Selective Scenarios for the Origin of Language

The issue of the origin of human language has provided fertile ground for speculation and alternative theories have been proposed (Box 1).

Most of the theories that suggest a given context for the evolution of human language attempt to account for its functional role. Given that, functionally, all of these theories are more or less plausible, it is almost impossible to decide on their usefulness based only on this criterion. However, recent game theoretical research can help us evaluate various contexts. These criteria concern the interest of communicating parties and the cost of equilibrium signals.

The central issue is whether early linguistic communication was honest. If signal cost is the same for all signallers, then honest cost-free signalling can be evolution-arily stable only if there is no conflict of interest between the participants (Maynard Smith, 1991). If the cost of signals varies with the quality of the signaller, then the situation is more complicated. In this case, it is possible to construct cost functions that give an arbitrarily low cost at equilibrium even if there is a conflict of interest (Hurd, 1995; Szamado, 1999; Lachmann et al., 2001) (Box 2).

In case of human language, the most obvious way to construct such a cost function is to punish dishonest signallers (Lachmann et al., 2001). However, this solution assumes that dishonest signallers can, on average, be detected (i.e. signals can be cross-checked); it also assumes that dishonest signallers are punished (which is a non-trivial assumption). Thus, one can conclude that, ' "conventional" signals will be used when communicating about (i) coincident interest or (ii) verifiable aspects of conflicting interest; "costly" signals will be used otherwise' (Lachmann et al., 2001). Although theory so far says nothing about the evolution of such systems of communication, there are a few computer simulations that suggest that honest cost-free communication evolves only if there is shared interest between the participants (Bullock, 1998; Noble, 2000; Harris and Bullock, 2002).

What does this tell us about the emergence of human language? The production cost of speech or gesturing appears to be low, thus human language consists of cost-free or low-cost signals at equilibrium (not counting time constraints). Thus, based on the above criteria, one should favour either those theories that propose a context with no conflict of interest (e.g. hunting, tool making, motherese, grooming, or the group bonding and/or ritual theory) or a context in which there might be a conflict

Box 1 Alternatives theories to explain language evolution Gossip: menstrual ritual can be a costly signal of commitment; hence participating in such rituals can create female groups of shared interest in which sharing information about the social life of others (i.e. gossiping) can be beneficial (Power, 1998).

Grooming hypothesis: language evolved as a substitution for physical grooming (Dunbar, 1998). The need for this substitution derived from the increasing size of the early hominid groups.

Group bonding and/or ritual: language evolved in the context of intergroup rituals, which first occurred as a kind of 'strike action' against non-provisioning males. Once such rituals were established, a 'safe' environment was created for further language evolution (Knight, 1998).

Hunting theories: 'our intellect, interests, emotions, and basic social life -all are evolutionary products of the success of the hunting adaptation.' (Washburn and Lancaster, 1968). Later, Hewes (1973) in his paper about the gestural origins of language takes up the idea and argues that the probable first use of language was to coordinate the hunting effort of the group. Language as a mental tool: language evolved primarily for the function of thinking and was only later coopted for the purpose of communication (Burling, 1993).

Mating contract and/or pair bonding: increasing size of the early hominid groups and the need for male provisioning also necessitated 'social contract' between males and females (Deacon, 1997).

Motherese: language evolved in the context of mother-child communication. Mothers had to put down their babies to collect food efficiently, and their only option to calm down babies was to use some form of vocal communication (Falk, 2004).

Sexual selection: language is a costly ornament that enables females to assess the fitness of a male. According to this theory, language is more elaborate than a pure survival function would require (Miller, 2001). Song hypothesis: language evolved rapidly and only recently by a process of cultural evolution. The theory assumes two important sets of pre-adaptations; one is the ability to sing; the other is better representation abilities (i.e. thinking and mental syntax) (Vaneechoutte and Skoyles, 1998). Status for information: language evolved in the context of an 'asymmetric cooperation', where information (that was beneficial to the group) was traded for status (Desalles, 1998).

Tool making: assumes a double homology: 'a homologous neural substrate for early ontogeny of the hierarchical organisations shared by two domains -language and manual object combination - and a homologous neural substrate and behavioural organisation shared by human and non-human primates in phylogeny.' (Greenfield, 1991).

Box 2 The problem of honesty

There is an ongoing debate about the honesty of animal communication, centred on the proposition that signals need to be costly to be honest (Zahavi, 1975). Although some models appear to provide support for this statement (Grafen, 1990; Godfray, 1991; Maynard Smith, 1991), there are exceptions. First, cost-free signals can be evolutionarily stable provided that there is no conflict of interest between the communicating parties (Maynard Smith, 1991). Second, even where a conflict of interest exists, cost-free signals can be evolutionarily stable provided that the cost of signals is a function of the quality being signalled (Hurd, 1995; Szamado, 1999; Lachmann et al., 2001). The most general case is when the fitness of the signaller depends on its state and the fitness of both players is influenced by the survival of the other. Assuming a discrete model with two states, two signals and two responses, the conditions of evolutionarily stable honest signalling are as follows (Eqs I-VI) (Szamado, 1999):

Wh+ rVh> 0

(Eq. I)

Wj+ rVl > 0

(Eq. II)

K + rWh> Ch

(Eq. III)

V + rWx> C

(Eq. IV)

Vh+ rWh> 0

(Eq. V)

V,+ rW > 0

(Eq. VI)

where W, V, and C denotes the fitness of the receiver and signaller, and the cost of signalling, respectively. l and h denote the quality of the signaller ('high' and 'low', respectively). The fitness of each player can be influenced by the survival of the other player (r). Equations I and II describe the conditions of the receiver for honest signalling; Eqs III and IV reflect are the condition of the signaller; and Eqs V and VI describe conflicts of interest. Reversing the inequality in Eq. VI would mean that there is no conflict of interest between signaller and receiver. If r = 1, then there can be no conflict of interest, assuming that signalling is beneficial for the receiver (because V + rWl = W + rVl, Eq. VI cannot be fulfilled). This implies that, in this case, Cl can equally zero or less than zero (given that the left-hand side of Eq. IV need not be greater than zero). However, if 0 < r < 1 then at least Cl should be greater than zero (Eq. IV). That is, signalling for low-quality individuals must be costly in case of a conflict of interest. Signalling for high-quality individuals need not be costly even in this case (Eq. III). Given that at the honest equilibrium only high-quality individuals signal, the observed cost can be zero. If, however, Cl = Ch, then both costs should be greater than zero for honest signalling to be stable. Figure 2 depicts the regions of honest signalling in case of conflict of interest. The same logic also applies and, thus, the same results hold, for continuous models (Lachmann et al., 2001).

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