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Tree of Origin: What Primate Behaviour Can Tell Us About Human Social Evolution

Authors: Frans de Waal, Richard Byrne, Robin Dunbar, William C. McGrew, Anne Pusey, Charles Snowdon, Craig Stanford, Karen B. Strier, and Richard W. Wrangham



Tree of Origin: What Primate Behaviour Can Tell Us About Human Social Evolution

Image: edited to show Pan species, a chimpanzee (left) next to a bonobo (right)



Purpose of Comparative Biology

"Depending on whether the main objective is to confirm human identity or to stress the common thread that runs through all forms of life"

1. Separating humans from animals

In anthropology, trying to define humanity, determining if a behaviour is exclusive to our species

2. Integrating humans with animals

As in the Darwinin approach, evolution, natural selection and behavioural origins/similarities

3. Understand human social evolution BY comparison to primates

Using observations of social organisation, communication, learned habits, subsistence, reproduction and cognition

4. Understand human behavioural evolution BY comparison to primates

Tool use etc.

Influential Factors

"All behaviour in all primates, including our own species, derives from a combination of evolved tendencies, environmental modification, development, learning, and cognition"

Behaviour is influenced by genetic and environmental factors

Kin selection, competition and cooperation, and food distribution important in determining affects on social behaviours

Comparisons & Beginnings

Pioneering comparisons of primate and human behaviour:

1967 Desmond Morris's 'The Naked Ape'

1971 Jane Goodall's 'In the Shadow of Man'

Reconsider culture, language etc.

Origins of politics, warfare, morality seen in primates

Image: credit from the book itself, 'Tree of Origin', showing the evolutionary primate tree



Of Genes and Apes: Chimpanzee Social Organisation and Reproduction

by Anne E. Pusey

Social structure: Patterns of kinship, residence, philopatry and mating

Influence: Patterns of cooperation and social behaviour

Observing Chimpanzees:

  • Typically difficult to observe wild

  • Provisioning use to be used to habituate chimpanzees

  • 1960 Jane Goodall, Gombe National Park: banana provisioning

  • 1965: Toshidada Nishida, Mahle Mountains: sugar cane provisioning

    • Adv: make close observations, habituate shy chimpanzees, follow individuals easily

    • Dis Adv: effect behaviour unnaturally, less valid results

  • 1970-1980: non-provisioned


  • Jane Goodall: chimpanzees move alone/small temporary parties, any gender/age, only sometimes gather in large communities, only mother and offspring stable group

  • Toshidada Nishida: chimpanzees rarely all together, live in large "unit groups" (communities), several adult males and more/same females and offspring

  • Now: chimpanzee communities 20-120 individuals

Fission-fusion society:

  • Individuals of a community spend time alone and time in temporary sub-groups

  • Females 50% time alone, males 18% time alone

  • Females localised but do visit all ranges, males large 8-15km range

  • Unusual among primates

Due to female receptivity

  • Sexual receptivity cycles by swellings lasting 13 days in a 36-day cycle

  • 5.2 year Gombe/7 year Kibale interbirth intervals

  • Swellings: females over large range/outgoing

  • No swellings: feed alone accompanied by dependent offspring over small range

  • Females distributed SO Males range over wide area in groups to protect access to receptive females

Due to feeding behaviour

  • Richard Wrangham: diet consists of seasonal/sparse fruits which cannot suit large groups, females minimise competition and maximise efficiency and reproductive success by feeding alone


  • 1960s Gombe: no evidence of lethal aggression

  • 1970s Gombe: habituated communities at feeding stations split into sub-groups, these groups became extreme separate communities, Kaesekela North and Kahamana South and they stopped travelling into the other range, patrolling boundaries and violent group attacks observed

  • Male chimpanzees participate in intergroup hostility


  • Feeding territory protection

  • Female access protection

  • Female access increase by expanding territory

Philopatry and dispersal:

  • Nishida: males and females belong to set particular group

  • Wrangham: male-only groups with female crossing both groups so male groups compete for territory with more females

  • BUT males attack both opposition/foreign community males and females

  • Goodall: 20 different strange females at Gombe attacked, 15/20 had infants

  • Females sensitive to community borders and expand core range with groups range

  • SO female group spatial allegiance means males don't gain more females by expanding range

  • So true dynamic: Male territoriality repel males and females of other communities

  • Males remain with kin and females disperse

Due to feeding

  • Goodall, Williams, Pusey: Males defend community feeding territory

  • Small range: smaller core area of females, chimpanzee density higher, lower body weight, longer interbirth interval

  • Expanding range: increase reproductive success and rates

Due to avoiding interbreeding

  • Male chimpanzees return to birth group from mothers stay within kin community range

  • Females mate with natal community and then disperse to avoid interbreeding with kin

  • Females show less sexual receptivity to male relatives, and scream to avoid them

  • This behaviour and social structure prevents interbreeding, reduced viability and fertility of offspring

  • Philopatry-dispersal of opposite sexes observed in many animals as interbreeding avoidance mechanism

  • Common pattern is for female philopatry and male dispersal

  • Chimpanzees have male philopatry and female dispersal

Due to kin selection

  • Male philopatry: cooperate with kin, genetics are passed on via relatives

  • Female dispersal: male kin remain for kin selection, so females must disperse to avoid interbreeding

  • Evidence: Gombe individuals shared mitochondrial DNA with individuals several km away which proves extensive female migration

Group dynamics and behaviour:

  • Friendlier behaviour towards relatives

  • Bonobos: females (which show philopatry in bonobos) have strict dominance hierarchy, with relatives and dominant females showing most support for one another and males have hierarchies with tense relationships and do not groom

  • Chimpanzees: male hierarchies depend on alliances with other males, many aggressive interaction but also more friendly male relationships, rival males groom and cooperate against external male threats, female rank has effect on reproductive success, offspring mature younger as they gain better access to food

But is it due to relatedness/kinship?

  • Mitochondrial DNA shows the most genetically related chimpanzees did not have close relationships

  • Chimpanzees with close behaviour relationships, cooperation and grooming were not maternally related

  • Relationships are opportunistic and not kinship related, for advantage of power

  • Philopatry-dispersal may have initially evolved for kinship

  • Levels of relatedness reduced by individual strategies and mating choice

Mating patterns and reproduction:

  • Female swell to advertise readiness to mate

  • Swollen females are centre of attention and subject to attack, wounding, less feeding time, infection

    • IMO: Though not advantages to the female. This feature is retained perhaps as it is those who have it who are more likely to reproduce and pass it on

      • Females can mate multiple times per day when receptive

Due to infanticide prevention

  • Infanticide is frequent but mating with many males confuses relation

  • Males will not want to kill their own offspring

  • By confusing who the father is, any male who has mated with the female will not kill the offspring, thus mating with more males offers greater protection from infanticide

  • Sarah Hrdy: Male primates taking over another group kill group infants so females return to receptivity

  • Mating with many males, even when pregnant, helps confuse paternity

  • e.g. Mahle: males kill boy offspring from newly immigrated females, though these males had mated with the females the fact that the females' ranging pattern was close to the border of their old group proved that infants were of dubious paternity, eventually females spend more time with the infanticidal males

Due to female gain

  • Females may mate with many males for personal gain

  • Males are more likely to protect and share food with females they have mated with

Due to genetic gain

  • Females mating with many males increase chance of high quality genes

  • More likely chance of high-quality male fertilisation

  • Increases male competition so more high-quality males mate with females

Male behaviour

  • Generally competition for female is low

  • Towards end of ovulation when fertilisation more likely high-ranking males become possessive of females

  • In large groups, high-ranking males work together to guard female and share copulation

  • Male may take female to edge of community range for long period, but the single pair at risk of neighbouring attacks

    • IMO: this last one sounds similar to human partnership behaviour

      • Males may also force females by attack to mate, or they may go willingly

Sexual dimorphism

  • Males much larger than female chimpanzees

  • Darwin: males compete aggressively over females

  • Male chimpanzees have large testes compared to other apes

  • As females mate with many males and house multiple male gametes, males who deposit more sperm are more likely to fertilise the female gamete

Paternity - who is the father?

  • Consortships high in chimpanzees (conceptions range from 50% Tutin and McGinnis, to 25% Goodall and Wallis)

  • Goodall: unlikely rank relates to paternity, even though alpha males may be possessive, females still mate with many males when likelihood of fertilisation high, so chance of paternity is varies

  • Other studies show rank and paternity correlation is strong but not absolute

  • Due to these mating patterns relatedness is rare, and few offspring are actually full siblings from the same parents

Human and Chimpanzee social structures


  • Hunter-gatherer bands consist of 150 members similar to chimpanzees

  • Female-biased dispersal and male philopatry (in agricultural, 2/3 hunter-gatherers)

  • Intergroup hostility

  • Patrolling

  • Kinship importance

  • Male philopatry- female dispersal so rare but occurs in chimps, bonobos and humans suggests it was also seen in our common ancestor

  • Sexual dimorphism with larger males


  • Human hostility is organised warfare and pitched battles with many opponents unlike chimpanzees

  • Kin bonds elaborated beyond group boundaries to extent of marriage in humans

  • Human copulations private unlike primates

  • Humans form conjugal marriage bonds, monogamy or man with many female partners unlike chimpanzees where females not bound and have multiple mates

  • Humans mate over more of total life span

  • Human mating occurs at lower frequencies, but at any time

  • Lower sexual dimorphism in humans (15%) than chimpanzees (25%)

  • Humans do not show cyclic changes to advertise ovulation

Why do humans form pair bonds and forgo high promiscuity?

  • Labour division: females gather vegetables and males hunt so the sexes share, infant dependency increased as brain size increased so males help protect and care for infant

  • Female benefits: Smuts suggests females gain protection by male partners and a male has increased mating opportunity, Wrangham proposes that after invention of cooking females needed males to guard their food



Apes from Venus: Bonobos and Human Social Evolution

by Frans B. M. de Waal

Use Bonobos and Chimpanzees as models

Bonobos as valuable as chimpanzees in studying human behaviour

  • Historically: Chimpanzee violence/male dominance means we adopt it as a model of the origins of human social behaviour and Bonobos more peaceful/female importance means they have been overlooked

  • Exaggerated idea: Chimpanzees not completely violent, Bonobos not completely peaceful

  • Use both as model: not wishful to compare to peaceful bonobo, or staying traditional to chimpanzee human social similarity


  • Bonobo changed very little since human-pan common ancestor

  • Chimpanzee adapted to semi-open woodlands

  • Bonobo stay adapted to equatorial regions

  • Chimpanzees adapted to new environment, bonobo similar environment to common ancestor = Bonobo model of the common ancestor?


  • Bonobos often walk upright (e.g. carrying food), with straight back more than other apes = very humanlike

  • Harold Coolidge state 1933 that bonobos "may approach more closely to the common ancestor of chimpanzees and man than does any living chimpanzee." = bonobo generalised anatomy as model of evolution, bridge gap between human and chimpanzee differences

  • Adrienne Zihlman measured body-weight distribution = bonobos resemble australopithecines (early hominid)

Image: credit Frans de Waal from the book itself, 'Tree of Origin', showing bonobos standing upright




Beyond the Apes: Reasons to Consider the Entire Primate Order

by Karen B. Strier



The Ape's Gift: Meat-eating, Meat-sharing, and Human Evolution

by Craig B. Stanford



Out of the Pan, Into the Fire: How Our Ancestors' Evolution Depended on What They Ate

by Richard W. Wrangham



by Richard W. Byrne

How did our cognition and intellectual capacities evolve?

Role of Comparative Biology

  • Behaviour of ancient ancestors hardly preserved archaeology

  • Behaviour of modern living species can provide an insight

  • We can look at:

  1. What form does a particular behaviour take?

  2. The functions of a particular behaviour now?

  3. Under what conditions is a particular behaviour seen?

  4. How is the behaviour related to the environment?

  5. How does the behaviour vary?

Relatives have evolved as much as we have since divergence

  • Modern primates they are not perfect prehistoric ape replicas

  • Two popular but wrong theories of using comparative biology:

  1. 19th Century: Great Chain of Being

    • Belief that studying modern primate is direct way our ancestors were like

    • View that monkeys evolved into apes evolved into humans

    • Ontogeny recapitulates phylogeny

  2. 20th Century: Radical Behaviourism

    • Belief that all species equal so rat can be used to compare with humans rather than ape

    • But if all species equal why need comparison

    • Claims that language sets humans apart


  • Both theories are overly simplistic

  • Language cannot be acquired suddenly, it is so complex it must have roots in other animal cognition

  • Capacities of all animals are not equal:

  1. Monkeys show more complex behaviour than most mammals

  2. This ability results from an enlarged neocortex that allows rapid learning

  3. Great apes demonstrate some understanding of intentions and causes.

  4. This comprehension is based on an ability to perceive, and to build, complex novel behaviour

What do we need to consider to answer evolutionary questions?

1. Function of evolved trait

  • Selection pressure = environmental challenge

  • Environmental challenge met by aptitude

  • How challenge increased inclusive fitness in carriers of allele/trait

  • How did those with trait produce more offspring with trait than those without the trait

Example: Cognition

  • "What environmental challenge caused ancestors to gain a fitness advantage from the cognitive adaption?"

    • The challenge may not have related to the function of cognition at the time

    • Are the challenges which cognition helps with today the same as the ones which led to its evolution, or does intelligence have new functions?

2. Form of evolved trait

  • Specific abilities for one use?

  • Multiple uses?

  • If intelligence is modular its function may have been the same

3. Chronology of trait

a) Sociocultural variation

  • Modern: cross-cultural variation

  • Evolutionary variety between cultures

  • Understand modern human psychology - individual and social environments

  • Recent evolution of trait

b) Evolution since divergence

  • Ancient: Evolutionary changes since divergence from last ancestor

  • Change in biological potential seen via archaeology, knowledge modern humans and last common ancestor

  • Behaviour leaves little material trace so archaeology rooted in past environment is more accurate

c) Evolution before divergence

  • Adaptions in our last ancestor with the potential for intelligence

  • Roots of intelligence have own evolutionary origins (challenges in past)

  • Presence and cause of intellect of the common ancestor can only seen via comparative study


As humans and chimpanzees/bonobos are more closely related to each other than any of them are to gorillas and orangutans, our last common ancestor is only 4.5m years old. This is too short a time span for any intellectual development. We need to look at its roots before our divergence. Modern comparative studies of animals shows wide intellectual differences, this provides evidence of the stages of intellectual development in human past.

Evolutionary Reconstruction

Evolutionary reconstruction uses comparative evidence to reconstruct the human past
  • The distribution of the characteristic in the living members is used to reconstruct its origin in the past

  • We can reconstruct the earliest phases of human behavioural evolution without fossil evidence

  • Similar to a family tree we use the phylogeny of primate relatives

    • Each biological branch on this family tree is a common ancestor

  • Existence of ancestors reliable from modern studies

  • Fossils/bones hard to attribute as lineage may have died out quickly

  • Phylogeny based on molecular and DNA similarities

  • Change occurs at constant rate overtime

  • So we can roughly calculate dates of ancestor

Traits shared with certain animals show its evolutionary origins:

  1. All monkeys = early evolutionary origin (approx. 25-30m ya)

  2. Great apes = trait evolves12-25m years ago

  3. Chimpanzees = trait evolves 4.5-6m years ago

  4. No species share trait = trait evolves after 4.5m years

  • Where trait is developed by convergent evolution (independently in different species) this cannot tell us how it evolved in humans but perhaps its function

  • Primate comparison allows time-frame for human behaviour

Complex Primate Behaviour

Social Support:

  • Monkeys and apes interact in third parties

  • Rely more on alliances to give power in competition

  • Alliances form among kin and non-kin


  • Important trade in alliances

  • Repaid by support in fights/tolerance at feeding site

Socially complex:

  • When major alliance threatened by minor argument, even opponents will reconcile

  • Obligation and influence of relationships key

Analysis of Social Rankings:

  • Socially knowledgable

  • Monkeys attacked by dominant animals react to assert their power

  • Attacked monkeys redirect aggression to weaker parties (like bullying)

  • Attack young relatives/subordinate females to gain power

  • Choice of victim shows awareness of social rank, opponents, dominant members and alliances

  • Calls and responses to monkeys shows they are aware of:

  1. Other kinships

  2. Dominance and social rank of other members

  3. Membership of groups they were never part of

Deception and Dominance:

  • Monkeys and apes use social knowledge for manipulative tactics (deception to get what they want)


  • Dominant male stops female gorillas from mating with subordinates

  • So she 'gets left' to be out of site to copulate

  • Invite subordinates and copulate quietly

Whiten and Byrne Survey of Deception:

  • Primatologists rarely publish 'anecdotes' of deception

  • Survey done to see cases of deception

  • Results suggest tactics varied but deception is used by primates


  • Young baboon screamed as if hurt when saw adult with food

  • Mother scare off 'aggressor'

  • Young gets food

  • AND Young aware of:

  1. Ensuring this is when mother out of sight

  2. Mother's rank is higher than adult

  3. Not reusing tactic often on same person

Why do primates have social complexity?

  • Quick at remembering socially relevant info

  • It is a species typical principle based on genetics

Trial and Error learning:

Because primates have fast social learning and connect social facts to environmental circumstances, they can quickly learn from situations


1. Event happens (Baboon attacked by adult)

2. Events follow (Mother protects young and gets food)

3. Application (Use situation is deceptive way next time)


Learn based off observing others

Ability to Learn:

  • Based off size of neocortex

  • Ratio neocortex linked to larger brains

  • Dunbar: ratio also linked to size of group/social complexity

  • Size neocortex may also be related to recognition (social or environmental - such as fruits)

  • Apes have brain x2 size of mammal of same body mass

Feedback Loop:

  • Increasing social complexity may select for brain size

  • Increasing brain size affects behavioural complexity

  • (neocortex ratio affect frequency of deception)


  • Larger brains evolved in response to a need for social skills, this increased brain size allowed for rapid learning which underlies the social sophistication of apes

Inability to understand mental state:

  • Tempting to assume animals using deception understand the situation

  • BUT comprehension of deceit and situation is not necessary

  • Rapid learning based off observation and application rather than understanding

Dorothy Cheney and Robert Seyfarth Experiment

  • Mother approached by "predator" when infant

  1. a) can see threat

  2. b) cannot see threat

  • In both circumstance mother alerted child of threat

  • Unaware of infants mental state of seeing or not

  • This explains why animals who are manipulated by deception do not understand what is happening

  • BUT Chimpanzees and apes show knowledge of intention:

Sarah Boysen Experiment

  • Chimpanzee approached by "predator" when friend

  1. a) can see threat

  2. b) cannot see threat

  • When chimpanzee realises its friend cannot see a call is raised and it acts to "protect"

  • When chimpanzee realises its friend can see there is no call raised

  • Chimpanzees can distinguish mental states

  • In the wild only chimpanzees teach offspring in a way that is aware of infants ignorance

Ability to understand mental state is unique to Great Apes:

  • Chimpanzees aware of when being deceived e.g. annoyed at a "look behind you" trick when there is nothing there

  • Cercopithecine monkeys use deceptive tactics more frequently than great apes

  • BUT the ways the apes use tactics imply awareness of what they were doing

  • Chimpanzees must have intentional social manoeuvring (e.g. switch allegiances)


  • Frans De Waal observe in Chimpanzee Politics:

  • Male who did not have qualities to become top ranked himself

  • Male used clever switches of allegiance to gain more effective power than held by either of the two males he supported

  • Once they began to solidify position with his support he defected to the other

Reasons behind social manoeuvring:

  1. Males who require powerful ally to hold top rank were always vulnerable

  2. A male who gradually built up a broad base of support among weaker males and females males was able to hold tenure far longer


  • Gordon Gallup notice chimpanzees, but not monkeys, show self-recognition

  • E.g. Looking in the mirror to examine hidden areas of their bodies such as teeth and gums, or making repeated, exaggerated gestures while watching their reflections

Complex understanding:

  • Few Old World monkeys regularly make/use tools

  • Chimpanzees in West Africa use two rocks as hammer and anvil to break nuts

  • Chimpanzees in East Africa use tools and bimanual coordination to capture ants

Manipulation of tools and techniques:

  • Capuchins use tools but in simplified situation - obvious visibility of hole of an open termite mound

  • Chimpanzees can termite fish with less clear mound, without a hole, and delicately guide tool - more abilities beyond use of tools


Chimpanzees can plan their tactics driven by knowledge not immediate stimuli and show awareness of possible outcomes


  • No suitable plant for fishing grows near termite mound, chimpanzees plan to make tools in advance to bring to site

  • No suitable rocks near nuts, chimpanzees carry a hammer-stone before they get to tree

Cultural Variation:

  • McGrew shows local traditions in chimpanzee tool use as in human cultures:

  1. Some discard frayed tools

  2. Some rotate tools to use less worn end

  3. Some 'resharpen' tool

  • The use of inefficient methods shows social learning by imitation

  • Trial-and-error requires progression towards optimal technique and cannot pin point inefficiency if there is no more efficient method attempted to compare to

Elaborate series of actions:

  • Tool manipulation and planning is a great ape trait


  • Gorilla diet is mainly of plants which use stings/hard castings/mechanics which make it hard to it

  • Gorillas use techniques to minimise stings from nettles

  • Remove leaf blades in gathered bunch by using half-open hand to strip stem

  • Remove stems/petioles with worst stings

  • Folds leaf bundle so only one leaf sting side is exposed

Why are great apes cognitively superior?

  • Efficient Skill Learning Mechanism: Imitation

  1. Gorilla: population consistent techniques and individual variation at detail

  2. Chimps: inter-population tool use differences not explained by ecology

  3. Orangutan: copy human traditions (e.g. building fire)

  • Comprehend objects and events - mental states and tool behaviour

  • Monkeys cannot acquire complex novel behaviour by observation and imitation

  • Great ape ancestor had cognitive potential to plan, imitate and understand others

    • Not due to further neocortex size (neocortex affects learning speed) so something else affect ape mental complexity (maybe structure?)

Selection Pressure:

  1. Complex Social interaction

  • 30m years ago: monkey and ape ancestor develop rapid learning and larger brain

  • Due to: Need for larger permanent groupings

  1. Monkey vs Ape Competition

  • 12m years ago: apes and human ancestor develop ability to take account of others' behaviour and understanding

  • Due to: Large size of apes adapted to brachiation (for trees) mean walk in awkward gait on knuckles

  • Old World monkeys have adv of eating unripe fruit

  • SO apes develop cognitive abilities such as foraging and planning to survive intense competition with monkeys

What cognitive skills were we left with?

  1. A causal understanding of complex behaviour, enabling novel schedules of actions to be put together and allowing the behaviour of others to be used as a source of ideas for new action schedules; in other words, nonverbal planning and skill learning by imitation

  2. A degree of understanding of the intentions of others-what they want, know, and think; in other words, possession of a theory of mind

Evolution of language would be impossible in a species in which individuals could not imagine that other individuals know things that they do not know themselves.
  • Language is acquired by imitation - not by direct copying

  • Children do not parrot our parents' words

  • Child "imitates" a word, the actual sounds are quite different and the use of the word is individual

  • An understanding of a novel behaviour is gained so an individual can express it in their way



Dunbar Brains on Two Legs: Group Size and the Evolution of Intelligence

by Robin I. M. Dunbar

Group size is a possible measure of social complexity

Social complexity then drives intelligence and neocortex size

The evolution of language allowed these large group sizes and brain advances



by Charles T. Snowdon



The Nature of Culture: Prospects and Pitfalls of Cultural Primatology

by William C. McGrew


My Reflections

What makes us human?

One would think our genes, looks, tools and intelligence makes us human but even those can be seen elsewhere.

It cannot be complex social structures, constructions, verbal communication and cultural material production, because we were human long before any of these.

Perhaps abstract thought and art? But I'm sure other species display that too.

Distinguishing us as a species by genetics?

We are Homo Sapiens based on our unique DNA - the genetic species theory.

However, what actually makes us human, humanity?

This has actually become harder to define as we learn more, as we compare humans with animals.

Rather than finding ways to distinguish ourselves from animals - as perhaps was the initial intention behind comparative biology - we find our behaviours and biology are identifiable in a wide variety of species.

From species close to us (primates) to even distantly related animals (dolphins), humans and not separate from nature, we are, like all animals, products of evolution and natural selection and thereby hold an equal place among all species.


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