EVOLUTION OF SOCIAL BEHAVIOUR PATTERNS IN PRIMATES AND MAN
A briefing document prepared for the Royal Society
and Association of British Science Writers
by Wendy Barnaby
August 1995
Summary and conclusions
The past few decades have seen bad blood between biologists and
social scientists. Each camp has seen social evolution as its own
preserve: the biologists confident that evolutionary biology would
deliver insights about culture and social behaviour; the social
anthropologists, psychologists, and linguists resenting intrusion
onto what they have seen as their patch. The two groups have
fiercely debated questions about genes and culture: how behaviour is
transmitted from one generation to another; where language comes
from and how it is learnt. This meeting was an attempt to bring the
two sides together. It set out to discuss how far the emergence of
new patterns of behaviour represents learning or results from
genetically-transmitted mechanisms; how cognitive development
interacted with social organisation during critical transitions in
human evolution; how far primates have culture but not language; and
when, and how, language itself developed in human but not other
primate species.
Summing up the meeting, Professor John Maynard Smith(1) said it had
been a pleasure to hear people from so many different disciplines -
genetics, linguistics, philosophy, archaeology, anthropology -
speaking without totally misunderstanding each other and without any
serious aggression. Understanding how human society evolved from
primate society would be hard, he contended, because the method of
transmitting information between the generations changed from
genetic to cultural mechanisms. While evolution or history may be
possible to understand, the intermediate stage, when the kind of
cultural behaviour depended on previous genetic evolution, would be
difficult. However, everyone agreed that there had been a change.
The problem was what sort of change it was.
Professor Maynard Smith thought that bringing together scientists of
human behaviour and biology could take us to the brink of an
important breakthrough in the study of human evolution, but that
each would have to learn the other's terminology - or, better, work
out a common one - if much more progress were to be made.
In his introductory remarks, Lord Runciman(2) predicted that we may
be at the beginning of a new synthesis involving primatologists,
archaeologists, linguists, psychologists, anthropologists and
biologists which may turn out to be as important as the old "new
synthesis" which integrated population genetics with evolutionary
theory.
The presentations Dr Carel van Schaik(3) is a social ecologist who
views social behaviour as adaptations maintained by natural
selection. He explains differences in social structures and
organisations - especially for females - in terms of ecological
factors, such as the distribution of food and predators. These lead
to two basic types of social structures among primates, which are
illustrated by related species with different ecologies, eg two
species of squirrel monkeys. One, in Costa Rica, live in a forest of
densely-packed, tiny trees, with minute food crops that they
disperse to harvest. They have no need for alliances and no
conflicts. The others, in Peru, live in a tropical rain-forest with
trees of all sizes in which conflicts and coalitions are common.
Males' behaviour, on the other hand, is more focused on increasing
their access to mates. They do not form alliances, because they
would not benefit from them. The ecological model has explained much
about behaviour, but it needs to take some social factors, eg
infanticide, into account. The best place to test these ideas is in
the field, but undisturbed habitats are disappearing fast and in a
few decades it may be impossible to find any.
Professor Robin Dunbar(4) argued that, in order to understand why
primates adopt particular social and reproductive strategies, we
need to understand the demographic context in which their decisions
are being made. Environmental factors in the form of the climate and
vegetation of a habitat will constrain the size of the group any
particular individual lives in, and therefore the social and
reproductive strategies it can adopt. Research on baboons in Africa
shows that three variables - rainfall, rainfall diversity and
temperature - predict accurately the size of sustainable
populations. Each population has its own ecological niche which
contains its preferred diet in quantities which allow it to feed,
rest and interact appropriately for the maintenance of the group.
Cognitive constraints are another determinant of group size. The
bigger the group, the more relationships the individuals have to
keep track of, and therefore the more brain power they need to do
it.
It turns out that, to sustain a given group size, apes need the
largest brain information processing capacity, particularly of the
type located in the cortex which deals with sensory perceptions and
the higher mental functions. Monkeys need less, and prosimians
(primitive pre-monkeys) need least. This could reflect the greater
complexity of the social relationships in ape and monkey groups.
Given a particular group size, males can adopt particular mating
strategies to optimise their fitness. Their behaviour can be
accurately predicted using mathematical models. It does therefore
seem possible to look at primate behaviour using a systems approach
from the top down as well as an optimisation approach from the
bottom up. This approach looks likely to produce, quite soon, a
general theory of primate social systems.
The next paper, given by Dr Dorothy Cheney(5), dealt with one of the
most difficult areas of culture: language and its meaning. It has
been argued that the sounds animals make can only be called language
when individuals recognise each other's mental states and understand
the causal relation between mental states and behaviour. Both
baboons and vervet monkeys give distinct calls which the other
members of the group recognise mean different things and which
affect behaviour. Baboons grunt to grease their social wheels,
especially between individuals of different ranks, and to reconcile
former enemies. They also give a louder call to maintain contact
among individuals when the group is widely dispersed. Vervet monkeys
give different sorts of alarm calls according to the danger they
perceive. They have one call for a leopard, and at this signal the
others climb into trees where they will be safe from leopards; and
another call for eagles, at which the others hide in bushes where it
will be hard for eagles to find them. But although the responses of
both baboons and monkeys are specific to each call, the sounds they
make cannot be called language. Experiments have shown that the
individual who makes the call has no intention of influencing the
others' mental states; in fact the caller has no conception of the
fact that the listener's mental state may be different from the
caller's. As baboons and monkeys cannot conceive of mental states
other than their own, they are unable to have culture in the sense
of language. When it comes to chimps, however, the evidence is far
less clear, and some would argue that chimps are able to impute
different states of mind to other individuals.
Professor Rob Boyd(6) asked why culture is so rare in nature, and
probably confined to humans. By "culture" he means differences
between individuals that are the result of information transmitted
by some kind of learning process. In human culture this would not
mean genetic or environmental variation, but ideas or beliefs, which
can vary between cultures. Many species have different ideas: from
food preferences in rats to foraging behaviour in pigeons. Their
cultural differences are maintained by a process known as social
enhancement, which occurs when the activity of the older animals
increases the chance that the younger ones will learn the behaviour
on their own. Cumulative cultural change however may be unique to
humans. Their artefacts (bows, arrows, knowledge of manufacturing
tents and clothing) have evolved over a long time and are too
complex for any individual to invent by themselves. It is humans'
ability to imitate which allows them to acquire culture in this way.
Most animals, with the possible exception of birds and chimpanzees,
aren't able to imitate. But why is imitation so rare? It could be
that it requires costly psychological mechanisms. Professor Boyd's
model of the evolution of imitation suggests that it may have
started very slowly and for some other reason; but once it began it
kept going.
Dr Robert Foley(7) tackled the evolution of human social behaviour
from the standpoint of paleobiology. He asked about the specific
conditions, such as group size, resources and social structure, that
would lead to social complexity. The best way of understanding why
humans have evolved at all but not several times over (there is
after all only one human species) is by an improbable combination of
ordinary biological conditions. For human evolution, this means
sociality which is the result of a bigger brain. Brains however are
expensive, and their size is constrained by metabolic cost: 20% of
our basal metabolic rate is used up simply keeping our brains going
(compared with about 8% for chimps). So unless the individuals have
a good, high-quality food supply to sustain the costs involved, they
will come up against an evolutionary brick wall. For most species,
the costs exceed the benefits and they're better off being more
fecund and less intelligent. Humans have been able to break through
this barrier because of the strategies they have adopted to cope
with the cost of the large brain. They have modified their life
history by maturing slowly and living longer, thus creating the
conditions for increased social complexity. They have also
established male kin-bonded groups to counter the increased energy
the females have had to expend on gestating and raising offspring
with large, energy-guzzling brains. These evolutionary strategies
have developed at different times over the past 50 million years.
Professor Leda Cosmides(8) takes an evolutionary approach to
psychology. She sees the mind as a Swiss army knife, which has
specialised structures for specialised functions. We do certain
things instinctively: see, speak, fall in love etc; and these are
often portrayed as being the opposite of reason because we seem to
do them without thinking. In fact, argues Cosmides, they are
rational strategies for survival, and only seem effortless because
they have become underpinned by a huge array of computational
machinery in the brain. She is studying social exchange and
reciprocation: functions which are crucial to human societies and
which have evolved over at least 2 million years. Social exchange
cannot evolve in a species unless its individuals have some way of
detecting cheats, in order to exclude them from future interactions.
Cheating on a social contract is different from simple
rule-breaking, and experiments show that only 25% of people can
manage the logical reasoning which would enable them to detect
rule-breakers. However, when it comes to detecting cheating in
social exchanges, in which the logic is the same, 75% detect
correctly. This suggests that we have specialised mechanisms for
detecting cheats. What these are is not known, but the next stage of
the research will be to look at the neural basis for them.
The specialised domains of the brain were also the subject of the
next paper, given by Dr Steven Mithen(9). As an archaeologist, he is
concerned with the relationship between the evolution of social
behaviour and the human mind. Modern hunter-gatherers, who lived 7
000-8 000 years ago, looked on the natural world as being full of
personal meanings and social relations, which they expressed through
totemism, symbolism concerning animals, personal adornment and
spacial organisation - the remains of which can be studied by
archaeologists today. These meanings are not found in the early
Paleolithic archaeological record, before about 60 000 years ago,
suggesting that complex social behaviour was absent among early
humans. Yet the fossil record of the late Neanderthals and other
people who lived before about 50 000 years ago shows they had brain
and body sizes which would suggest that their social structure would
have been more complex. Mithen suggested that this paradox can be
resolved by the way we regard the human mind. He postulated that
early humans had specialised cognitive domains for language and
intelligence about social relations, technical know-how and natural
history, but that (unlike modern humans) there was only a limited
degree of contact between them. Consequently, early humans were
unable to integrate their various intelligences to use them as media
for social interaction.
He suggested that the primate mind began as generalised but became
more specialised until 60 000-40 000 years ago, with a return to an
integrated intelligence which marked the start of the Upper
Paleolithic and is so evident in modern hunter-gatherers.
Dr Paul Mellars(10) presented a paper that dealt with what happened
in Europe and probably Asia, but not Africa, between 40 000 and 35
000 years ago, in the middle of the last glaciation. This period saw
the replacement of the Neanderthal by anatomically-modern Cro-Magnon
populations, and behavioural changes commonly referred to as the
Upper Paleolithic Revolution (new and more complex tools, personal
ornamentations, representational art, musical instruments, etc). In
cognitive terms, there may have been a change in the neurological
structure that gave abilities for long-range planning and
problem-solving, or the brains may have stayed the same but produced
new cognitive structures, eg a major increase in the development of
language. Mellars speculated about the social changes which might
have accompanied this transformation: an increase of population and
the size of residential groups, more bounded demographic groups and
associated ethnicity, increased specialisation of personal roles and
more complex descent and kinship systems. He suggested various prime
factors which could have precipitated the changes - population
increase, technological change, language development - but as yet
nobody knows which came first.
As a social anthropologist, Dr Monique Borgerhoff Mulder(11)
switched focus and spoke about how an evolutionary perspective can
throw light on contemporary human cultural diversity. She argued for
the role of behavioural ecology, which explores the function of
behaviour in contemporary populations, as against evolutionary
psychology, which takes Pleistocene conditions as its baseline of
psychological adaptation and ignores intriguing cultural
differences. Field anthropologists have tested hypotheses based on
the assumption that humans would behave in ways that would maximise
their fitness - ie the numbers of their descendants. Her own work
with the Kipsigis in Kenya has been on bridewealth: the payment that
a man makes to his wife's family when he wants to marry her. In
1982-3, men were more likely to pay much more for girls who reached
menarche at an early age. Demographic data showed that these girls
were more likely to have longer reproductive lifespans and to
produce significantly more surviving offspring. Men also paid more
for women who came from far away, and who were therefore not
distracted by close relatives and worked harder for their husbands.
By 1991, however, things had changed. The Kipsigis were no longer
paying over the odds for girls who reached menarche early. They were
having fewer children and investing more in the education of those
they did produce. Borgerhoff Mulder argued that the change in
bridewealth reflected the devaluation of fertility in this new
situation. The relationship between bridewealth and the distance of
the bride's home had also disappeared. The price of maize, which the
women had tended, had crashed, and men's contribution to the
household budget had increased. She argued that men were modifying
their exertion of mating effort, as reflected in the bridewealth, as
an adaptation to their new circumstances.
Professor Myrna Gopnik(12) presented a paper on language and genes.
Since the 1960s linguists have recognised that the language instinct
which enables children to learn to speak without instruction also
enables them to build appropriate grammars. In some people, however,
this instinct for grammar seems to be impaired. They then use their
general-purpose cognitive mechanisms to learn language, and although
they can simulate language, the grammar they use is very different
from usual. Children who come to language pathology clinics appear
to have normal intelligence and no perceptual or emotional problems,
yet they have very late onset of language and a great deal of
difficulty developing normal language properties, even under intense
instruction. Gopnik has looked at English, Japanese and Greek to
study the general nature of the disorder. She presented evidence to
support the hypothesis that impaired speakers cannot make an
abstract grammatical rule and apply it to novel forms of words. With
hard work they can learn explicit rules and vocabulary items one at
a time, but they never develop normal language. Epidemiological data
suggests that this is a genetic disorder: if a child is
language-impaired, there is a significantly higher probability (an
incidence of 20-40%) that a relative will also be language-impaired.
Gopnik did not however think the disorder is caused by anything so
crude as a grammar gene. She guessed that the condition disturbs the
neuronal arrangement and might very well impair other domains as
well. Until the cause is found it is impossible to say what kinds of
biological consequences it has and how it affects the brain.
Biologist Dr Christophe Boesch(13) gave a paper about the
possibility of culture amongst wild chimpanzees. By "culture" he
means the transfer of information between generations by non-genetic
means. Different groups of chimpanzees behave differently in
different situations. Psychologists see different behaviours as the
result of the chimps' attempts to learn under different ecological
constraints, and conclude that they do not have culture.
Primatologists have argued that the variations are the result of a
social learning process, which means they do have culture; and
Boesch's observations of chimp behaviour lead him to agree. Chimps
in Tanzania for example are four times more efficient in fishing
ants out of their nests than chimps elsewhere, and it may be that
social norms are constraining the less efficient chimps from finding
the more efficient way of fishing. Chimps use leaf-stripping to mean
different things in different populations; they deal with the
parasites they pick off each other in different ways, and the males'
drumming means different things too (in one group it is a code for
instructions to the group: how long they should rest and which
direction they should move in). Boesch thinks that the reason humans
think culture is so rare in other animals is that our definition of
what constitutes culture is too restrictive. If it were looser we
would allow these examples to indicate culture.
The last paper was presented by paleontologist Dr Leslie Aiello(14),
who addressed the tangible evidence from the fossil record over the
last 4 million years on the evolution of language. There is a
tendency for the development of modern human language to be
associated with the appearance of the Upper Paleolithic in Europe
about 48 000 years ago, and for the previous population, the
Neanderthals, to be thought anatomically incapable of modern
language. Recent research however has overturned this view. Aiello
argues that the pre-conditions for vocalisation were associated with
early man changing, about 2 million years ago, from living in trees
to living on the ground. This change was accompanied by an increase
in the size of groups, increased brain size and changes in
anatomical features of the jaw and throat, all of which provided
spin-offs for the later development of language.
Fossil evidence from 1.6 million years ago, half way between the
apes and us, suggests that the people living then would not have
been able to make the sophisticated sounds we can make today. But
why did language begin to evolve? Aiello suspected that it might
have had something to do with the cost to the female. She had to
maintain a large-brained fetus, care for a large-brained offspring
for a long period, eat a high-quality diet to nourish her
newly-evolved larger brain, and therefore expend more energy in
finding food. Females would therefore have been put under increasing
strain, and the development of language might have had something to
do with changes to the social pattern in which the males were
brought in to provide for the females and offspring more than they
had before. The evidence suggests that our early human ancestors
might have been capable of making some sounds at an early stage in
their evolution, and would probably have spoken in the modern sense
about 250 000 years ago.
Contacts
1. Professor John Maynard Smith F.R.S.
School of Biological Sciences
University of Sussex
Falmer
Brighton tel: 01273 606755
BN1 9QG fax: 01273 678433
2. Lord Runciman F.B.A.
Trinity College
Cambridge tel: 01223 338515
CB2 1TQ fax: 01223 338564
3. Dr Carel van Schaik
Department of Biological Anthropology
and Anatomy
Duke University
3705 Erwin Road
Durham
North Carolina 27705 tel: 00 1 919 490 6286
UNITED STATES OF AMERICA fax: 00 1 919 490 6718
4. Professor Robin Dunbar
Department of Psychology
University of Liverpool
Eleanor Rathbone Building
Liverpool tel: 0151 794 2957
L69 3BX fax: 0151 794 2945
5. Dr Dorothy Cheney
Leidy Laboratory of Biology
University of Pennsylvania
Philadelphia
PA 19104-6018 tel: 00 1 610 527 9641
UNITED STATES OF AMERICA fax: 00 1 215 898 8780
6. Professor Rob Boyd
Department of Anthropology
University of California
Los Angeles
CA 90024 tel: 00 1 310 206 8008
UNITED STATES OF AMERICA fax: 00 1 310 206 7833
7. Dr Robert Foley
Director
Studies in Archaeology and Anthropology
King's College
Cambridge tel: 01223 335455
CB2 1ST fax: 01223 335460
8. Professor Leda Cosmides
883 Windsor Way
Santa Barbara
CA 93105 tel: 00 1 805 569 3126
UNITED STATES OF AMERICA fax: 00 1 805 569 5806
9. Dr Steven Mithen
Department of Archaeology
University of Reading
Reading tel: 01734 875123
RG6 2AA fax: 01734 314404
10. Dr Paul Mellars F.B.A.
President
Corpus Christi College
Cambridge tel: 01223 338006
CB2 1RH fax: 01223 338061
11. Dr Monique Borgerhoff Mulder
Department of Anthropology
University of California
Davis
California 95616-8522 tel: 00 1 916 752 0659
UNITED STATES OF AMERICA fax: 00 1 916 752 8885
12. Professor Myrna Gopnik
Department of Linguistics
McGill University
1001 Sherbrooke Street West
Montreal
Quebec H3A 1G5 tel: 00 1 514 398 4224
CANADA fax: 00 1 514 398 0788
13. Dr Christophe Boesch
Institute of Zoology
University of Basel
Rheinsprung 9
4051 Basel tel: 00 41 61 267 3438
SWITZERLAND fax: 00 41 61 267 3457
14. Dr Leslie Aiello
Department of Anthropology
University College London
Gower Street
London tel: 0171 387 7050
WC1E 6BT fax: 0171 380 7728
Enquiries to: Ref: PR 32 (95)
Miss Anna Link
Science Promotion Section
The Royal Society
6 Carlton House Terrace
London
SW1Y 5AG
Telephone: 0171 839 5561 ext 315 16th August 1995
INFORMATION NOTE
Evolution of social behaviour patterns in primates and Man
On 4th and 5th April 1995, the Royal Society and British Academy
held a scientific meeting on Evolution of social behaviour patterns
in primates and Man. The enclosed document was prepared afterwards
to summarize key issues raised by the speakers and to provide a list
of helpful contacts for future reference.
This document does not necessarily constitute the views of the Royal
Society or the British Academy, and views expressed in it should not
be attributed to the Society or the Academy. The document is free of
copyright and may be used without reference to source.