Biology and Philosophy 19: 721–737, 2004.
© 2004 Kluwer Academic Publishers. Printed in the Netherlands.
Hominid cultural transmission and the evolution of
language
LAUREANO CASTRO1, ALFONSO MEDINA2 and MIGUEL A. TORO3
1 Departamento de Mejora Genética Animal, SGIT-INIA, Carretera de La Coruña km. 7,
28040 Madrid, Spain; 2 Facultad de Psicología, UNED, C.A. de Madrid, Faculty of Social
Sciences, Saint Louis University, Madrid, Spain; 3 Departamento de Mejora Genética
Animal, SGIT-INIA, Carretera de La Coruña km. 7, 28040 Madrid, Spain (e-mail:
[email protected])
Key words: cultural transmission, evolution of language, human evolution
Abstract. This paper presents the hypothesis that linguistic capacity evolved through
the action of natural selection as an instrument which increased the efficiency of the
cultural transmission system of early hominids. We suggest that during the early stages of
hominization, hominid social learning, based on indirect social learning mechanisms and
true imitation, came to constitute cumulative cultural transmission based on true imitation
and the approval or disapproval of the learned behaviour of offspring. A key factor for
this transformation was the development of a conceptual capacity for categorizing learned
behaviour in value terms – positive or negative, good or bad. We believe that some hominids
developed this capacity for categorizing behaviour, and such an ability allowed them to
approve or disapprove of their offsprings’ learned behaviour. With such an ability, hominids
were favoured, as they could transmit to their offspring all their behavioural experience about
what can and cannot be done. This capacity triggered a cultural transmission system similar
to the human one, though pre-linguistic. We suggest that the adaptive advantage provided by
this new system of social learning generated a selection pressure in favour of the development
of a linguistic capacity allowing children to better understand the new kind of evaluative
information received from parents.
1. Introduction
“I suggest that advanced mental qualities might possibly be produced
as an incidental effect of selection for the ability to understand and
remember simple verbal instructions early in life”
G.C. Williams (Adaptation and Natural Selection)
The origin and evolution of language is a controversial topic within developmental biology. According to Darwin, linguistic capacity went through
an evolutionary process driven by natural selection which led from those
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primates who were able to produce unarticulated screams, to humans with
articulated language and a number of syntactic rules. Max Muller, a notable
linguist at the time of Darwin, defended the opposite position, inspired in
the Cartesian tradition which considered that language lacks any animal
antecedent and constitutes the true Rubicon separating our kind from the
other species. During the 20th century, the development of psychology as
a science did not help to bring language and evolution closer to one another.
Behaviourism, the dominant paradigm for a long period, made some progress
towards shortening the gap between psychology and biology, but its notion of
an ontogenetic acquisition of language as just one more learned skill resulted
in no special treatment or study for language, apart from the unfruitful attempt
to justify verbal overt behaviour. Confronted with the conceptions of the
behaviourists, Noam Chomsky (e.g., 1972, 1988) suggested the existence of
an innate mental ability for language and a specific mechanism for acquiring
it (L.A.D., Language Acquisition Device), which might explain the ease with
which an average child can learn any language. Each unit of the natural
languages is generated by applying a finite set of rules that constitute the
so-called Generative Grammar. Chomsky defended the presence of a deep
logical structure common to all languages, and which reflects our innate
capacity for language.
Support for an innate linguistic capacity could have helped to narrow
the gap between linguistics and Neodarwinian theory of evolution. However,
there was no such rapprochement. Chomsky’s refutation of behaviourism was
taken by many biologists as the Cartesian reaffirmation of a qualitative lag
between the rational and linguistic capacities of our kind and the absence
of them in the rest of the species. This posture was indeed encouraged by
Chomsky himself, who repeatedly held that the origin and presence of the
innate and universal linguistic capacity in our kind could not be explained by
natural selection (Chomsky 1972, 1988). Moreover, some evolutionists, most
prominently Stephen Jay Gould, suggested language to be the by-product
of other evolutionary processes (a spandrel), rather than a special adaptation arising through ordinary natural selection acting on mutations. In the
early 1990s, Pinker and Bloom (1990) and Pinker (1994) wittily criticized
these rationales, and defended the hypothesis that natural selection has a
central role in the origin of linguistic capacities. The kernel of their argument focuses on the fact that language is far too complex a structure for us
to accept that it could have emerged without a specific functional purpose.
These authors believe that language shows some signs of the presence of
a complex design aimed at fulfilling some function – signs that cannot be
explained in a convincing manner by any other theory. Even more plainly,
language serves many everyday functions. We devote a great deal of time and
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energy to mastering it and using it. Hence, it becomes necessary to accept
that natural selection is involved in language evolution.
In the last few years, considerable evidence supporting the idea of a
gradual evolution of linguistic capacity during the hominization process has
been provided by palaeontology, and most of all from genetics (see, for
example, Lai et al. 2001; Enard et al. 2002; Zhang et al. 2002). The acceptance of the Darwinian thesis led to a number of hypotheses attempting
to discern the possible adaptive advantage that allowed the evolution of
language through natural selection (Dunbar 1997; Deacon 1997; Calvin and
Bickerton 2000). For example, Deacon (1997) holds that humans are “the
symbolic species”. Language is not merely a mode of communication, it is
also the outward expression of an unusual mode of thought – symbolic representation. Symbolic communication was a response to a reproductive problem
that only symbols could resolve: the regulation of reproductive relationships
between early hominids. For Deacon, our ancestors could only take advantage
of a hunting-foraging subsistence strategy if they could regulate marriage, and
symbolic communication was subsequently improved, gradually, because of
the myriad advantages it provided for other forms of communication: motherinfant communication, passing on both foraging tricks and tool-making skills,
manipulating competitors, collective warfare and defence, and sharing of past
experiences.
Other theories link the development of language and the human mind to
social life. Dunbar (1997) suggests that social interactions are complex in
primates, and demand big brains because so much has to be remembered.
Individuals need to remember who did what to whom, and how strong every
alliance is at a particular moment. These complex relationships are maintained by grooming for many primates. In other words, the real function
of grooming is to keep social groups together. For Dunbar our hominid
ancestors faced increasing predation when they moved into the grassland, so
that increasing the size of the group would have been a valuable strategy for
survival. But as groups get larger the time requirements for grooming become
impossibly high. This is where language sprung up, to allow the cementing
and maintenance of our human relationships. The function of language is
gossip, and gossip is an ultra-efficient form of grooming because we can talk
to more than one person at a time.
The theory proposed by Bickerton (1990) and Calvin and Bickerton
(2000) is especially relevant for us. These authors support the hypothesis
that the evolution of language occurred in two stages: in the beginning,
a kind of “proto-language” would have emerged, in which words had no
syntactic organization and worked as labels for signalling concepts in an
organism’s mind; later, a true language with defined syntactic rules would
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have evolved. Proto-language would have emerged under the selection pressure of the need to transmit information about the location of food resources
and to alert offspring to potential risks they faced. Accordingly, the developmental ancestor of human language would be not the set of previous
animal communication systems, but the previous conceptual representation
systems such as those which some primates appear to have. The development of syntax would have occurred at a later stage, as a consequence of
the establishment of reciprocal altruism in those hominid groups that already
had a proto-language. Reciprocal altruism requires the development of a kind
of social intelligence allowing clarification of “who did what to whom?”,
in order to identify and reject those individuals who cheat, and in order to
maintain alliances. For Bickerton, episodic memory required to recall these
actions is useful for giving labels to those elements involved in a particular
action that play the roles of agent, topic and goal, which are precisely the core
of the basic syntactic categories.
All of these hypotheses about the evolution of language face the same
problem: identifying the triggering factor in the hominization process. The
lifestyle of our hominid ancestors was, as far as we know, quite similar to that
of the direct ancestors of the chimpanzees, and the problem is to determine
why there were selection pressures that caused the development of the brain
and of linguistic abilities in the hominid line, and why these did not act in
the same way in other primates. In the present study, we adopt Bickerton and
Calvin’s position on the fact that the ancestors of human language are the
primates’ conceptual representation systems, and we defend the hypothesis
that language was selected as an instrument that increased the efficiency of
the cultural transmission system possessed by the first hominids.
Our rationale starts out from a consideration of the adaptive significance
and evolution of social learning in animals. First, we discuss in section
2 the evolution of social learning from the perspective of dual-inheritance
theory, which suggests that social learning may have evolved as far as human
complexity, when it could make individual learning less costly and more
accurate. Later, in section 3, we suggest that a key factor enabling such a
transformation was the fact that some hominids developed the capacity to
conceptually categorize their own learned behaviour in terms of values –
positive or negative, good or bad – and that such a capacity allowed them
to approve or disapprove of their offspring’s learned behaviour (Castro 1992;
Castro and Toro 1995). We propose that during the early stages in human
evolution, hominid social learning, which was probably rather similar to that
of today’s chimpanzees, based on social indirect learning mechanisms and
perhaps true imitation, turned into a cumulative cultural transmission based
on true imitation and the approval or disapproval of offspring’s learned behav-
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iour. We believe that natural selection favoured those hominids equipped with
this capacity to approve or disapprove of their children’s behaviour, and thus
transmit all their behavioural experience to their offspring. By valuing some
of their experiences positively and others negatively, and by communicating
these valences to their offspring, parents could become active teachers, rather
than relying simply on the imitative abilities of their children. Similarly, if
naive offspring acquired, by individual or social learning, negatively-valued
traits, parents’ disapproval could be used to extinguish them. This means that
communication in hominids embedded a new type of information referring
to the adaptive value of a given learned behaviour, a kind of information
that, as far as we understand, is not transmitted among non-human primates
(Seyfarth and Cheney 1992; Tomasello and Call 1997; Premack 2004). With
the help of a mathematical model, we have shown that the capacity to
approve or disapprove of offspring’s behaviour may have become adaptive
and evolved in hominids by making individual learning more precise and
less expensive (Castro and Toro 2002). Finally, in section 4, we suggest that
the adaptive advantage provided by this new system of cultural transmission
generated a selection pressure in favour of the development of a more efficient communication system, allowing children to better understand this new
kind of information received from parents about the value of a given learned
behaviour. In other words, it generated a selection pressure in favour of a
linguistic capacity.
2. The evolution of social learning
Although non-learned behaviour is a sufficient tool for the life of a large
number of animal species, many others have also developed the capacity
to learn. Individual learning evolves with the aim of allowing organisms to
acquire a degree of phenotypic plasticity that may yield a better adjustment
to the unpredictable elements in the environment. There are many different
kinds of individual learning: from habituation and perceptual learning to classical conditioning and trial-and-error learning. The last of these is of special
importance in the quest for phenotypic plasticity, since it allows the animal to
learn to associate a voluntary activity with the consequences that follow.
In species with some trial-and-error learning capacity, the development of
a system of cultural transmission can turn out to be adaptive, since it allows a
reduction in the time and cost involved in such learning (Boyd and Richerson
1985), and takes advantage of the range of behaviour developed by the
previous generation (Lumsden and Wilson 1981). However, although culture
defined as variation acquired and maintained by social learning is common
in nature, it is nowhere as important in any other species as it is in humans,
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where it led to a cumulative cultural evolution process with great adaptive
value (Tomasello 1999). Moreover, it is not clear how cultural transmission
has improved human adaptability while other primates with social learning
abilities show comparably restricted ranges. Boyd and Richerson (1995) have
shown that social learning can be adaptive if it makes individual learning
less costly or more accurate. The first condition is satisfied if individuals use
individual learning when it is cheap and reliable, and switch to social learning
when individual learning is expensive. The second allows the accumulation
of behaviours that no individual learner could acquire by him/herself from
one generation to the next. That is, it allows cumulative cultural evolution.
The evolution of the first condition poses an interesting problem, because
although individual and social learning should not be regarded as mutually
exclusive, their co-evolution implies some kind of evolutionary trade-off.
Most of the theoretical models used to analyse the evolution of social learning
consider that the two processes work in a mutually exclusive way (Boyd and
Richerson 1995; Feldman et al. 1996; Laland et al. 1996; Henrich and Boyd
1998; Aoki 2001, for a review). It is assumed in these models that when
individuals use trial-and-error learning, they learn the appropriate behaviour
but they face learning costs. When they use imitation, individuals adopt the
behaviour of the one that they use as a model and, therefore, avoid the
costs of individual learning, but they incur a cost if they imitate an inappropriate model. Steady equilibrium might be achieved through a mixed strategy
involving either polymorphism between pure imitators and pure individual
learners, or more realistically, individuals displaying both modes of behaviour
in a fixed proportion (Boyd and Richerson 1995; Feldman et al. 1996; Henrich
and Boyd 1998). Social learning would work as an inheritance system alternative to the genetic one, and would be adaptive if individuals used social
learning when evaluation of the behaviour to be adopted was costly and
unreliable – that is, when individual learning was costly.
The evolution of the second condition also appears problematic, and in
fact, cumulative cultural evolution resulting in behaviours that no individual
could invent on their own is rare – it is limited to humans, song birds, and
perhaps chimpanzees (Boyd and Richerson 1996). All primate cultural traditions may be maintained through indirect social learning, while the evidence
suggests that cumulative cultural change is only feasible with true imitation,
and not with indirect social learning, since with the latter the behaviour is not
truly imitated, but rather has to be re-invented (Boyd and Richerson 1995).
More precisely, imitation and teaching are direct social learning modalities,
whose possible presence in primates (especially in the case of teaching) is
questionable. As far as imitation is concerned, there is a consensus among
researchers that either it does not exist at all or it is very rare to find true
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imitation in primates living in their natural environment (Galef 1992; Visalberghi and Fragaszy 1990; Tomasello 1996); however there is clear evidence
that chimpanzees are able to imitate when they are bred in captivity and with
a high level of cultural training (Tomasello and Call 1997). With respect to
teaching there is practically no evidence in primates (Premack and Premack
2003); Tomasello et al. (1993) hold that chimpanzees do not actively teach
their offspring in their natural habitat. The most thoughtful study providing
some evidence for the presence of teaching in primates was made by Boesch
(1991) with chimpanzees. Boesch observed just two possible instances of
active instructions over many years of observation.
For Boyd and Richerson (1996) cumulative cultural evolution is not
present in chimpanzee culture because they deploy their imitative learning
abilities in a less consistent manner than humans. That is, although a
given individual may learn somebody else’s innovation by imitation, there
will be no other individual that could later imitate it in a precise manner.
Consequently, this innovation cannot be propagated, and becomes lost until
someone else re-invents it. More recently Tomasello (1999) claims that
the key to the transformation of social learning in hominids into a human
cultural inheritance system was a qualitative change in imitation ability,
which requires as a previous step that individuals develop a capacity for “a
theory of mind”, enabling them to perceive their peers as intentional beings
with a similar mind. For Tomasello, true imitation, rooted in an evolved
capacity for “theory of mind”, generated human cultural evolution, including
language evolution. However, our opinion is that true imitation is not sufficient to explain the transformation that occurred in primates’ social learning
during hominization.
3. The evolution of social learning: a different perspective
Cultural transmission in our own species works most of the time as a true
cumulative inheritance system. Hence, the question is to identify the types
of changes that occurred during the hominization process and transformed
the typical social learning in primates into a cumulative cultural inheritance
system similar to the human one.
Our thesis is that a key factor enabling such transformation was the fact
that hominids developed both a capacity for “theory of mind” and a capacity
to conceptually categorize learned behaviour. A capacity for “theory of mind”
enables individuals to make a connection between the acts of other animals
and their own acts and goal states, and such ability was useful for increasing
qualitatively (and quantitatively) the effectiveness of imitation. The conceptual capacity to categorize is defined as the ability to categorize one’s own
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and others’ behaviour in terms of values – positive or negative, good or
bad –, which in turn allow the transmission of information about the value
of learned behaviour between parents and offspring, hence facilitating and
orienting learning in the latter. We believe that such a capacity to approve or
disapprove of the behaviour of offspring helped to make individual learning
both less costly and more accurate, making possible the transformation from
primate social learning to human cultural transmission.
We suggest that the simultaneous presence of the two capacities – imitation and approval/disapproval of the offspring’s learned behaviour – in our
hominid ancestors represented a radical change in the rudimentary cultural
transmission of the first hominids. Individuals with both abilities, which
we call assessors, generated a more efficient cultural inheritance system,
since they could transmit information about the behaviour their children were
learning by conditioning the final acceptance or rejection of it.
Approval or disapproval of behaviour works as a new behaviour evaluation
criterion for the learning individual that is particularly useful when evaluation
is difficult and has low reliability. This means children can take advantage of
parental expertise, and transforms social learning into an inheritance system
in a rigorous sense, since young individuals reproduce the phenotypic structure in their parent’s generation, thereby avoiding learning costs. Approval
or disapproval of behaviour permits the transmission of information about
those behaviours the individual learned to avoid. A great part of what we
learn is what we must not do. This type of information cannot be transmitted through imitation, except on very rare occasions and in an indirect
way. Individuals discover through individual learning this type of “negative”
environmental information, but often at a high cost. Approval and disapproval of behaviour may greatly contribute to avoiding the cost of learning
wrong behaviour, and to saving time when having to choose among different
behavioural alternatives that are difficult to evaluate.
The capacity for approving or disapproving of individually or socially
learned behaviour may be considered as an elementary form of teaching,
since individuals with such capacity (assessors) can transmit to their children
the emotional value they gave to those behaviours they have previously
learned and categorized. Note the implication here that individuals, when
adults and far from parental influence, can re-evaluate their behaviour if its
result is unsatisfactory. Transmission of values turns imitation into a parentchild inheritance system, without eliminating children’s capacity to innovate.
Hence, the possibility to learn from other individuals does not constrain their
evaluation system either; it simply feeds it through approval and disapproval
while individuals are young. Moreover, this cultural inheritance system is
flexible, in spite of its reliability, since it does not impede dissemination
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of behavioural innovations when these were not negatively evaluated by the
parental generation.
We showed in a previous study that the capacity to approve or disapprove of behaviour may be adaptive in a wide range of situations (Castro
and Toro 2002). We can summarize the adaptive advantage of approving or
disapproving of behaviour in the following points:
a) The approval and disapproval of offspring’s learned behaviour increases
phenotypic resemblance among parents and offspring for those behaviours
that parents know and have previously categorized, which is adaptive as long
as the environment does not change so fast that it cancels out the biological
value of what was learned by the previous generation.
b) The approval and disapproval of offspring’s learned behaviour increases
reliability in imitation processes which allowed the generation of a cumulative cultural inheritance system. The need for fidelity in the copy for
imitation to work as a cumulative inheritance system is a serious handicap that
also affects cultural transmission in humans (see, for example, Blackmore
1999). Disapproval of badly-imitated behaviour forces the imitator to repeat
the process, and this makes the increase of fidelity in the copy possible.
c) Disapproval allows the offspring to acquire information about behaviours they are self-discovering, so that they can reject risky responses without
having to experience all their negative consequences. That is, although
negative assessment of a given behaviour may be easy, with no need
for parental help, disapproval may help to avoid or diminish the cost of
experiencing it.
d) Disapproval impedes or makes difficult for the offspring to imitate tasks
for which they are not qualified due to their youth. Disapproval represents
an extension of parental care to learned behaviour, and decreases the risk
in young people that they will tend to imitate other individuals in the same
population.
e) Approval favours the implementation of behaviour that has no immediate positive evaluation for the individual experiencing it, decreasing the
costs and time involved in adopting a given behaviour when the outcome
of its evaluation is not immediately obtained.
Approval or disapproval of offspring’s behaviour involves a cost for the
teaching assessor, as do many other forms of parental care. The cost derives
from the time and energy devoted to influencing the offspring’s behaviour.
Evolution of approval/disapproval of behaviour will depend on whether the
benefit of parental orientation outweighs the cost or not. However, it is
important to bear in mind that a given amount of parental investment can be
used to control simultaneously several behaviours learned by the offspring.
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Thus, the cost will be attenuated because it will be offset by the benefits
obtained from the transmission of information on several learned behaviours.
If we try to analyse the situation from the perspective of the offspring,
they, in principle, do not have the possibility of choosing whether or not to be
orientated by their parents. What appears to be true is that, in environments
that change little, whatever is learned by one generation may be used for
the next, so that parental orientation must be adaptive. The sensitivity of a
learner to disapproval reduces the cost of error, by substituting an objectively
harmless parental signal of error for a potentially dangerous signal from the
world. In fact, the evolution of cultural transmission across generations would
never have occurred without a sufficiently stable environment. Moreover,
acceptance of parental orientation could hardly be negative if we consider that
the docility of offspring does not cancel out the possibility that children keep
their innovative tendencies, imitate new behaviour ignored by the parents or
change their behaviour when they are adults. In other words, approval or
disapproval of behaviour has an adaptive advantage: decreasing the costs of
learning and increasing reliability in the replication of imitated behaviour.
In any case, were it not adaptive for the infant to receive parental guidance,
a tendency to rebellion could have evolved. However, it seems that humans
have developed psychological mechanisms that enable the cultural transmission of values by making ourselves more receptive to parental directions
(approval and disapproval of behaviour). For example, Baum (1994) states
that humans are unusually sensitive to expressions of approval and disapproval by parents and others. Simon (1990) suggested that humans possess
a tendency to accept social influence that can be called “human docility”.
Such a tendency has been favourably selected, according to Simon, because
it allows the individual to use adaptive advantages stored in the cultural
tradition. Waddington (1960) defines human beings as “authority acceptors”,
because children need to have the ability to be taught and they need to develop
authority systems in their minds in order to do so.
The fact that approval or disapproval of learned behaviour only evolved
in the hominid line appears to be related not to the importance of the implied
costs, but rather to the need for a previous complex cognitive development,
similar at least to that possessed by chimpanzees within primates. According
to some authors, true imitation is still present in chimpanzees (Whiten 1998),
but not the ability to approve or disapprove of offspring’s learned behaviour (Premack 2004). In fact, it seems that there is no animal antecedent
of transmission of information about the value of the learned behaviour. In
some species, parental care involves innate responses limiting the offspring’s
movements in order to help them avoid certain risky situations, but these are
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not prohibitions of responses that parents learned to avoid and categorized as
bad.
Today’s primates can emulate other individuals’ behaviour, but they do
not try to modify that behaviour when they are not directly affected by
it. This sort of indifference to other individuals’ behaviour when they are
not directly affected extends to mother-child relationships. A chimpanzee
mother could not teach her infant anything, because, although the infant
watches her problem-solving intently, she never returns the infant’s observation (Inoue-Nakamura and Matsuzawa 1997). Cheney and Seyfarth (1990)
proved experimentally that macaque mothers are unable to transmit to their
offspring information about the existence of hidden food or the presence of a
predator. The authors attributed this far-from-human behaviour to the fact
that monkeys lack of a theory of mind, and so are unable to distinguish
between what they know and what others (including their offspring) know.
This inability is also found in human infants under 3 years old. Nonetheless, it
is difficult to accept that such incapacity could by itself explain such maternal
indifference in monkeys, since it would be more logical to expect that mothers
show some kind of concern when their offspring move closer to where they
shouldn’t, regardless of how sure they are about whether the offspring know
the risks. It seems more reasonable to assume that mothers are unable to
evaluate their offspring’s behaviour by approving or disapproving of it and
taking their interests into account.
Seyfarth and Cheney (1992) reported that when Vervet monkey offspring
start to utter alarm signals or to answer alarms from others they make many
mistakes; under such conditions, adults might be expected to try and help
offspring learn about aspects related to predators. However, after extensive
observation, they have never seen adults selectively stimulating those youngsters who screamed the right alarm signals, or correcting the behaviour of
those who respond inappropriately to vocalizations related to danger. Thus,
offspring learn by mere observation without explicit tutoring. According to
these authors, while human communication greatly helps to condition the
knowledge, beliefs, and motives underlying behaviour, monkeys (and perhaps
other primates, too) cannot communicate with the intention of influencing
another animal’s mental states, probably because they do not recognize the
existence of these states.
On the other hand, on the basis of reports of people who have tried to
raise chimpanzee infants in their homes as children, a companion evolutionary step seems to have been a greater sensitivity to social reinforcement in
children than in chimpanzee infants. Good-bad feedback from human parents
to chimpanzee children results in some socialization, but chimpanzees remain
quite wild and troublesome compared to human children (Baum 1994). Thus,
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in line with our hypothesis, a chimpanzee may classify another individual’s
behaviour as favourable or unfavourable with respect to him/herself, and may
act accordingly, but he/she is unable to attribute a proto-concept of good or
bad to his/her own behaviour, and this impedes the categorization of other
individuals’ behaviour as good or bad for those individuals.
4. Language evolution and the conceptual capacity to categorize
behaviour
It seems reasonable to assume that first ancestors in the hominid line,
Australopithecus, had some kind of cognitive capacity similar to current
chimpanzees, and lacked any linguistic ability. They probably acquired
most of their behaviours through trial-and-error learning and social indirect
learning, though some of them may have been able to learn by imitation.
They may also have had a rudimentary capacity for “theory of mind”.
Consequently, early hominid culture would not have been much different
from that of current chimpanzees. The emergence of Homo habilis and, most
of all, Homo ergaster, meant a considerable increase in relative brain size
and level of encephalization, which seems to correlate with a meaningful
increase in intellectual capacity. The hypothesis we suggest is that at an early
stage of hominid evolution – possibly some ancestor of Homo ergaster –
the development of a new capacity was arising: the capacity to categorize
learned behaviour as good or bad. We defined such a capacity as the ability
of an individual to categorize a given behaviour by means of an essentially
dichotomous conceptual evaluative code (Castro and Toro 1995; Castro and
Toro 1998). This implies the transformation of an automatic mechanism for
categorizing behaviour used for individual learning into a conceptual categorization mechanism. Behaviour rewarded during learning will receive, at least
initially, a positive conceptual categorization, while behaviour provoking
some external rejection will receive a negative evaluation.
The idea we defend is that hominids with the capacity to categorize behaviour (assessors) may approve or disapprove of the behaviour learned by their
offspring. Hence, hominid assessors could transmit to their offspring the
emotional value attributed to those responses they have previously learned
and categorized. Our hypothesis is that the adaptive advantage providing the
possibility to transmit information about learned behaviour exerted a selection pressure in favour of the development of a more efficient communication
system between parents and children.
Our thesis suggests that what initially occurred during hominization
was modification of the kind of information transmitted, but not of the
communication system itself. The set of information that any primate can
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acquire is ample, including information about locations, objects, animals,
individuals and actions that individuals have to face throughout their life span.
The adaptive relevance of some of this knowledge is beyond doubt, and a
communication system based on gestures and unarticulated screams is used
to deal with it. If evolution of language has been the response to a need to
improve such a nonverbal communication system in primates, we should find
differences in fitness, which would be directly related to the ability to transmit
or receive information, and a given evolution should be perceived in all the
different groups of primates. This does not seem to occur: the communication
system of current primates is adequate for communicating the kind of information they transmit. We propose that, apart from transmitting information
about the same types of issues as other primates, our hominid ancestors were
the first primates to be able to transmit information about the value of the
learned behaviour, by approving or disapproving of such a behaviour when
executed by their offspring. Note that such information transmission may
begin working from the very beginning, given that approval or rejection of
other individual’s behaviour is common in the behaviour of chimpanzees and
other primates (for example, a movement of the head or the expression of
some emotion through the face may be sufficient for communicating approval
and disapproval), even though it always refers to behaviour directly affecting
the individual expressing the emotions. However, despite the fact that the
system may have initially started working through gestured communication,
it is reasonable to think that it soon became inadequate for the efficient transmission of information about what behaviour to follow. It is difficult to forbid
children to do something they believe they can do, or something they see
others within the same population do: explanation is necessary.
According to Calvin and Bickerton (2000), what first arose was probably
a proto-language, in which words had no syntactic organization and worked
as labels for designating nonverbal concepts in the organism’s mind. From
our point of view, the two crucial primary concepts were good and bad,
which allowed categorisation of actions performed and objects with which
offspring interact. This introduces a qualitative change in intentionality of
communication: parents are not indifferent to what their children learn. The
important thing now is to evaluate and keep control of what they do, and
parents are thus forced to transmit information to them about how to behave
at each moment. Information transmitted arises from an evaluative conception
of behaviour through which the assessor may determine what must or mustn’t
be done, and the intention of communication is to modify other individuals’
behaviour for their own interest.
Our thesis states that the genesis of an arbitrary system of propositionally organized linguistic signs was probably the adaptive response explored
734
by hominids. The propositional structure allowed the creative flexibility
necessary for transmitting a wide range of information items. Its origin,
though, may be linked to the transmission of information about the value
of behaviours and objects: “This is right, this is wrong”, “This is good,
this is bad”, “Do this, don’t do this”, where the presence of basic syntactic
categories (agent, topic, object) can be identified. Thus, a developmental
scenery allowing the gradual evolution of the human language capacity was
shaped. Recently, Nowak and co-workers have shown that evolutionary game
theory provides a framework in which the evolution of linguistic elements,
such as word formation, lexical structures, syntax and universal grammar,
can be studied (Nowak and Krakauer 1999; Nowak et al. 2000; Nowak et al.
2001).
Greater efficiency in communication would have made investment in individual learning costs more profitable, and may have favoured intellectual
development. The cultural transmission of values increased the fitness of
those assessor parents with greater cognitive capacity because they were able
to transmit all their behavioural experience to offspring – that is, they were
able to transmit information that cannot be imitated by others about what
must not be done. Moreover, the cultural transmission of values modified
the adaptive meaning of intelligence. The pressure of natural selection on
intellectual development in a system of value transmission derives from its
ability to generate asymmetries of value among different types of behaviour,
and consequently to reflect preferences, rather than from the innovative power
of intelligence (Castro and Toro 1998). Innovative and creative aspects of
intelligence lost adaptive importance compared to the ability to weigh up
rationally the advantages and disadvantages of different cultural alternatives.
The adaptive value of intelligence would be associated with the capacity
to recognise and positively categorize behaviour with adaptive importance
that sporadically arises among the population, helping to avoid its disappearance as a result of cultural drift or inadequacies in the hereditary process.
All of this, in turn, made the capacity to transmit the learned information
more advantageous. Hence, there emerged a self-catalytic process favouring
development of the intellect and of a communication system capable of
transmitting previously-learned information. Thus any kind of evolutionary
trade-off between individual and social learning disappears.
Some ideas about the ontogenetic development of ethical capacity appear
to lend support to our thesis. For example, Kohlberg (1981) identified a
sequence of six different stages, grouped in three levels, within the maturation
process of moral judgement; these categories, as regards their basic features,
have been confirmed in many other studies. The first level, predominant in
children up to age 10 years, refers to two types of moral reasoning based on
735
the refusal to do certain actions, either to avoid being punished (stage 1), or
to obtain a reward (stage 2). Good and bad is assessed in terms of what can
and cannot be done, and is linked to receipt of a punishment or a reward.
This may be interpreted from our perspective as unmistakable evidence of
the importance of approval and disapproval of behaviour as a guide to child
behaviour in our species.
Finally, it is worthy of note that some important aspects of our hypothesis can be verified empirically. For example, the lack of a true capacity to
approve or disapprove of offspring behaviour in primates when it does not
directly affect them; the importance of approval/disapproval of behaviour in
current hunting and gathering cultures; the effect of the value associated with
different behaviours on cultural transmission; and perhaps even, in the near
future, the neurological foundations of the conceptual capacity to categorize
behaviour.
Acknowledgements
This work was developed within the R+D Project: Estudio interdisciplinar de
rasgos funcionales del grado humano (BSO2000-116-C04). The work was
also assisted by the concession to L. Castro of a “licencia por estudios” by
the Madrid local government.
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