau [1992];Nietzsche [1967]; see also Korsgaard [2010]).
Another example is how the evolutionary genealogy of our moral faculties has
become a way to vindicate (Kitcher [2006],[2011]) or debunk (Ruse and
Wilson [1986];Ruse [1998];Joyce [2006]) morality. As a result, genealogical
projects in philosophy become highly sensivitive to different assumptions
1
Something similar can be said about the man-the-hunter hypothesis (Lee and DeVore [1969]).
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.
Ivan Gonzalez-Cabrera2
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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
[1996]) and the killer ape hypothesis (Dart [1953]), we evolved from a
chimpanzee-like hominin whose basic social nature was characterized by hos-
tile 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 con-
trolled 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 [2011]).
This picture radically changes, however, if a different ape species—for
instance, the bonobo instead of the chimpanzee—turns out to be a compara-
tively 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 [2006];Miller [2007];
Kitcher [2011]). But all these theories are built on the assumption that the
social organization of early hominins closely resembled the social organiza-
tion of the chimpanzee. If the sexual behaviour of these hominins was less
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 de-
pends more directly on features of the stimulus input (for a more detailed discussion, see Rauss
and Pourtois [2013]).
Evolution of Human Normative Guidance 3
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characterized by high levels of intermale and intersex aggression than in chim-
panzees, 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 ([2011]) hypothesis about the evo-
lution of our capacity for normative guidance. According to Kitcher, the origins
of the ethical project cannot be understood in terms ofbiological 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 de-
sires 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 mo-
tivational psychological mechanisms underlying them (see also Sober and
Wilson [1998]). This presupposes a form of belief–desire psychology, for ‘altru-
ists are intentional agents whose effective desires are other-directed’ (Sober and
Wilson [1998], 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 re-
spond 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 [2011], p. 74; see also Kitcher [2006], p. 172).
Kitcher’s ([2011], 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.
Ivan Gonzalez-Cabrera4
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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 unconven-
tional 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 ([2011], 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 eth-
ical 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 ([2011], 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 punish-
ment 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.
([2011], 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 pun-
ished 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. [2012]), and a ver-
sion of the demonic male view—the idea that dominance and male aggression
were the cardinal challenges in the evolution of human sociality. His vindicat-
ing 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 pro-
social tendencies of our lineage. As we will see later, if the model of our early
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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 philo-
sophical 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 capa-
cities. This baseline can be established through research in comparative psych-
ology. 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 (Pru
¨fer et al. [2012]), 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 [1997]). They also display relatively
low levels of cooperation (Hirata and Fuwa [2007]). In contrast, hierarchical
relationships among bonobos are not always clearly defined (Kano
¯[1992]).
Female dominance is common, and it is based on female alliances against
aggressive males (Vervaecke et al. [2000]). Moreover, experimental evidence
also suggests that bonobos are more similar to humans in the way they solve
various cooperative problems (Hare et al. [2007]).
Sexual and play behaviours are different as well. In bonobos, sexual inter-
actions occur in mixed- and same-sex pairings, and it is also used for conflict
resolution (de Waal and Lanting [1997];de Waal [2001]). Play behaviour is
common in adult bonobos, especially among females (Palagi [2006]). In con-
trast, chimpanzee sexual behaviour is less rich and diverse (Goodall [1986]).
Sexual interaction does not typically occur in same-sex pairings, and (as in
other primates) high-ranking males monopolize oestrus females. Unlike bo-
nobos, 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
Ivan Gonzalez-Cabrera
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homologies (traits inherited from a common ancestor), analogies (traits that
have evolved independently due to similar selective pressures), or a combin-
ation 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 ex-
planation of these differences. As we will soon see, comparative studies in Pan
show that neuro-anatomical differences may be responsible for these behav-
iours, 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 differ-
ences 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 bo-
nobos 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
3
Evolutionary lability can lead to these neuro-anatomical differences. In plasticity-first hypoth-
eses, phenotypic plasticity can produce developmental variants that might increase fitness (Levis
and Pfennig [2016]). 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 [1992];Waddington [1953];West-Eberhard [2003]). 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.
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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 evolution-
ary trajectory of the lineage when the selective pressures for increased cooper-
ation escalated. Adaptations for tolerance and prosociality make the
evolutionary trajectory toward seemingly distinctive human traits such as
imitative learning (Galef [1996],[2009];Tomasello [2009]) or collective fora-
ging (Tomasello et al. [2012]) 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,
Pru
¨fer 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 (Pru
¨fer et al. [2012], pp. 2–3). Given the behav-
ioural 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 ([2012], 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 bo-
nobos 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:
(1) group hunting behaviour;
(2) enhanced emotional control;
(3) increased aversion against aggression (specially intermale and inter-
group aggression);
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(4) enhanced brain connectivity for empathy (top-down and bottom-up
control of aggressive impulses);
(5) increased mind reading skills;
(6) increased cooperative and sharing tendencies;
(7) a non-linear or ill-defined hierarchy; and
(8) non-exclusive male dominance.
Traits (2)–(4) are comparative features, that is, they are traits of early homi-
nins 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 ag-
gressive and dominant tendencies, such that overcoming them would be dis-
placed (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 guid-
ance. 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 behav-
iours if such behaviours are present in all of its living descendants. This argu-
ment 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 [2008]). The same goes for some aspects of physical
cognition such as tool manufacture and use (Ingmanson [1996];Gruber et al.
[2010]).
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
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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 conser-
vative 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. [2010]), 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 chim-
panzees. Bonobos and humans have a similarly organized orbitofrontal cortex
and a relatively smaller area 13 (Semendeferi et al. [1998]). 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. [1999]). In humans, their hypothesized functions include
self- and social awareness, self-control, and empathy (Allman et al. [2010],
[2011]), which would be crucial for bonobo social organization and its typic-
ally weak dominance hierarchy. They are also thought to be an important part
of the circuitry responsible for rapid, intuitive choice in complex social situ-
ations (Allman et al. [2005]).
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. [2012]). The former is implicated
in emotion regulation in humans, while the latter enables the restraint of ag-
gression via top-down suppression of aggressive impulses from the amygdala
(Davidson et al. [2000];Pezawas et al. [2005];Meyer-Lindenberg et al. [2006]).
The same pathway may also be involved in controlling aggressive impulses via
a bottom-up relay of perceived distress in others to the ventromedial pre-
frontal cortex that inhibits anti-social behaviour (Blair [2007],[2008]).
Insofar as the above neurobiological traits are examples of fine-grained
similarities, then parsimony suggests that the early hominins possessed (2)
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. [2005];Santos et al. [2011]), frontotemporal dementia
(Seeley et al. [2006];Santillo and Englund [2014]), and schizophrenia (Bru
¨ne et al. [2010]) 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. [2014]).
Ivan Gonzalez-Cabrera10
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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 physio-
logical responses that are especially geared to cooperative breeding
(Fernandez-Duque et al. [2009]). 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. [2007]). They show a stronger stress hormone response
to feeding competition (Wobber et al. [2010]). They have also been described
as more nervous and shy than chimpanzees (de Waal and Lanting [1997]). 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.
[2011]). Similarly, bonobos outperform chimpanzees in tasks related to mind-
reading, while chimpanzees are more skilled at tasks requiring the use of tools
and an understanding of physical causality (Herrmann et al. [2010]).
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 impli-
cated in mindreading capabilities in humans (Gallagher and Frith [2003];Saxe
and Kanwisher [2003]). 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 competi-
tive contexts (Call and Tomasello [2008]), 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 [2004]) or when
the food is not valuable and not monopolizable (Blurton-Jones [1987];
Gilby [2006]). However, peaceful food sharing in wild bonobos seems to
contradict the usual sharing-under-pressure hypothesis (Yamamoto [2015]).
Under experimental conditions, active and voluntary food sharing also seems
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to be present in bonobos (Hare and Kwetuenda [2010]), even among strangers
and when food is easily monopolizable (Tan and Hare [2013]).
Moreover, recent studies suggest that selection on emotional reactivity crit-
ically shapes a species’ ability to solve social problems (Hare et al. [2005];Hare
and Tomasello [2005]). This hypothesis, for instance, predicts that bonobos
will cooperate more successfully in food-retrieval tasks than chimpanzees be-
cause 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 bo-
nobos are much more prone to cooperation than chimpanzees (Hare et al.
[2007]).
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 de-
serves 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 differ-
ences. 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 in-
creases bonding between individuals. Bonobos with lower testosterone levels
and attenuated testosterone responses engage more often in amicable relation-
ships with unrelated females and have greater reproductive success (Surbeck
et al. [2012]). Therefore, bonobo males benefit from affiliative long-term as-
sociation with females (Surbeck et al. [2012]), which facilitates more egalitar-
ian 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 [2006]). Bonobos—females more than males—seem to use play to
assess physical skills, the willingness of other individuals to invest in a rela-
tionship, and to strengthen already existing social bonds. Adult bonobos play
much more frequently than chimpanzees. This asymmetry is important be-
cause it has been shown in experiments that both species use grooming and
play as social currency (Schroepfer-Walker et al. [2015]). 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
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could also have a crucial role in the bonobo social organization and its typ-
ically 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 [1996]). 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. [2015]).
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 refer-
ential doctrine suggests (Sayers et al. [2012]).
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 Zrzavy´ [2013]). 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 [1981]) or analogy
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 [1996];Wrangham [1993]). 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.
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through convergent evolution (DeVore and Washburn [1963];Fernandes
[1991];Jolly [2001];Perry et al. [2003]). The above model can be considered
rather neutral regarding this issue.
Palaeoanthropological evidence, however, suggests that even if some as-
pects 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 [2001];Brunet
et al. [2002];Haile-Selassie et al. [2004];White et al. [2009]).
Early hominins and the Pan/Homo LCA could also have been very differ-
ent 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. [2009]). 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. [2009]). 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. [1994],[2015]). 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. [2009]). 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. [2002], p. 150), Orrorin (Senut et al. [2001]), and Ardipithecus kadabba
(Haile-Selassie [2001]). Since the canine tooth is usually used as a weapon in
intermale and intergroup conflicts, the less pronounced upper canine teeth
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suggest that early hominins, including A. ramidus, were characterized by rela-
tively little intermale and intergroup aggression compared to chimpanzees.
Similarly, A. ramidus is also expected to have shown little sexual dimorph-
ism in body size—comparable to that of chimpanzees or humans, as opposed
to orangutans or gorillas (White et al. [2009]). In higher primates, body size
dimorphism is usually coupled with strong canine dimorphism. Using di-
morphism 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 [1997]). 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 chimpan-
zees, A. ramidus possessed low levels of agonistic male–male competition
(Clark and Henneberg [2015])—and even, perhaps, male–female codominance
as in bonobos (Suwa et al. [2009], 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 [2009]).
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. [2008];Van Arsdale and Wolpoff
[2013])—which is true in terms of body size but not canine size (Leutenegger
and Shell [1987];McHenry [1992];Plavcan and van Schaik [1992]). 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 australo-
pithecines 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
7
A similar problem occurs with diet. A. ramidus and modern humans are omnivorous, but aus-
tralopithecines 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.
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(Reno et al. [2003],[2010]). 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).
It is not clear whether body size dimorphism in australopithecines is a con-
sequence of male–male competition, since their canines have a variety of fea-
tures inconsistent with their use as a weapon (Greenfield [1992]). There are
multiple reasons that could potentially explain the increase in body size di-
morphism in australopithecines, such as reduction in female body size (Leigh
and Shea [1995]), predator defence (Clutton-Brock et al. [1977]), and Rench’s
rule (see Fairbairn [1997]).
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 [2015]).
7 Philosophical Consequences
The model I have defended in this article has important philosophical conse-
quences 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 [1998],
[2006],[2011]).
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, dom-
inant 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 expan-
sion 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,
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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 conse-
quences 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 guid-
ance 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
Figure 1. 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 inter-
group 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.
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demonic male view nor Kitcher’s ([2011]) account of our capacity for norma-
tive 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 capaci-
ties (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 in-
dividuals to monopolize resources and to impose social order through aggres-
sion 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 avail-
able 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 [2007]). 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 atten-
tion, 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 [2014]). 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. [2003]), 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 [2009]).
Primates do form social expectations, but they lack the capacity to form
normative ones (von Rohr et al. [2011]). 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. [2006],[2007],[2012]). Thus, aban-
doning the joint activity naturally entails a risk of reprisal (Gilbert [1989]).
Similarly, it has been argued that shared intentionality also has straightfor-
ward consequences for moral cognition (Tomasello [2015]) since, as Christine
Korsgaard ([1993], 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’.
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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 previ-
ous step for the full emergence of shared intentionality (Hawkes [2012],[2014])
because although cooperative breeding leads to greater prosocial skills, it does
not, in itself, entail higher cognition (Burkart et al. [2014]). 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. [2012]).
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 cooper-
ation could be partially explained by the role of normative thinking in facil-
itating coordination explains why some (Sterelny [2012];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. [2012]).
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 com-
ments on earlier versions of this article. I also thank the two anonymous
referees for this journal for further improvements. I am pleased to acknow-
ledge the financial support for this work provided by the Australian National
Evolution of Human Normative Guidance 19
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University, the Max Planck Institute for Evolutionary Anthropology, and the
Konrad Lorenz Institute for Evolution and Cognition Research.
School of Philosophy
Research School of the Social Sciences
Australian National University, Canberra
Australia
and
Department of Comparative and Developmental Psychology
Max Planck Institute for Evolutionary Anthropology
Leipzig, Germany
ivan.gonzalez-cabrera@anu.edu.au
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