19 Evolutionary origins of great ape intelligence
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19 • Evolutionary origins of great ape intelligence:
an integrated view
A N N E E . RU S S O N 1 A N D DAV I D R . B E G U N 2
1 Psychology Department, Glendon College of York University, Toronto
2 Anthropology Department, University of Toronto, Toronto
Among the great apes that once ranged the forests of While the challenges these abilities address may have
the Old World, only four species survive. Their evolu- provided the evolutionary impetus to enhancing great
tionary history reveals a huge range of morphological ape cognition, evolutionary reconstructions have more
and behavioral diversity, all of which must be consid- to explain than these. No single ability, combination of
ered successful adaptations in their own time. Some of abilities, or cognitive domain encompasses what sets
these attributes (large brains, sclerocarp and hard-object great ape cognition apart. In the physical domain,
feeding, frugivory, folivory, gigantism, terrestriality, and great apes do use tools in ways that require their
suspensory positional behavior) survive in modern great grade of cognition (Yamakoshi, Chapter 9, this volume)
apes. Our questions are: what combination of behav- but they devise equally complex manual techniques
iors and attributes characterized the ancestor of living (Byrne, Chapter 3, this volume) and solve equally
great apes? what was the significance of this suite of complex spatial problems (Hunt, Chapter 10, Russon,
features for cognition? and how did it arise in evolu- Chapter 6, this volume). They show exceptionally com-
tion? To that end, we offer our model of a distinct great plex social cognition in social routines, scripts, and
ape cognition along with its biological underpinnings fission–fusion flexibility, as well as in imitation, teaching,
and environmental challenges, then attempt to trace the self-concept, perspective-taking, deception, and pre-
evolutionary origins of this ensemble of features. tense (in this volume see Blake, Chapter 5, Parker,
Chapter 4, Russon, Chapter 6, van Schaik et al.,
Chapter 14, Yamagiwa, Chapter 12). Their communi-
COGNITION
cation reaches rudimentary symbolic levels, even con-
All living great apes express a distinctive grade of cog- sidering only strictly defined gestures and language
nition intermediate between other nonhuman primates (Blake, Chapter 5, this volume), as does their logico-
and humans. Their cognition normally reaches rudi- mathematical cognition (e.g., analogical reasoning, clas-
mentary symbolic levels, where symbolic means using sification, quantification) (e.g., Langer 2000; Thomp-
internal signs like mental images to stand for referents son & Oden 2000). The latter has not figured in
or solving problems mentally. It supports rudimentary evolutionary reconstructions but perhaps it should.
cognitive hierarchization or metarepresentation to levels Enhanced logico-mathematical capacities offer impor-
of complexity in the range of human 2 to 3.5 year olds, tant advantages; classification and quantification, for
but not beyond (in this volume, see Blake, Chapter 5, example, may aid in managing great apes’ broad diets
Byrne, Chapter 3, Parker, Chapter 4, Russon, Chapter 6, and social exchange (Russon 2002), and analogical rea-
Yamakoshi, Chapter 9). soning may support limited cognitive interconnections
Great apes’ high-level cognitive achievements are (see below). Others have also emphasized generalized
generalized in that they manifest system wide and features of great apes’ cognitive enhancements (in this
relatively evenly across cognitive domains (Russon, volume, Byrne, Chapter 3, Parker, Chapter 4, Russon,
Chapter 6, this volume). Evolutionary reconstructions, Chapter 6, van Schaik et al., Chapter 11, Yamakoshi,
however, have typically fixed on specific high-level abil- Chapter 9). Features our contributors identify include
ities, singly or in combination, such as self-concept or regular, sequential plans of many actions, hierarchi-
intelligent tool use (see Russon, Chapter 1, this volume). cal organization, bimanual role differentiation, complex
Copyright Anne Russon and David Begun 2004.
353354 A . E . RU S S O N & D. R . B E G U N
event representations, scripts and routines, and coordi- and ecological challenges compared with other anthro-
nating more components in solving a task. poid primates (Byrne, Chapter 3, Parker, Chapter 4,
Individual great apes can also interconnect abili- van Schaik et al., Chapter 11, Yamagiwa, Chapter 12,
ties from different domains to solve a single problem or Yamakoshi, Chapter 9, this volume) coupled with the
use one ability to facilitate another (Russon, Chapter 6, longer time they need to grow their exceptionally large
this volume). This is an important source of cognitive brains (Ross, Chapter 8, this volume). Great apes’
power because it enables solving multifaceted problems enhanced cultural potential (e.g., more powerful social
and boosts problem-specific abilities. It is not commonly learning, greater social tolerance) is considered essential
recognized in great ape cognition but evidence for its to their cognitive development, underlining how diffi-
role in exceptionally complex achievements in the wild, cult these challenges must be. Even with larger brains
for example Taı̈ chimpanzees’ cooperative hunting, sug- and more time to learn, immature great apes need more
gests that it should. It is important in evolutionary per- sophisticated and extensive social support than other
spective because it is a plausible source of the “fluidity of anthropoid primates.
thought” or “multiple intelligences working together” Many cognitive features believed critical to hominin
that stands out in humans. Its appearance in great apes evolution are then shared by great apes, including sym-
ties well with evidence that some of their species-typical bolism, generativity, and cognitive fluidity as well as
problems require coordinating abilities across cognitive specific abilities like complex tool use and manufac-
domains, for example adjusting foraging strategies as ture, mental representation of absent items, perspective
social needs, feeding needs, and their interactions fluc- taking, cooperative hunting, food sharing, and symbolic
tuate (Yamagiwa, Chapter 12, this volume). It also speaks communication. While great apes share these features
to claims that only humans have this capacity. only to rudimentary symbolic levels, these achievements
Great apes’ cognitive achievements appear to be are significant comparatively. Rudimentary symbolism
products of generative systems, i.e., systems that con- in particular has been taken as an exclusively human leap
struct problem-specific cognitive structures to suit the forward in cognitive evolution. If great apes share this
particular challenges encountered. Their skills in stone capacity, however, it must have evolved with ancestral
nut cracking (Inoue-Nakamura & Matsuzawa 1997), lan- hominids.
guage (Miles 1991; Miles, Mitchell & Harper 1996),
and classification (Langer 1996) all show construc-
tive processes. Models characterizing great ape cog- B I O L O G I C A L BA S E S O F G R E AT
nition in terms of centralized constructive processes APE COGNITION
like hierarchization or hierarchical mental construc-
The brain
tion take this position (Byrne 1995; Gibson 1993).
Hierarchization is especially important because hier- Efforts to establish what in the brain confers high cogni-
archical cognitive systems may be intrinsically gener- tive potential have focused on brain size because it pre-
ative (Gibson 1990; Rumbaugh, Washburn & Hillix dicts many other brain features (e.g., structures, gyrifi-
1996). Generativity helps explain several ostensibly cation, organization). The picture for great apes remains
anomalous features of great ape cognition that have unclear because all available size measures are prob-
incited debate – notably, achievement variability across lematic as indices of cognitive potential and samples
individuals, tasks, rearing/testing conditions, and com- of great ape brains have typically been very small (see
munities, and “atypical” abilities that emerge with Begun & Kordos, Chapter 14, MacLeod, Chapter 7,
special rearing. If great apes’ cognitive systems are gen- Ross, Chapter 8, this volume). As larger samples are
erative, these “anomalies” may simply be normal expres- becoming available, within-species variation is appear-
sions of generative cognitive systems. ing to be extensive, so the many published findings based
Development is a defining feature of primate cog- on small samples must now be treated as suggestive.
nition. Distinctive in great apes is prolonging cognitive These limitations in mind, modern great ape brains sug-
development beyond infancy and emergence of their gest the following cognitive characterization.
distinctively complex achievements during juvenility Great apes brains appear to follow a distinctively
(Parker & McKinney 1999). Prolonged cognitive devel- “ape” design (MacLeod, Chapter 7, this volume). All
opment probably relates to their more complex social apes, compared with other nonhuman anthropoids,Evolutionary origins of great ape intelligence 355
show more complex cerebral convolutions and an aug- structures, magnifying the cognitive advantages of an
mented neocerebellum. The neocerebellum connects ape cerebellum. This cerebellar advantage may con-
extensively with the cerebral cortex, and primarily tribute to handling the more severe tasks that great
through it the cerebellum contributes to cognitive pro- apes face as extremely large-bodied suspensory pri-
cesses such as planning complex motor patterns, visuo- mates. Large brain size also increases demands on cere-
spatial problem solving, and procedural learning. These bral cortical connectivity that, in humans, may have
cognitive processes support skills apes need as suspen- favored neocortical reorganization towards lateraliza-
sory frugivores, for example spatial memory, mapping, tion and locally specialized functional units (Deacon
and complex manipulation. A large sample of primate 1990; Hopkins & Rilling 2000). Great ape brains, all
brains also suggests that apes may have disproportion- weighing over 250 g, appear to be large enough to experi-
ately larger brains for their body size than other anthro- ence similar effects: they show two specialized structures
poids; this finding is tentative and runs counter to implicated in sophisticated communication, a planum
standard views, but it is consistent with these structural temporale and spindle neurons of the anterior cingulate
distinctions (see MacLeod, Chapter 7, Ross, Chapter 8, cortex, which are otherwise found only in humans. That
this volume). A distinctive ape brain is also consistent the allometric effects of large brain size likely brought
with apes’ distinctive life histories: living apes have dis- specialized structures along with greater interconnect-
proportionately prolonged immaturity with delay con- edness may be related to the co-occurrence of problem-
centrated in the juvenile period (Ross, Chapter 8, this specific and interconnected cognitive structures in great
volume); fossil hominoids may have shared this pattern apes and humans.
(Kelley 1997, Chapter 15, this volume).
Great apes’ higher cognitive potential over lesser
Life histories
apes, system wide, may well be a function of absolutely
large brain size and its allometric effects on morphology. Life history traits are fundamental attributes of a species’
Large brains provide more “extra” neurons for cognition biology that govern the pattern of maturation from con-
(Gibson, Rumbaugh & Beran 2001; Rumbaugh 1995). ception to death (e.g., gestation period, age at weaning,
Lesser apes’ brains resemble great ape brains morpho- maturation rate – age of female first reproduction, inter-
logically but resemble typical anthropoid brains in abso- birth interval, longevity). These traits typically occur in
lute size (Begun, Chapter 2, this volume), and do not packages that fall roughly along a continuum of fast–
show these cognitive enhancements. Large brains are slow rates of life. They correlate highly with body and
also more extensively interconnected; this may enable brain size, but some taxa depart dramatically from the
more complex cortical processing by enabling parallel predicted life history–body size relationship. For their
processing and distributed networks, and so enhance body sizes, primates have greatly protracted life his-
problem solving via simultaneous processing in mul- tories with notably delayed maturation compared with
tiple areas of the cortex and their connecting structures most other mammals. Links between the brain and life
(Gibson 1990). This fits well with great apes’ capacity for histories may suggest broader biological factors associ-
solving complex problems by interconnecting multiple ated with high cognitive potential. Reasons for specific
cognitive structures. scaling factors are typically explored by assessing links
Many specific brain features that distinguish great among ecological, brain, and life-history features.
apes can also be explained by their brains’ absolutely Anthropoid brain size is linked with delayed matu-
large size (e.g., greater lateralization, neocortex expan- ration, in particular prolonged juvenility. Anthropoids
sion, specialized areas). Even if these features owe prin- may then make tradeoffs against juvenile growth rates to
cipally to larger brain size, they can translate into impor- support their large brains, diverting energy away from
tant differences in cognitive potential. Brain structures body growth to support the brain. Even after removing
that increase in size with increases in overall brain size body size effects, juvenility appears to be further pro-
do so at differential rates. Structures implicated in cog- longed relative to body size in apes. Great apes may do
nition (e.g., neocortex, cerebellum) typically increase at the same thing to a greater degree. Slower body growth
higher rates, so they come to represent a larger per- probably affects juveniles, even though most primate
centage of the brain in larger-brained species. For this brain growth occurs in infancy, because caregivers with-
reason great apes have relatively larger neocerebellar draw support at weaning (Ross, Chapter 8, this volume).356 A . E . RU S S O N & D. R . B E G U N
Juveniles’ immature foraging skills and the slow rate at exploring related behavioral challenges. Behavioral chal-
which great apes learn, added to withdrawal of care- lenges affecting modern great apes are often used to
giver nutritional subsidies, can only prolong the period suggest evolutionary selection pressures that may have
in which their energy intake does not meet the energetic shaped their cognitive enhancement. Their counter-
needs of supporting the brain and body growth. Espe- parts in evolutionary history are inferred from indi-
cially in apes, prolonged juvenility may be best explained rect indices, for example diet from dental morphology.
as an unavoidable but bearable cost imposed by large Ecological challenges that primarily tap physical cog-
brains, rather than as directly adaptive (Ross, Chapter 8, nition include diet/foraging (Parker & Gibson 1979),
van Schaik et al., Chapter 11, this volume). No clear links diverse “technical” difficulties (Byrne 1997), and ar-
occur between the brain and life history in great apes as boreality (Povinelli & Cant 1995). Social challenges,
a distinct group (Ross, Chapter 8, this volume). which tap both social and communicative cognition,
involve both competition and cooperation (e.g., Byrne &
Whiten 1988; Parker 1996; van Schaik et al., Chapter 11,
Body size
this volume). In light of our characterization of great ape
There is no question about great ape body sizes – all are cognition and contributions to this volume, we recon-
exceptionally large for primates – or about correlations sider these challenges.
between their large body size and their large brain size
(Ward et al., Chapter 18, this volume). Yet the reasons
Ecological challenges
for this relationship are unresolved: direct cause–effect
in one direction or the other, parallel adaptations to other Food is considered a primary limiting ecological factor
selection pressures, or byproducts of selection on related of primate populations because of its sparse distribu-
factors. tion and anti-predator defenses (Yamagiwa, Chapter 12,
Because brains scale to body size, ratios between this volume). Features considered to challenge cognition
the two have been used to index a species’ “encephaliza- include eclectic frugivory, very large dietary repertoires
tion,” the extent to which its brain has increased in cog- and correspondingly large ranges, and essential “tech-
nitive potential, by assessing its enlargement beyond the nically difficult” foods. Interest in difficult foods has
size predicted by its body size. By these measures, great focused on embedded foods, especially those that elicit
apes appear no more encephalized than other anthro- tool use, but foods protected by other defenses such
poids: their brains are not relatively larger given their as barbs or noxious chemicals and obtained manually
body size, even if they are absolutely larger (but see present comparable cognitive challenges (e.g., Byrne &
MacLeod, Chapter 7, this volume). This has prompted Byrne 1991, 1993; Russon 1998; Stokes & Byrne 2001).
some to suggest that body size is the driving evolution- The distribution of tool use in the wild (chimpanzees
ary adaptation and that great apes’ large brains are mere and orangutans) probably reflects opportunity and not
side effects of their large bodies (e.g., see MacLeod, differential hominid cognitive potential. Bonobos and
Chapter 7, this volume). Analyses that simply seek to gorillas can both use tools when opportunities arise.
“remove body size effects” implicitly take this view. Fallback foods on which great apes rely during fruit
Brain–body mass relationships are much more com- scarcities are often difficult to obtain. This may be espe-
plex than such corrections suggest (Begun & Kordos, cially true of the fallback foods on which orangutans and
Chapter 14, Ward et al., Chapter 18, this volume) and chimpanzees rely, some of which elicit use of foraging
no acceptable method has yet been developed to appor- tools in the wild (Yamakoshi 1998; Yamagiwa, Chapter
tion relative percentages of brain mass related directly to 12, this volume). Seasonal fruit scarcities also prob-
body mass and to selection for absolutely bigger brains. ably contribute to great apes’ extremely broad dietary
repertoires and their flexibility in using individual foods.
Cognitively, the latter may require interpreting local
E N V I RO N M E N TA L P R E S S U R E S
indices of change to detect the availability of particular
ON COGNITION
foods, given that great apes inhabit the tropics where sea-
Establishing the function and evolution of complex sonal change can be irregular. The last common ances-
cognition and its biological underpinnings involves tor (LCA) was also a generalized frugivore that may alsoEvolutionary origins of great ape intelligence 357
have consumed hard foods needing preparation prior the puzzle, great ape species differ widely in their social
to ingestion and inhabited seasonal forest habitats that systems but are very similar in cognitive potential.
probably imposed periodic fruit scarcities. By implica- Van Schaik et al. propose social challenges in great
tion, the same dietary pressures affecting modern great apes that may help explain their enhanced social cog-
apes also affected the LCA: seasonality, dietary breadth, nition: fission–fusion tendencies with individuals out
and the need for fallback foods. of contact with conspecifics for lengthy periods and
Arboreal locomotion and navigation, two spatial foraging females solitary; relatively high subordinate
problems, present extreme cognitive challenges to great leverage leading to less rigid dominance and enhanced
apes because of their extremely large bodies and for- social tolerance; greater intrasexual bonds with non-kin,
est habitats. Navigating large ranges effectively and effi- and extensive flexibility in social organization and affil-
ciently may require mapping skills sophisticated enough iation. These are clearly shared by chimpanzees and
to calculate routes and distances mentally. Povinelli and orangutans, and perhaps by the other species. Most
Cant (1995) hypothesized that the great apes’ work-it- are consequences of large size and exceptionally slow
out-as-you-go, non-stereotypic modes of arboreal loco- life histories, which reduce vulnerability to predators,
motion, for example cautious clambering and gap cross- increase vulnerability to hostile conspecifics, increase
ing, require minds with the representational capacity to the potential for contest competition (especially for
figure in the self. These “cognitive” positional modes females and in species unable to switch to high-fiber fall-
are neither shared among nor unique to all living great back foods), and favor non-kin bonding. They require
apes, however (Gebo, Chapter 17, Hunt, Chapter 10, more complex cognition to handle greater flexibility in
this volume). They are prominent in orangutans and social relations and interactions and in the interplay
lesser apes but not African great apes. They could have among a more complex array of labile factors (e.g., bal-
influenced great ape cognitive evolution if the LCA was ance rivalry with interdependence, or social with pre-
a large arboreal clamberer but this is uncertain, per- dation or foraging pressures). Rejoining conspecifics
haps even unlikely (Gebo, Chapter 17, this volume). after lengthy absences increases needs for sophisticated
Povinelli and Cant suggested Oreopithecus as a model navigation, distance communication, and renegotiat-
of that ancestor, with the requisite large size and body ing relationships. Two examples of complex commu-
plan for arboreal clambering. Oreopithecus was other- nication in wild great apes concern rejoining compan-
wise very unlike other hominids, however (e.g., folivo- ions: tree drumming (Boesch & Boesch-Achermann
rous versus frugivorous, unusually small brained), and 2000) and placing indicators of travel direction (Savage-
probably represents an isolated adaptation to a refugium Rumbaugh et al. 1996). Higher subordinate leverage,
rather than great apes’ ancestral condition (see Begun & less rigid dominance, and enhanced social tolerance are
Kordos, Chapter 14, Gebo, Chapter 17, Potts, Chapter likely to improve opportunities for social learning, cul-
13, Singleton, Chapter 16, this volume). Even if arboreal tural transmission, and more flexible use of eye con-
locomotion demands complex cognition in orangutans, tact (Yamagiwa, Chapter 12, this volume). Similar social
there is little to indicate that it does, or did, in the great complexities also occur in some monkeys (capuchins,
ape lineage. some macaques), however, so alone they cannot explain
the enhanced cognition seen in great apes.
Great ape sociality should be affected by diet
Social challenges
because social groups must adjust to ecological condi-
Primate social life is recognized as having high poten- tions. Effects probably differ more in great apes than
tial for cognitive complexity. It is puzzling about great in other anthropoids because of great apes’ broad, tech-
apes that they use more complex cognition than other nically difficult, and seasonally varying diet (Yamagiwa,
anthropoids to solve social problems, but the problems Chapter 12, this volume). Social foraging strategies dur-
themselves are not obviously more complex. Their social ing fruit scarcities, when dietary and social competi-
unit sizes are well within the range of other anthropoids, tion pressures are at their worst, expose these effects.
their demographic composition is no more complex, and Significant to cognition is that great ape foraging groups
few if any more complex social phenomena are known change as a function of food availability, although pat-
(van Schaik et al., Chapter 11, this volume). To add to terns differ between species depending in part on the358 A . E . RU S S O N & D. R . B E G U N
preferred type of fallback food. This is consistent with 2001). In later hominids, the ability to exploit these
suggestions that fission–fusion in Pan functions to allow resources may have served as an important parachute
flexibility in handling challenges that vary over time during times of scarcity in more seasonal environments
and space (Boesch & Boesch-Achermann 2000), great when soft fruits, generally preferred by hominids, are
ape life allows and requires facultative switches between more difficult to find. Greater seasonality is indicated in
solitary and gregarious foraging (van Schaik et al., Chap- both Europe and Asia in the late Miocene, suggesting
ter 11, this volume), and ephemeral activity subgroups fruit scarcities with hard objects serving as fallback foods
show exceptional flexibility relative to ecological con- in some taxa. Sivapithecus is often reconstructed as hav-
ditions (Parker, Chapter 4, this volume). All great apes ing had an essentially soft fruit diet based on microwear
then share the challenge, as a normal circumstance, of (Teaford & Walker 1984), although morphologically it
complex problems wherein pressures from two distinct shared many features with hard-object feeders (thick
cognitive domains interact. enamel, low, rounded cusps, large molars, thick, massive
mandibles), suggesting an ability to exploit hard objects
when needed. Dryopithecus was not a hard-object feeder
E VO LU T I O NA RY R E C O N S T RU C T I O N S
and may have lived in less seasonal environments than
The origin of great ape cognitive capabilities is to be Sivapithecus (Andrews et al. 1997; Begun 1994; Single-
found in the Miocene, when the great apes originated ton, Chapter 16, this volume; Potts, Chapter 13, this vol-
and diversified. Here, we examine the evidence of brain ume). However, seasonality was probably greater in envi-
size and morphology, life history, body size, positional ronments inhabited by Dryopithecus than in most early
behavior, diet, and environment in ancestral hominoids Miocene hominoid environments, and evidence of the
as they relate to the evolution of great ape intelligence. anterior dentition suggests enhanced abilities for pre-
Patterns are summarized in Table 19.1. ingestive processing of embedded foods (Begun 1992).
Either way, late Miocene hominids probably extended
their frugivory with fallback foods during fruit scarcities.
Ecology: habitat and diet
Their large body size may also represent a response to
The local habitats of early Miocene hominoids were increased seasonality because it enhances energy-storing
most likely warm, moist forests in tropical and sub- capacities for surviving periods of fruit scarcity (Knott
tropical zones that enjoyed low seasonality and cli- 1998; Yamagiwa, Chapter 12, this volume). Living great
matic stability (Andrews, Begun & Zylstra 1997; Potts, apes show similar dietary breadth. Species differ in how
Chapter 13, this volume). Soft fruit, their dietary main- they adjust to fruit scarcities, but all share the overall
stay (Singleton, Chapter 16, this volume), would have pattern of relying on fallback foods. Orangutans and
been available year-round, albeit patchily distributed chimpanzees use “hard” fallback foods (e.g., embed-
spatially and temporally. ded, barks, pith), perhaps analogous to Sivapithecus, and
Hominid emergence in the late middle Miocene, gorillas and bonobos lean to folivory (although bono-
14–12 Ma, coincides with increasing climatic fluctua- bos appear to enjoy especially rich habitats abundant
tion, especially increasing seasonality (Potts, Chapter 13, with THV, which may or may not serve as fallback
this volume). This may have restricted soft fruit avail- foods), possibly more similar to the Dryopithecus strat-
ability for several months annually, at least in some egy. These environmental pressures and species traits
regions. The earliest Eurasian hominoid, Griphopithecus, imply considerable cognitive–behavioral adaptation, all
which is more modern in dental anatomy than Proconsul, in the direction of increased flexibility or adaptability
shows for the first time a fully developed suite of mastica- (Potts, Chapter 13, this volume).
tory characters indicative of hard-object feeding (Güleç The latest Miocene experienced cooling, drying,
& Begun 2003; Heizmann & Begun 2001; Singleton, and more pronounced seasonality, causing a worldwide
Chapter 16, this volume). The ability of the ancestors of shift from moist, warm forest to drier, open grass-
hominids to exploit hard objects may have allowed their land and a corresponding shift in vegetation (Cerling
expansion into Eurasia at the end of the early Miocene, as et al. 1997; Potts, Chapter 13, this volume). Effects
a way of avoiding competition with the many frugivores on hominids’ preferred habitats, moist warm forests,
making the same trip northward (Heizmann & Begun include shrinkage, fragmentation, and retreat. PreferredTable 19.1. Selection pressures and biological traits in modern great apes and ancestral hominoids
Brain4,5,8 Body Environments Sociality
Size Size Regional & Sexual
Epoch Ma Species (g) Morphology Life history (kg-f) Anatomy3,7 Diet2 Local global dimorph8 Organization
Modern
Chimpanzee 325 GA DM PJ 34–60 Semi-terrestrial Fruit + hard F–W to O, High seasonal Low (f–f+)
knucklewalk fallback + S+, T Fruit scarcity
& climb omnivory
Bonobo 314 GA DM PJ 33–45 Mixed Fruit + THV F, S−, T Low (f–f+)
knucklewalk
& climb
Gorilla 426 GA DM PJ 71–175 Semi-terrestrial Fruit + folivory F, S+, T High (f–f+)
knucklewalk fallback,
& climb folivory
Orangutan 288 GA DM PJ 36–79 Suspensory Fruit + hard F, S+, T High (f–f+)
clamber & fallback
climb
Early Miocene
20.6 Morotopithecus 30–35? ?Suspensory Soft fruit Less seasonal High
(E Africa) primitive fluctuation
brachiate
& climb
20–18 Proconsul 146 H4 (DM) 10–15 Pronograde Soft fruit F−W, S− High
(E Africa) arboreal
quadruped
& climb
18–17 Afropithecus (DM) 25–30 Pronograde Hard object F−W, S− High
(E Africa) arboreal
quadruped
& climbTable 19.1. (cont.)
Brain4,5,8 Body Environments Sociality
Size Size Regional & Sexual
Epoch Ma Species (g) Morphology Life history (kg-f) Anatomy3,7 Diet2 Local global dimorph8 Organization
Middle to Late Miocene
16.5– Griphopithecus* 20–25 Pronograde Hard object W−OP, S− Increasing High
15 (W Asia, arboreal /S+, ST fluctuation
C Europe, quadruped
E Africa) & climb
15.5– Kenyapithecus* 30 Pronograde Hard object W+GP, Increasing High
14 (E Africa) arboreal S+, T, fluctuation
quadruped dry + cooling
& climb
12–9 Dryopithecus 289 GA? DM5,8 20–30 Suspensory Soft fruit, F+W/SW, Seasonal fruit High
(Europe) brachiate eclectic S+, ST scarcity
& climb
12.3–7 Sivapithecus DM5 20–40 Suspensory Fruit + hard F+W, S+, Seasonal fruit High
(S Asia) arboreal or object warm scarcity
semi-
terrestrial
quadruped
10–9.5 Ouranopithecus 35? Partly Hard object F to O, S+ Seasonal fruit High
(S Europe) terrestrial? specialist scarcity
8–7 Oreopithecus 112 15 Suspensory Folivory F−SW, Insular High
(S Europe) brachiate S+, moist,
& climb ST
Notes:
Epoch: Early Miocene (22–17 Ma), Mid Miocene (17–11.5 Ma), Late Miocene (11.5–5.5 Ma).
Species: underlined, good hominid candidate; *, possible hominid candidate.
Brain: Size (for females, where available): weight in grams; (GA), estimated weight in the modern great ape range; Morphology: H, hominoid; GA, great ape (hominid);?, unknown.
Body size: size (for females, where available): size > 25 kg is interpreted as within the hominid range.
Life history: DM, delayed maturation; PJ, prolonged juvenile period; bracketed values are estimates.
Habitat: Type: F, forest, FP, forest patches; W, Woodland; WG, wooded, more grassy; GP, grassy patches; SW, swamp; O, open; OP, open patches; Seasonality: S−, low; S+,
higher; Zone: T, tropical; ST, subtropical.
Social: Sexual dimorphism: High (roughly 2:1 M:F); Low (closer to 1:1 M:F); Organization: f–f+ = flexible fission–fusion.
Shading highlights significant traits in living great apes and their appearance in the fossil hominoid record.
Sources: 1, Potts, Chapter 13; 2, Singleton, Chapter 16; 3, Gebo, Chapter 17; 4, Begun & Kordos, Chapter 14; 5, Kelley 1997, Chapter 15; 6, MacLeod, Chapter 7; 7, Hunt,
Chapter 10; 8, Begun, Chapter 2; 9, Ward, Flinn & Begun, Chapter 18 this volume.Evolutionary origins of great ape intelligence 361
foods would have been available in smaller patches and to greater omnivory, including increased meat con-
more dispersed in distribution, an ecological situation sumption, and use of savanna habitats. Bonobos main-
less suitable to large-bodied hominid foragers – espe- tained forest habitats and increased THV consumption.
cially in groups. Hominid presence is indicated in iso- Orangutans maintained earlier diets and remained in
lated moist forest and swamp refuges and forest–open tropical moist forests of southeast Asia, which persisted
woodland mosaics, suggesting they tracked their pre- in large blocks on Borneo and Sumatra until this cen-
ferred habitats where possible. Overall, their presence tury. Hominins became increasingly dependent on ter-
was increasingly restricted southward (Begun 2001, restrial resources and developed a variety of approaches
2002; Harrison & Rook 1997; Potts, Chapter 13, this (megadontia, tools) to maximize dietary breadth and
volume). ecological flexibility.
Fewer hominids are known in the fossil record of
the late Miocene after the last occurrence of Dryopithe-
The brain
cus and Sivapithecus between about 9.5 and 7 Ma. They
appear to have become extinct locally while their descen- Great apes’ distinctive brains seem be defined by their
dants may have moved south at this time (Begun 2001). large absolute size and hominoid morphology. Recon-
In Europe, the most ecologically specialized hominids structing their evolutionary origins comes down to when
are known from this time. Oreopithecus from Tuscany and why these features evolved. This exercise remains
had an exceptionally small brain, well-developed sus- hampered by the dearth of fossil material on ancestral
pensory positional behavior and highly folivorous diet, apes, especially crania.
while Ouranopithecus was among the largest of the Proconsulids, early Miocene stem hominoids, were
Miocene hominids and had a specialized hard-food diet. relatively unspecialized pronograde quadrupeds but
It is also most likely during this time that the ancestors were distinguished from primitive anthropoids by their
of the African apes and humans arrived in Africa large size, taillessness, powerful appendages, and brains
and that gorillas shortly thereafter diverged from the with a few hominoid features (Begun & Kordos, Chapter
chimpanzee–human clade. Recent evidence from Thai- 14, this volume; Kelley 1997, Chapter 15, this volume;
land suggests that orangutan ancestors may have first Ward et al. 1991, Chapter 18, this volume). The early
appeared in Southeast Asia at this time as well (Chaima- hominids, Dryopithecus in Europe and Sivapithecus in
nee et al. 2003). These patterns overall also suggest habi- South Asia, are either known or supposed on indirect
tat tracking, i.e., maintaining established habitat and but solid grounds to have had brain sizes in the range
fruit preferences (Potts, Chapter 13, this volume). of modern great apes; where known, their endocasts
In the Plio-Pleistocene, worldwide climate was show greater resemblances to modern hominids than
marked by strong arid–moist and temperature oscil- do Proconsul endocasts (Begun & Kordos, Chapter 14,
lations, wider climatic fluctuation, instability, pro- this volume). Sivapithecus neurocrania are not known.
found habitat variability, and arid–monsoon seasonal- For Dryopithecus, partial neurocrania yield brain size
ity (Potts, Chapter 13, this volume). Hominids would estimates at the low end in absolute size but at the high
have experienced increased episodic disturbance, intra- end relative to body mass compared with the ranges for
annual variability in food availability, and repeated forest modern great apes. The fact that great apes with brains
contraction–expansion and fragmentation–coalescence. ranging from 280 to 700 cc, or humans with brains rang-
Predictable effects include impoverished habitat in size ing from 1000 to 2000 cc, have not been shown to differ in
and quality, even greater variability in food availability cognitive capacity could be taken to indicate that there is
and abundance, changing species communities, chang- a loose causal relationship between brain mass and cog-
ing competitor and predator patterns, and variable pop- nitive capacity (Kelley, Chapter 15, this volume). On the
ulation densities (Potts, Chapter 13, this volume). Plio- other hand, the fact that there is no overlap in brain mass
Pleistocene pressures likely led to further diversification between monkeys and great apes or between great apes
of strategies to augment capacities for handling unpre- and humans suggests that normal brain mass minima
dictable habitat instabilities. Gorillas shifted towards in each taxon represent thresholds for cognitive change
folivory, especially for fallback foods, smaller ranges, beyond which cognition is not affected, until the next
and reduced foraging complexity. Chimpanzees shifted threshold is attained. If this is the case, and absolutely362 A . E . RU S S O N & D. R . B E G U N
large brains are what generate great apes’ grade of cogni- probably smaller on average than the smallest living
tion, then the rubicon represented by Dryopithecus and great apes, the smallest females possibly weighing about
the smallest extant great apes (280–350 cc) evolved in the 20 kg (Begun, Chapter 2, Ward et al., Chapter 18, this
late middle Miocene with Dryopithecus and Sivapithecus. volume). The smallest Sivapithecus, female S. punjabicus,
The emergence of the hominid-sized brain is associ- probably ranged from close to Dryopithecus in body size
ated with increasing seasonality, seasonal fruit scarcities, to as large as the smallest living hominids. Other clearly
and frugivorous diet enhanced with hard foods. Though hominid taxa such as Ouranopithecus, other species of
there are indications of hominid-like cerebral reorga- Sivapithecus, and Lufengpithecus are in the size range of
nization in Dryopithecus, its endocast is distinct from large chimpanzees and small gorillas; so is Morotopithe-
that of extant hominids so it is not clear whether their cus, though it is less clearly a great ape. The LCA was
brains provided equivalent cognitive potential. At a min- therefore almost certainly large compared with most pri-
imum however, the cognitive potential of late Miocene mates. Hylobatids are small bodied, but this is probably
hominids spans the considerable gap between great apes a result of secondary reduction in size compared with
and other nonhuman primates, probably coming closer the common hominoid ancestor (Begun, Chapter 2, this
to the former. volume). The range of body sizes in the proconsulids is
The Plio-Pleistocene is likely to have exerted fur- broad and overlaps with the hominids.
ther selection pressures on hominid cognition given its In addition to being the size of an extant great
negative effects of great ape habitats. Brain size has not ape, Sivapithecus and Dryopithecus M1 emergence age
changed, but organizational differences between extant estimates suggest life history prolongation roughly
and Miocene hominids probably occurred at this time equivalent to that of modern great apes (Kelley 1997,
(Begun & Kordos, Chapter 14, Potts, Chapter 13, this Chapter 15, this volume). The stem hominoids Proconsul
volume). The most telling findings from the fossil record and Afropithecus may show the first signs of life history
may be that (1) partial de-coupling of size and mor- prolongation. Proconsul may have been intermediate
phology is a common feature in the evolution of catar- between hominids and non-hominids in M1 emergence
rhine brains, and (2) hominoid brain evolution is highly age (Kelley 1997, Chapter 15, this volume), although
diverse, with reduction in some lineages and increases Begun & Kordos (Chapter 14, this volume) and Kelley
in others. Some lineages experience brain mass loss in (Chapter 15, this volume) both also find Proconsul to
connection with body mass reduction (e.g., Hylobates) be equivalent to Papio in M1 emergence and brain size.
or independent of body mass change (e.g., Oreopithe- Afropithecus may have been within the great ape ranges
cus). The pattern of brain size diversity in fossil great for M1 emergence and brain size (Kelley & Smith, 2003;
apes more closely matches broad patterns of diet than of Kelley, Chapter 15, this volume). However, in our view
size (Begun & Kordos, Chapter 14, this volume), espe- the poorly preserved neurocranium of Afropithecus ten-
cially frugivory extended (seasonally) with challenging tatively suggests a somewhat smaller brain than in a simi-
fallback foods. Brain size has been surprisingly stable in larly sized chimpanzee, which would be consistent with
hominid evolution until Homo, despite dramatic changes the lower end of the range of estimates of M1 emer-
and diversity in body mass, diet, positional behavior, and gence and brain size provided by Kelley (Chapter 15, this
ecological conditions. It may be that a hominoid brain volume). Either way, the implication is that prolonged
size at least 250 g represents a rubicon that generates immaturity emerged with the hominoids but became
hominid levels of cognitive and behavioral complexity. more clearly prolonged as brain size increased with the
Conversely, although large bodies do not always imply first hominids, because of energetic constraints, social
large brains in hominoids, large brains always co-occur constraints, or both. There is likely a complex inter-
with large bodies. relationship among life history, body mass, and body
size that has yet to be fully understood in vertebrates in
general (Ward et al., Chapter 18, this volume).
Body size and life history
Sociality
Fossil hominids were predominantly large bodied but
somewhat smaller than living great apes. The smallest Characterizing sociality in the LCA is a highly spec-
Dryopithecus (female D. laietanus and D. brancoi) was ulative exercise resting exclusively on indirect indices.Evolutionary origins of great ape intelligence 363
Several features of great ape sociality result from large Ecology
size and exceptionally slow life histories, both of which
Compared with the first hominoids, the first well-known
characterize early hominids (many Chapters in this vol-
fossil hominids, Dryopithecus and Sivapithecus, inhab-
ume). If, as argued for living great apes, large size and
ited middle to late Miocene moist tropical forests with
slow life histories give impetus to these social features,
greater seasonality, frugivory extended in the direction
then hominids should share them. All great apes but
of challenging foods, polygyny/high male–male compe-
no lesser apes also share fission–fusion tendencies that
tition, life histories with prolonged immaturity and pro-
are affected by fruit scarcities and fallback foods; early
longed juvenility, and larger bodies and brains reaching
hominids likely experienced similar dietary pressures, so
into the modern great ape range. The greater season-
they too may have had a fission–fusion form of sociality.
ality combined with incorporation of hard or otherwise
The main influence on female sociality, food
challenging foods in the diet suggests a dietary shift
availability, depends on fallback foods in great apes
towards adding fallback foods requiring pre-ingestive
(Yamagiwa, Chapter 12, this volume). In chimpanzees
preparation as diet supplements during fruit scarcities.
and orangutans, which rely on similar hard fallback
Increased absolute brain size indicates increased cog-
foods, females restrict their social grouping during fruit
nitive potential. Altogether, this suggests that seasonal-
scarcities and increase it during periods of abundance.
ity resulted in a more cognitively challenging diet that
In gorillas and bonobos, which have more folivorous fall-
favored larger brains. Ecological pressures on hominids
back patterns, females grouping patterns remain more
intensified under the increasingly seasonal and unpre-
stable. The main influence on male sociality, access to
dictable conditions of the latest Miocene and Plio-
females, is primarily shaped by sexual dimorphism in
Pleistocene. Their effects on cognitive evolution were
great apes. In highly dimorphic orangutans and gorillas,
perhaps constrained by habitat tracking, with the great
males tend to be solitary and corral females for mat-
apes adopting a more conservative ecological approach
ing. In less dimorphic chimpanzees and bonobos, males
and the hominins exploiting more radically different
associate with one another via dominance ranking sys-
environments.
tems. In early hominids, challenging fallback foods co-
occur with high sexual dimorphism (Begun, Chapter 2,
Singleton, Chapter 16, Ward et al., Chapter 18, this vol- Brain–Body–Sociality
ume), so their social systems may have resembled the
In the anthropoid/hominoid phylogenetic context,
orangutan’s, perhaps in less dispersed form. Attributes
hominid large brain and body size likely co-occurred
include polygynous mating systems with solitary males
with slow life histories, prolonged immaturity, lower
attempting to monopolize multiple females or female
predation risk, higher vulnerability to hostile con-
ranges, male dominance based on size, and female asso-
specifics, stronger relations with non-kin, high subordi-
ciations waxing and waning with the seasons.
nate leverage, and relaxed dominance. Which came first
is neither interesting intellectually nor a useful question
processually. We will never know, and these variables
DISCUSSION
were probably a package as soon as they appeared in
Stem hominoids lived in moist tropical forest habi- early hominids.
tat with low seasonality, and probably exhibited dedi- Socially, this package is consistent with unusually
cated frugivory, social complexity commensurate with flexible fission–fusion tendencies and enhanced social
frugivory, polygynous social structures with relatively tolerance (van Schaik et al., Chapter 11, this volume).
high male–male competition, life histories with some- The former would have favored larger brains for more
what prolonged immaturity, brains mostly of anthro- complex social problem-solving; the latter may have
poid size and design, and body mass somewhere in the further boosted cognition by enhancing conditions for
range between monkeys and great apes (10–25 kg). From socio-cultural learning. Some social intelligence mod-
this starting point and considering the many factors dis- els argue for an “arms race” in cognition, once cog-
cussed in this book, we suggest the following patterns nitive solutions to social problems take hold, because
and processes in the evolution of great ape intelligence competing successfully depends on outwitting increas-
(Figure 19.1). ingly savvy conspecifics (e.g., Ward et al., Chapter 18,364 A . E . RU S S O N & D. R . B E G U N
Figure 19.1. Factors implicated in the evolution of great ape intel- came first may never be known, and may not even be important.
ligence. Early hominids are distinguished from early hominoids The combination of characters is unique to hominids. While auto-
mostly by body and brain size and slowed growth. Ecological catalytic, directionality is not inevitable, as we see in the exam-
changes may have been the catalyst for a feedback reaction between ples of hominoids that have smaller brains and presumably less
larger bodies and slower growth on the one hand and ecological intelligence.
challenges on the other. Which response typical of extant hominids
this volume). Biological and ecological variables exert pressures on caregivers, especially mothers. These pres-
similar dynamic effects, and in concert with social pres- sures have been linked with enhancing apprenticeship
sures they feed back and contribute to further cognitive (e.g., imitation, teaching) as a means of speeding their
evolution. skill acquisition (e.g., Parker 1996).
Additional pressures between the brain and social-
ity may have arisen through prolonging juvenility, which
has been linked with their large brains’ higher energy Body–diet–brain
demands (Kelley, Chapter 15, Ross, Chapter 8, this vol- Brain size correlates with diet more closely than with
ume). Prolongation increases vulnerability for juveniles, body size (Begun & Kordos, Chapter 14, this volume).
who are handicapped by poor foraging skills and small Large bodies are none the less linked with diet. The
size. Learning foraging skills is exceptionally slow and hominid combination of body size, diet, and brain size
difficult because of great apes’ difficult diets; complex probably aggravated cognitive challenges.
skills for obtaining their most difficult foods, some of Hominids, exceptionally large bodied, would have
them fallback foods, may not be mastered until near required more and/or better food than smaller-bodied
adulthood. Juveniles’ poor foraging skills and slow learn- hominoids, although not proportional to their greater
ing essentially extend their dependency, aggravating size because of their lower metabolic rates. FruitEvolutionary origins of great ape intelligence 365
specialists’ diets are typically diversified because fruits to the degree expressed in hominids. First causes may
are energy rich but poor in important nutrients like then be less important to present day outcomes than
proteins and fat; hominids in particular are too large changes induced by multiple interdependencies among
to be dedicated frugivores, and at some point they these factors.
diversified their diets to include foods richer in protein It is also probable that in the evolution of hominid
and fat (Waterman 1984; Yamagiwa, Chapter 12, this brains, this attribute package entailed “arms races”
volume). Whenever large body size appeared between involving both ecological (dietary) and social pressures.
stem hominoids and early hominids, broadening the Arms races are always constrained by initial conditions.
diet was one probable avenue of obtaining more food. As Ward et al., Chapter 18, this volume note, within
Compared with stem hominoids, early hominid den- a taxon individuals compete mainly with conspecifics.
tition indicates expanding beyond soft fruits to eclec- Pressures on a hominid come from other hominids in
tic frugivory or additional hard foods. If modern great their ecological and social context. Given their different
apes are any index, their broader diets increased cogni- evolutionary trajectories, arms races in different social,
tive challenges by increasing foraging complexity, which biological, and evolutionary contexts should produce
increases memory load and the range and complexity of different outcomes. This is the reason we do not see
skills needed to locate and obtain food. monkeys, even capuchins and baboons, as intelligent as
Large brains, with their high energetic costs, favor great apes and humans. Monkeys experience different
better-quality diets (e.g., meat in hominins). Non-fruit ecological conditions and do not need to be as intelli-
foods are generally differently distributed and more gent as great apes to compete with other monkeys. For
highly defended against predators than fruits. Effects the hominids, diet and moist tropical forests are good
on behavior include broadening and/or shifting foraging candidates for constraints. The great apes never really
ranges and foraging skill repertoires; this increases the got out of the fruit market and that may have limited
variety and especially the complexity of foraging skills, their capacity to take in enough energy to enlarge their
which translates into greater cognitive challenges. In brains beyond some ceiling. Their persistent tracking of
hominids, then, improving diets to support large brains moist tropical forests would impose other constraints on
likely generated new pressures to enlarge the brain even their adaptation, especially given ever-dwindling forest
more. In other words, hominid diets and large brains size and productivity. The possibility that some sort of
may have generated their own dietary cognitive arms systemic equilibrium sets in is suggested by the distinct
race. “grades” of intelligence and brain–body size scaling pat-
terns that are evident within the primates, as opposed to
Diet–Sociality continuous gradation.
Hominid diets and sociality mutually affect one another,
as shown by great apes’ foraging strategies during sea-
C O N C LU S I O N S
sonal fruit scarcities. Foraging strategies are affected by
both fruit scarcities (through females) and social pres- Our interpretation of available evidence is that the evo-
sures (through male competition). For cognition, this lution of a great ape grade of intelligence involved a web
is the sort of intertwined tangle of complex social and of factors, causally interrelated and mutually adjusted.
ecological demands that requires interconnected cogni- Constituent pressures and traits may have affected one
tion, that is, handling diverse demands in one integrated another in spiraling or arms race fashion before reach-
solution; it is a recurrent feature of normal great ape life. ing the particular combination seen in the hominids.
Potts, Chapter 13, and Ward et al., Chapter 18, this vol- Great ape adaptation constitutes an integrated pack-
ume also recognize this situation. age of cognitive–behavioral–social–morphological traits
This myriad of interdependent biological, social, dovetailed to a particular constellation of ecological and
and ecological factors affecting intelligence in hominids social pressures and possibilities, rather than an assem-
is complicated beyond our ability to discern first causes blage of individual traits adapted independently to spe-
or prime movers. We do know, however, that these cific pressures. Their cognitive system, one component
attributes co-occur only in hominids. Some of them of this package, was shaped by all these traits and shaped
occur in other mammals, but never all together and never all these traits in turn.366 A . E . RU S S O N & D. R . B E G U N
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