The term "social learning", as currently applied to animals, describes a ragbag of heterogeneous processes, with a variety of functions, found in a broad array of vertebrate and invertebrate species. A more narrow use of the term would restrict it to those processes that might reasonably be regarded as homologous to processes operating in human social learning, and that mediate a general capacity to acquire information from others, regardless of the nature of the information, its function, or the sensory modality employed. Within the narrow category of social learning, humans probably transmit more information vertically from parent to offspring than all other species. Protocultural species typically depend primarily on horizontal transmissions based on social enhancement, rather than on imitation or teaching (Galef, 1988; Laland et al., 1993). A comparative perspective thus implies that the earliest forms of social transmission were probably horizontal. In contrast, humans appear to acquire large amounts of information from their parents, and from the parental generation (Hewlett & Cavalli-Sforza, 1986; Guglielmino et al., 1995), suggesting that the lineage leading to Homo sapiens has been selected for increasing reliance on vertical and oblique cultural transmission. The theoretical analyses described above imply that a shift from transient horizontal traditions towards increased transgenerational cultural transmission reflects a greater constancy in the environment over time. Such a shift is difficult to reconcile with the traditional evolutionary perspective, since there is no evidence to suggest that environments have become more constant over the last few million years, but rather the opposite. Moreover, even if they had, other proto-cultural species would be expected to show more vertical transmission too. However, the increasing reliance of hominids on vertical transmission is consistent with our perspective, since here, a significant component of the selective environment is self-constructed by the species concerned, and this component could have favoured vertical transmission. We are suggesting that our ancestors constructed niches in which it "paid" them to transmit more information to their offspring. The more an organism controls and regulates its environment, and the environment of its offspring, the greater should be the advantage of transmitting cultural information from parent to offspring. For instance, by tracking or anticipating the movements of migrating or dispersing prey, populations of hominids may have increased the chances that a specific food source was available in their environments, that the same tools used for hunting would always be needed, and that the skin, bones and other materials from these animals would always be at hand to use in the manufacture of further tools. Such activities create the kind of stable social environment in which related technologies, such as food preparation or skin processing methods, would be advantageous from one generation to the next, and could be repeatedly socially transmitted from parent to offspring. It is possible that, once started, vertical cultural transmission may become an autocatalytic process: greater culturally generated environmental regulation leading to increasing homogeneity of environment as experienced by parent and offspring, favouring further vertical transmission. With new cultural traits responding to, or building on, earlier cultural traditions, niche construction sets the scene for an accumulatory culture. This might result in offspring learning higher-order "packages" of cultural traits from their parents, as appears to be the case in pre-industrial societies (Hewlett & Cavalli-Sforza, 1986; Guglielmino et al., 1995). Clearly, the transition from animal proto-culture to human culture involved much more than a shift towards vertical transmission of information and the development of an accumulatory culture. Nonetheless, it is clear that the new evolutionary perspective portrayed in Figure 2c can generate novel hypotheses that may help to reconstruct some aspects of the evolution of human culture. 2.2. Example 2: Human Altruism, Cooperation and Conflict. At present, standard evolutionary theory provides two principal explanations for "altruistic" cooperation in organisms, kin selection (Hamilton, 1964) and reciprocity (Trivers, 1985; Axelrod, 1984), as exemplified by solutions such as tit-for-tat to prisoner�s dilemma type games (Axelrod, 1984) . These ideas have been used to account for a lot of cooperation in nature but neither is sufficient to explain the full gamut of human cooperation (Boyd & Richerson, 1985; Richerson & Boyd, In Press) . Kin selection is restricted to kin, while the evolution of cooperation based on reciprocity is probably limited to group sizes of less than ten, because increasing the size of interacting social groups reduces the likelihood that selection will favour reciprocating strategies (Boyd & Richerson, 1988)3. It is also hard to account for certain forms of human altruism, such as military heroism, in terms of reciprocity or kin selection (Boyd & Richerson, 1985). Our evolutionary framework, however, indicates that the suite of processes that may be regarded as feasible evolutionary explanations for human cooperation is considerably larger than kin selection and reciprocity alone. We suggest that any organism, O1, should be prepared to cooperate in ways that benefit any other organism, O2, provided the total niche-constructing outputs of O2, or any of O2�s decendents, modify resources in the environment of O1, or any of O1�s decendents, with resulting fitness benefits to O1 that exceed the cost of O1�s cooperation. This reasoning applies to relatives, and implies that cooperation should be more likely among kin that niche construct in mutually beneficial ways, and less likely among kin that niche construct in mutually detrimental ways, than a strict interpretation of Hamilton�s rule might suggest. The same logic also applies to non kin. In fact it is obvious that reciprocal altruism is a special case in which O1 and O2 are unrelated individuals of the same species that directly modify each other�s environment. Recent analyses here suggest that most cases currently described as reciprocal altruism, and many cases of kin selection, are actually forms of intra-specific mutualism that are the incidental outcomes of selfish behaviour (Mesterton-Gibbon & Dugatkin, 1992; Connor, 1995). In addition our statement justifies a variant of reciprocal altruism in which individuals do not trade altruistic acts with each other, but rather an individual aids a second individual if the latter acts in ways that benefit the first individual�s decendents. Mutualisms and commensalisms also slot into this framework. Like altruism, mutualism also depends on the modification of the selective environments of recipient organisms by the niche-constructing activities of donor organisms. However, in the case of mutualisms, O1 and O2 are individuals in different species. The same is true of commensalisms, which for our purpose can be regarded as asymmetric versions of mutualism. In commensalisms O1 cooperates with O2 because O2 benefits O1 by ameliorating O1�s environment, but O2 is unnaffected by O1, so O2 does not cooperate with O1. Our general statement sketching the conditions under which organisms should cooperate, may, under restricted circumstances, generalise to cases in which O1, and O2 are groups of organisms, in one or more species. In many cases, the niche-constructed by-products of several organisms are exploited by a population. For example, shoals of fish, flocks of birds, and herds of animals enjoy reduced predation risks, relative to solitary animals, merely because their combined presence changes the selective environment of each individual (Hamilton, 1971). Coordinated fish driving by cormorants, or seal hunting by killer whales, provide examples of individuals coordinating their niche construction so that it results in more effective food acquisition for everyone (Connor, 1995). Mutualistic interactions that result from the exploitation of incidental by-products are more evolutionarily robust than altruistic interactions as, because each individual is already acting selfishly, they are less likely to be broken down by selfish cheaters. Organisms may also invest in others so as to enhance the benefits which eventually return to themselves (Connor, 1995). A large amount of human cooperation can probably be explained in terms of the exploitation and investment in the by-products of others, in other words in terms of mutualisms resulting from human niche construction. For example, barter and exchange are mutualistic interactions in which individuals or organizations trade products for more desirable alternatives. Moreover, human individuals and institutions "invest", metaphorically, or even literally, in other individuals or institutions in order to enhance their own returns. One other possible explanation of cooperation in animals is the revised group selection hypothesis proposed by Wilson and Sober (1994), about which we have some reservations. In the past, much confusion has been caused by a failure to distinguish between group selection for group or for individual advantage (Wynne Edwards, 1962), or between "replicators" and "vehicles" 4 as the object of group selection (Wilson & Sober, 1994). Wilson and Sober maintain that the fundamental question concerning group selection really turns on whether social groups, or any other higher-level entities, can be vehicles of selection, and not on whether they are replicators. They propose a nested hierarchy of vehicles for genes (individual, group, metapopulation) in which each level also includes a population of lower-level units. Selection then acts at the lowest level for which there are fitness differences. Thus, if there are no fitness differences between individuals, there should be a "frame shift" (p. 592) to selection at the group level. The original, or "naive", group selection hypothesis failed, primarily because the processes that maintain group differences and select between groups are typically weak compared with the processes that break down group differences and select within groups (Williams, 1966). If group-level adaptations are based on the cooperation of altruists, then any individual who refuses to cooperate can reap the benefits without paying the costs, and selfish strategies should be favoured by natural selection. It is not clear how Wilson and Sober can surmount this obstacle to explain the cooperation of large groups of individuals, since they have not proposed any feasible process that could have reduced individual differences within groups, yet promoted differences between groups, during hominid evolution. Here it may be worth reflecting on the fact that group-level cooperation typically depends on niche construction, because group-level adaptations are generally expressed outside of human bodies. Groups may remain cohesive because it pays each individual to invest in mutually beneficial niche construction. Although we recognize some utility in the replicator-vehicle distinction, and in Wilson and Sober�s hierarchical approach, we regard it as a distortion. The entities that are selected, and between which there are fitness differences, are not well described as "vehicles", or even "interactors" (Hull, 1988), but rather are "organism-environment systems". Similarly, what is replicated, from one generation to the next, is a complex of information (both genetic and cultural) and some environmental (including developmental) resources (Gray, 1992; Oyama, 1985). Once these distortions are removed, it becomes easier to see how a form of group selection could help account for some human cooperation. The hypothesis that comes closest to a solution comes from gene-culture coevolutionary theory, and places explicit emphasis on culturally inherited niche construction. In this hypothesis, proposed by Boyd and Richerson (1985), group selection works at the cultural level, with group-level cultural traits being selected. They place emphasis on a "when in Rome do as the Romans do" conformity, where individuals adopt the behaviour of the majority. The significance of this "conformist transmission" is that it minimises behavioural differences within groups, while maintaining differences between groups. Thus group selection of cultural variation "for the good of the group" is possible because if an altruistic cultural trait becomes frequent in a cultural group, the transmission process should subsequently discriminate against selfish individuals. Group selection of cultural rather than genetic variation, requires a "frame shift" of replicator, because it is not genes that are selected for, but rather groups of individuals expressing a particular culturally transmitted idea. Since cultural selection between groups may favour beliefs that benefit the group at the expense of the individual, Boyd and Richerson provide a new explanation for human cooperation. Several properties of cultural inheritance, as opposed to genetic inheritance, make Boyd and Richerson�s idea attractive. One is that cultural inheritance, unlike genetic inheritance, may depend on more than two parents. It is therefore possible for individuals to be sensitive to the most frequent cultural traits in their society, and to conform to them. Second, group selection of cultural variants can be faster than group selection of genetic variants, because cultural death does not imply the physical death of all the people in a culture. A threatened or defeated people may switch to the traits of a new conquering culture, either voluntarily, or under duress. Thus, unlike Wilson and Sober�s group selection, here migration will not weaken the process. Third, symbolic group marker systems, such as totem animals, human languages, and flags, make it considerably easier for cultures to maintain their identities, and to resist imported cultural traits from immigrants, than it is for local gene pools, or demes, to maintain their identity by resisting gene flow (Boyd & Richerson, 1985). Fourth, cultural transmission of information about cheaters (e.g. gossip) reduces the efficacy of non-cooperative strategies (Dugatkin, 1992). More negatively, conformist transmission may potentially be exploited by powerful individuals, groups, or institutions, that dominate the dissemination of information through societies, to promote their own interests. Powerful "cultural parents" may stand to gain from persuading other less powerful humans to conform, perhaps by recruiting extra assistance in modifying environments in ways in which they, rather than the helpers benefit. These processes may be amplified by tool use, for instance by the technology of the modern media, by weapons, by art, or by deceit. Religious, commercial and political propaganda, for example, may all be used to persuade, trick or coerce conformity from others against their own individual interests, yet in favour of the interests of a dominant class of cultural transmitters. We can also reverse our earlier logic to suggest that any organism O1, should act in a hostile manner, to the disadvantage of any other organism O2, provided the total niche-constructing outputs of O2, or of any of O2�s descendents, modify resources in the environment of O1, or any of O1�s descendents, to the detriment of O1, if the resulting reduction in the fitness costs to O1 of O2�s outputs exceeds the cost of O1�s agonistic behaviour. It is easy to see how this reasoning might account for a great deal of aggressive behaviour, including a form of reciprocal hostility, in which individuals and their descendents trade antagonistic acts. In other words, we predict that organisms should actively harm other organisms by investing in niche construction that destroys other organism�s selective environments, provided the fitness benefits that accrue to the investing organisms from doing so, are greater than their fitness costs. Since this is a general idea, it should extend to the human cultural level, with the qualification that at this level other processes may be operating. 2.3 Example 3: Selfish Phenogenotypes and Multiple-Process Adaptation 2.3.1. Units of Selection in Human Evolution: A prima facie problem with our multiple-processes-in-evolution approach is that it raises questions about the currency of human evolution. Should cultural traits be measured in terms of reproductive success only, as the sociobiologists advocate, or should they be measured by a cultural transmission rate parameter as well as, or instead of genes? This problem was one of the earliest hurdles faced by contemporary gene-culture theory. Initially genetic and cultural processes were treated as independent, with separate fitness scores and transmission coefficients allocated for natural selection and cultural transmission. However, it rapidly became clear that genes and cultural traits can interact in the same way that two genetic loci can interact, to generate associations of genotype and cultural phenotype in non-random frequencies (Feldman & Cavalli-Sforza, 1984). This means that treating genetic and cultural processes as independent is a distortion. A straightforward, pragmatic solution is to allocate fitnesses and transmission rate parameters directly to combinations of genotypes and cultural traits, a package known as a 'phenogenotype'. For instance, in Feldman and Cavalli-Sforza�s (1989) theoretical exploration of the coevolution of lactose tolerance and milk usage, one phenogenotype was a milk using individual expressing two copies of the allele conferring lactose tolerance, while another was a non-milk using individual which no such alleles. For human sociobiologists, the most appropriate way to think about evolution is from the perspective of the gene: those characteristics that have been favoured by selection are the expression of the "selfish genes"(Dawkins, 1989) that were best able to increase their representation in the next generation. For gene-culture coevolutionary theory, the logic is the same, but the replicator is different: Instead of the selfish gene there is the "phenogenotype". Those human characteristics that have been favoured in the face of both natural selection and cultural transmission are the expression of the phenogenotypes that were best able to increase their representation in the next generation, by whatever process. As an intuitive shorthand, a phenogenotype can be thought of as a human with a package of genes and experience. In this sense, the phenogenotype approach re-establishes the organism (or rather, classes of organism) as the central unit of human evolution, not as vehicle but as replicator. In fact, what is really replicated is a bio-cultural complex, with a composite array of information (acquired through multiple processes, and stored at different levels) and inherited resources. However, we recognize the need for simple conceptual and formal models that have the utility to explore and shed light on the dynamics of such systems, and we regard the phenogenotype approach as the best method currently on the market5. 2.3.2. Multiple-Process Adaptation: Controversy has surrounded the sociobiological postulate that human beings typically behave in ways that increase their inclusive genetic fitness (Sahlins, 1976; Montagu, 1980). It is trite to point out that the processes that underlie culture are adaptations, and that socially learned information, and cultural inheritance, may increase reproductive success. Mathematical models that have explored the evolution of social learning reveal that it is a truism of the modelling exercise that the capacity for social learning cannot be favoured unless it generally increases some measure of fitness. Obviously, the same is true of knowledge-gaining ontogenetic processes. So what characteristics of human culture could allow humans to behave in a maladaptive way, or to transmit maladaptive information? One of the most important findings to emerge from gene-culture coevolutionary theory is that there are a variety of mechanisms by which culture can lead to the transmission of information that result in a fitness cost relative to alternatives. Cavalli-Sforza and Feldman (1981) provided theoretical confirmation of the intuitive notion that cultural traits associated with a viability or fecundity deficit may still increase in frequency in a population if there is strong conversion of individuals to the same trait. Boyd and Richerson (1985) found that, where individuals adopt the behaviour of influential or successful members of their society, maladaptive cultural variants can spread, even if associated with a substantial viability disadvantage. Other gene-culture models reach the same conclusion (Feldman & Laland, 1996). Our perspective suggests that, in each generation, populations of organisms persistently construct or reconstruct significant components of their environments. This means that, as they evolve, organisms may, in effect, drag part of their own environments along with them, thereby transforming their own "adaptive landscapes". If ontogenetic processes, culture, and counteractive niche construction in general, have consistently damped out the need for a genetic response to changes in the population�s environment, hominid populations may have become increasingly divorced from their ecological environments. At the same time, our hominid ancestors may increasingly have responded to novel selection pressures initially generated by inceptive niche construction, and subsequently dominated by cultural traditions. In this case, the common conception that modern human populations are adapted to an ancestral Pleistocene environment (Barkow et al., 1992) can only be partly correct. In particular, components of the social environment, for example, traits related to family, kinship and social stratification, may have been increasingly vertically transmitted by culture to the extent that contemporary human populations may have become largely divorced from local ecological pressures. Support for this argument comes from Guglielmino�s et al.�s (1995) study of variation in cultural traits among 277 contemporary African societies, in which most traits examined correlated with cultural (linguistic) history, rather than ecology. In the short term organisms typically niche construct in ways that enhance their immediate fitness, but in the long term organisms can also "niche destruct" relative to their own genes. For example, they can build up polluting detritus, or strip their environments of non-renewable, or too slowly renewing, resources, until they have made their own environments hostile to themselves and to their offspring (Diamond 1993). Among plants, this process typically leads to auto-ecological succession, while animals typically respond by dispersing to other environments. Failure to respond to the feedback from negative niche construction is a possible recipe for extinction. Could humans drive themselves to extinction? There are two reasons for supposing that this is a possibility. First, culture greatly enhances the human capacity for niche construction. For example, science based technology is currently making an enormous impact on the human environment. It has made many new resources available via both agriculture and industry, it has influenced human population size and structure via hygiene, medicine and birth control, it has drastically changed human warfare, it is drastically reducing biodiversity, and it may already have resulted in the degradation of large areas of our global environment. These are all potential sources of modified natural selection pressures. Second, human cultural processes can work much faster than human genetic processes, generating new adaptive problems at a faster rate than the human genetic processes can respond to. In these circumstances, human culture might drive either local, or general, self-induced extinctions. In many respects, this is a reoccurrence of an old evolutionary problem. Many relatively long-lived species encounter rates and types of environmental change, whether self-induced or independent, that exceed the capacity of their genes to handle, and they frequently go extinct. Clearly, one way in which human beings could adapt to culturally induced environmental changes is through quicker acting responses at some non-genetic level, especially through further cultural change. Unfortunately there are well known snags with this kind of solution. First, the population may not recognise the source of the novel, culturally induced selection pressure. This was the case with the Fore of Papua New Guinea who maintained a cannibalistic tradition despite the fact that it perpetuated a deadly disease (Durham 1991). Second, the required corrective technology may not always be available, or may be too costly to introduce. For example, in theory the Kwa could have responded to the increased selection by malaria by the cultural control of this disease, but in fact they lacked the technology to do so. Third, the feedback from cultural niche construction may be indirect, which may make it difficult to recognise any longer term negative consequences of the niche construction. Rogers (1995) documents how the adoption of wet rice cultivation in Madagascar had a range of diffuse indirect effects only manifest several generations later, including changes in tribal government, patterns of warfare and the role of the father. Fourth, responding to cultural change with further cultural change always risks introducing a "runaway" situation, in which each new solution generates the next problem, at an ever accelerating rate. The phenomenon of antibiotic resistance is a recent example (Ewald, 1994). 3.0 CONCLUDING REMARKS In the preceding sections we have begun to develop a new type of evolutionary framework for the human sciences by emphasing niche construction, and ontogenetic and cultural processes. We have also illustrated the conceptual model with a number of ideas, related to sample topics. Our hope is that these suggestions will encourage others to use the evolutionary framework we are proposing in this paper, either to take some of the ideas we have already discussed further, to the point where they can be empirically tested, or to generate other hypotheses of their own. Notes. 1 Here "construction" refers to a physical modification of the environment and not to the perceptual processes responsible for constructing a mental representation of the world from sensory inputs. 2 Although in their book, Genes, Mind and Culture, Lumsden and Wilson (1981) labelled their models "gene-culture coevolutionary theory", their approach had more in common with conventional sociobiology than the modern gene-culture coevolutionary theory (Feldman & Cavalli-Sforza, 1976; Boyd & Richerson, 1985). 3 Boyd and Richerson (1992) established that punishment allows reciprocity to evolve in large groups, if reciprocators respond to noncooperation by withholding future cooperation, and also punish others that do not punish noncooperators. However, they also established that there is no guarantee that the cultural traits stabilized by punishment will enhance individual or group fitness. 4 Dawkins (1989) coined the terms "replicator" and "vehicle" to distinguish between the �immortal� genes, that are replicated each generation, and the transient, vehicular organisms that house them. Dawkins also makes the point that there may be other kinds of replicator, for instance, culturally learned beliefs or traits, or "memes", which may be selected by processes analogous to natural selection, a point anticipated by Cavalli-Sforza & Feldman (1973), and central to contemporary gene-culture coevolutionary theory. 5 The "phenogenotype" is simply a convenient tool for operationalizing the modelling of gene-culture coevolution. We anticipate that, in some circumstances, the relationship between genetic information, culturally acquired information, and the behavioural phenotype may eventually prove too complex to be handled in this way. For instance, in the past, gene-culture modellers have chosen to parameterize either the frequency of a behaviour pattern or of acquired information, as convenient. Such switching between symbolically encoded information and phenotype is legitimate only where there is a tight correspondence between information and behaviour (Cronk, 1995). Where this correspondence is weak, gene-culture methods might have to be developed further, for example, by introducing a coefficient into the models that represents the extent to which individuals with a particular combination of genes and acquired information are likely to express a particular behavioural phenotype. Acknowledgements We are grateful to Robert Hinde, Adam Kuper, Richard Lewontin, Henry Plotkin, Peter Richerson, and Kate Robson-Brown for helpful comments on an earlier draft of this manuscript. Kevin Laland was supported by a Royal Society University Research Fellowship, and Marcus Feldman by NIH grant GM28016. 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