The target article epitomizes a growing appetite for interdisciplinary research, bridging balkanized fields such as psychology, economics, anthropology, and biology. This integration requires “a research program that can explain the manifest patterns of similarity and variation by clarifying the underlying evolutionary and development processes” (sect. 7.2, para. 2). Development must be a foundation stone upon which this new program is built – if not, to stretch our titular metaphor, the conceived integration will be stillborn.
Confronted with a catalogue of human universals (e.g., social exchange) and cultural variation (e.g., rituals), social scientists traditionally link universals with evolutionary processes and variation with developmental ones. Echoing Henrich et al., we believe this is a false choice. Among other reasons, developmental mechanisms, including the learning abilities giving rise to cultural transmission, are products of natural selection; universality can arise for several reasons, natural selection being one of them (Jablonka & Lamb Reference Jablonka and Lamb2005); and, evolutionary processes can result in adaptive plasticity, developmental systems capable of constructing a range of adaptive phenotypes, contingent on the local ecology.
To illustrate how natural selection can tailor development to local conditions, let us consider the soapberry bug, a half-inch-long, seed-eating insect dwelling in the Southeastern United States (Carroll Reference Carroll1993; Carroll & Corneli Reference Carroll and Corneli1995). While it takes a male soapberry bug only ten minutes to copulate with a female, he may spend hours more anchored to her by means of specially designed genital hooks. This mate guarding increases fitness by preventing rival males from copulating with a female before she lays her eggs. However, guarding comes at a price – males could be copulating with additional females. The local sex ratio arbitrates this opportunity cost: fewer females leads to more mate guarding.
In Oklahoma, where sex ratios vary between populations, males exhibit adaptive plasticity, calibrating the amount of mate guarding to the sex ratio experienced during development. By contrast, in Florida, where sex ratios don't vary, male soapberry bugs engage in a fixed amount of mate guarding, and, when raised in lab conditions with variable sex ratios, are incapable of calibrating. Soapberry bugs teach us why natural selection and development should not be seen as opposites. Natural selection designs developmental mechanisms, and these mechanisms give birth to phenotypes adapted to their local ecologies. When environments routinely vary, natural selection can engineer developmental mechanisms that use experience to facultatively adjust behavior.
Some of the variation across human cultures may be due to calibration, analogous to mate guarding in soapberry bugs. For example, women growing up in harsh environments – where life-expectancies are lower – exhibit earlier onset of menarche and younger age of first birth than women growing up in safe environments (Ellis et al. Reference Ellis, McFadyen-Ketchum, Dodge, Pettit and Bates1999). The explanation for this accelerated reproductive strategy, seen in other animals as well, may be a quantity/quality trade-off: When long life is a given, organisms invest in prolonged growth and development, resulting in fewer but higher quality offspring; when life is short, organisms forgo further growth and development, focusing instead on maximizing the number of offspring (Belsky et al. Reference Belsky, Steinberg and Draper1991). The developmental system, in this case determining the timing of reproduction, can thereby produce a correlation between ecology and behavior. Variation in cultural practices, such as coming of age rituals, may then partially reflect the interaction between evolved developmental processes and the state of the environment.
Of course, humans are more complex than soapberry bugs. In addition to calibration, human developmental systems can be “open,” enabling the acquisition of novel skills and information. When combined with culture, a repository of wisdom accumulated across generations, novel skills and information can be passed directly to other individuals, bypassing genetic transmission (Richerson & Boyd Reference Richerson and Boyd2005). Infants and children are thus tasked with extracting adaptive cultural information in order to become competent adults; while adults are tasked with teaching them. The lesson, here, is not that social learning precludes evolutionary explanation; instead, the psychological mechanisms subserving cultural transmission should be viewed as adaptations (e.g., Csibra & Gergely Reference Csibra and Gergely2009).
However, culture is more than a consequence of social learning adaptations; culture can impose selection pressures on developmental processes, altering their genetic compositions (i.e., gene-culture coevolution; Richerson & Boyd Reference Richerson and Boyd2005). As Henrich et al. discuss, the ability of some people to consume milk into adulthood, particularly those from European and some African populations, provides a clear-cut example: A cultural adaptation, pastoralism, and its consequence, the prevalence of milk, created a novel selection pressure on genes, prolonging the production of lactase, an enzyme needed to digest milk sugars. Research on how coevolutionary processes shaped human cognition and development is still in its infancy.
Although Henrich et al. survey the rich breadth of the human experience, it is worth underscoring just how special we are: No other animal occupies as many different ecologies, no other animal deploys a comparable range of subsistence techniques, and no other animal exhibits as wide a range of social structures. The propensity for this plasticity makes sense only in the light of evolution. Fear not: An evolutionary explanation need not be simplistic; a thorough explanation of human plasticity requires, at a minimum, phylogenetic, paleo-ecological, cross-cultural, and adaptationist considerations. For example, recent research on past climates points not to a static evolutionary ecology, but to one in which climate change was the norm (reviewed in Richerson et al. Reference Richerson, Boyd and Bettinger2001). Further, the timescale of this climatic variation was short, particularly during the late Pleistocene (120,000 to 10,000 years ago) when environments changed radically on the order of hundreds to thousands of years, a situation best tracked by cultural adaptation, rather than genetic evolution (too slow) or individual learning (too error prone). Our human nature, housing a rich array of evolved developmental mechanisms capable of open-ended, facultative adaptation, may have been conceived in this nurturing cradle of change.
Learning precisely how the human mind emerged from the evolutionary process poses a challenge that some believe insurmountable (e.g., Lewontin Reference Lewontin, Scarborough and Sternberg1998). We remain optimistic. Progress will be made as research becomes increasingly interdisciplinary. Scholars interested in developmental processes will benefit from attending to cross-cultural studies, as processes often reveal themselves through their manifestations in different ecological contexts. An understanding of developmental processes will benefit students of culture, as development links ecology to behavior. An evolutionary perspective can illuminate why humans have the particular developmental mechanisms they do, given our species' evolutionary history.
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The target article epitomizes a growing appetite for interdisciplinary research, bridging balkanized fields such as psychology, economics, anthropology, and biology. This integration requires “a research program that can explain the manifest patterns of similarity and variation by clarifying the underlying evolutionary and development processes” (sect. 7.2, para. 2). Development must be a foundation stone upon which this new program is built – if not, to stretch our titular metaphor, the conceived integration will be stillborn.
Confronted with a catalogue of human universals (e.g., social exchange) and cultural variation (e.g., rituals), social scientists traditionally link universals with evolutionary processes and variation with developmental ones. Echoing Henrich et al., we believe this is a false choice. Among other reasons, developmental mechanisms, including the learning abilities giving rise to cultural transmission, are products of natural selection; universality can arise for several reasons, natural selection being one of them (Jablonka & Lamb Reference Jablonka and Lamb2005); and, evolutionary processes can result in adaptive plasticity, developmental systems capable of constructing a range of adaptive phenotypes, contingent on the local ecology.
To illustrate how natural selection can tailor development to local conditions, let us consider the soapberry bug, a half-inch-long, seed-eating insect dwelling in the Southeastern United States (Carroll Reference Carroll1993; Carroll & Corneli Reference Carroll and Corneli1995). While it takes a male soapberry bug only ten minutes to copulate with a female, he may spend hours more anchored to her by means of specially designed genital hooks. This mate guarding increases fitness by preventing rival males from copulating with a female before she lays her eggs. However, guarding comes at a price – males could be copulating with additional females. The local sex ratio arbitrates this opportunity cost: fewer females leads to more mate guarding.
In Oklahoma, where sex ratios vary between populations, males exhibit adaptive plasticity, calibrating the amount of mate guarding to the sex ratio experienced during development. By contrast, in Florida, where sex ratios don't vary, male soapberry bugs engage in a fixed amount of mate guarding, and, when raised in lab conditions with variable sex ratios, are incapable of calibrating. Soapberry bugs teach us why natural selection and development should not be seen as opposites. Natural selection designs developmental mechanisms, and these mechanisms give birth to phenotypes adapted to their local ecologies. When environments routinely vary, natural selection can engineer developmental mechanisms that use experience to facultatively adjust behavior.
Some of the variation across human cultures may be due to calibration, analogous to mate guarding in soapberry bugs. For example, women growing up in harsh environments – where life-expectancies are lower – exhibit earlier onset of menarche and younger age of first birth than women growing up in safe environments (Ellis et al. Reference Ellis, McFadyen-Ketchum, Dodge, Pettit and Bates1999). The explanation for this accelerated reproductive strategy, seen in other animals as well, may be a quantity/quality trade-off: When long life is a given, organisms invest in prolonged growth and development, resulting in fewer but higher quality offspring; when life is short, organisms forgo further growth and development, focusing instead on maximizing the number of offspring (Belsky et al. Reference Belsky, Steinberg and Draper1991). The developmental system, in this case determining the timing of reproduction, can thereby produce a correlation between ecology and behavior. Variation in cultural practices, such as coming of age rituals, may then partially reflect the interaction between evolved developmental processes and the state of the environment.
Of course, humans are more complex than soapberry bugs. In addition to calibration, human developmental systems can be “open,” enabling the acquisition of novel skills and information. When combined with culture, a repository of wisdom accumulated across generations, novel skills and information can be passed directly to other individuals, bypassing genetic transmission (Richerson & Boyd Reference Richerson and Boyd2005). Infants and children are thus tasked with extracting adaptive cultural information in order to become competent adults; while adults are tasked with teaching them. The lesson, here, is not that social learning precludes evolutionary explanation; instead, the psychological mechanisms subserving cultural transmission should be viewed as adaptations (e.g., Csibra & Gergely Reference Csibra and Gergely2009).
However, culture is more than a consequence of social learning adaptations; culture can impose selection pressures on developmental processes, altering their genetic compositions (i.e., gene-culture coevolution; Richerson & Boyd Reference Richerson and Boyd2005). As Henrich et al. discuss, the ability of some people to consume milk into adulthood, particularly those from European and some African populations, provides a clear-cut example: A cultural adaptation, pastoralism, and its consequence, the prevalence of milk, created a novel selection pressure on genes, prolonging the production of lactase, an enzyme needed to digest milk sugars. Research on how coevolutionary processes shaped human cognition and development is still in its infancy.
Although Henrich et al. survey the rich breadth of the human experience, it is worth underscoring just how special we are: No other animal occupies as many different ecologies, no other animal deploys a comparable range of subsistence techniques, and no other animal exhibits as wide a range of social structures. The propensity for this plasticity makes sense only in the light of evolution. Fear not: An evolutionary explanation need not be simplistic; a thorough explanation of human plasticity requires, at a minimum, phylogenetic, paleo-ecological, cross-cultural, and adaptationist considerations. For example, recent research on past climates points not to a static evolutionary ecology, but to one in which climate change was the norm (reviewed in Richerson et al. Reference Richerson, Boyd and Bettinger2001). Further, the timescale of this climatic variation was short, particularly during the late Pleistocene (120,000 to 10,000 years ago) when environments changed radically on the order of hundreds to thousands of years, a situation best tracked by cultural adaptation, rather than genetic evolution (too slow) or individual learning (too error prone). Our human nature, housing a rich array of evolved developmental mechanisms capable of open-ended, facultative adaptation, may have been conceived in this nurturing cradle of change.
Learning precisely how the human mind emerged from the evolutionary process poses a challenge that some believe insurmountable (e.g., Lewontin Reference Lewontin, Scarborough and Sternberg1998). We remain optimistic. Progress will be made as research becomes increasingly interdisciplinary. Scholars interested in developmental processes will benefit from attending to cross-cultural studies, as processes often reveal themselves through their manifestations in different ecological contexts. An understanding of developmental processes will benefit students of culture, as development links ecology to behavior. An evolutionary perspective can illuminate why humans have the particular developmental mechanisms they do, given our species' evolutionary history.