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Plasticity in Human Life History Strategy Implications

for Contemporary Human Variation and the Evolution of Genus Homo

by Christopher W. Kuzawa and Jared M. Bragg

Jurnal JSTOR

ABSTRACT

The life history of Homo sapiens is characterized by a lengthy period of juvenile dependence that requires extensive allocare, short interbirth intervals with concomitantly high fertility rates, and a life span much longer than that of other extant great apes. Although recognized as species-defining, the traits that make up human life history are also notable for their extensive within- and between-population variation, which appears to trace largely to phenotypic and developmental plasticity. In this review, we first discuss the adaptive origins of plasticity in life history strategy and its influence on traits such as growth rate, maturational tempo, reproductive scheduling,and lifes panin modern human populations. Second, we consider the likely contributions of this plasticity to evolutionary diversification and speciation within genus Homo. Contrary to traditional assumptions that plasticity slows the pace of genetic adaptation, current empirical work and theory point to the potential for plasticity-induced phenotypes to “lead the way” and accelerate subsequent genetic adaptation. Building from this work, we propose a “phenotype-first” model of the evolution of human life history in which novel phenotypes were first generated by behaviorally or environmentally driven plasticity and were later gradually stabilized into species-defining traits through genetic accommodation.

INTRODUCTION

Modern humans are characterized by a life history strategy with features that are distinct from other nonhuman primates, including slow childhood growth, early weaning followed by a long period of dependence, a shortened inter birth interval, and lengthy life spans (Hawkes et al. 1998; Hill 1993; Hill and Kaplan 1999; Robson, van Schaik, and Hawkes 2006)

1.1 The Unusual Human Life History

Organisms vary remarkably in the size and pace of life, which is reflected in body size, growth rate, fertility rate, and life span. These traits help define a species’ life history, which may be viewed as a life-cycle strategy that optimizes expenditures in service of reproductive success(Stearns1992).

1.2 The Timescales of Human Adaptation and the Role of Developmental Plasticity

While this strategy is characteristic of the human life history generally nutritional and mortality conditions can vary widely across environments and through time. Although genetic adaptation by natural selection helps explain the durability of species-level characteristics that differentiate us from other great apes, the transience of many of the ecological challenges that populations confront may not be dealt with effectively by changes in gene frequencies, which require many generations and hundreds if not thousands of years to accrue in the gene pool. As such, population variation in life history parameters is largely traceable to developmental plasticity.

1.3 Intergenerational Phenotypic Inertia

There is a growing list of biological systems that are not modified in response to the environment itself but to hormonal or nutrient signals or cues of past environments as experienced by ancestors, most typically the mother (Bateson 2001; Gluckman and Hanson 2004; Kuzawa 2001, 2005; Wells 2007; Worthman 1999). Brief “critical” or “sensitive” periods in early development often overlap with ages of direct nutrient, hormone, or behavioral dependence on the mother (e.g., via placenta, breast milk, or emotional attachment), which facilitate the transfer of integrated cues of past maternal or matrilineal experience (Kuzawa and Quinn 2009).

1.4 Variation in Modern Human Life Histories Traces Primarily to Environment Driven Plasticity

We have reviewed the key derived characteristics of the human life history strategy. We have also considered the need for a capacity to modify priorities “on the fly” as an important dimension of life history strategy for most organisms in light of changing nutritional and demographic/mortality conditions. Consistent with this expectation, most human life history traits exhibit extensive sensitivity to ecological context.

1.5 Somatic Growth and Adult Body Size

Growth during the postnatal period can be divided into several periods of distinct hormonal regulation that vary in sensitivity to environmental influence and that ultimately determine age-specific contributions to adult size and sexual dimorphism (Karlberg 1989).

1.6 Nutrition as an Influence on Age at Reproductive Maturity

Age at maturity represents an important life history transition because it marks the age at which the body shunts energy previously allocated to somatic growth into reproduction (Charnov 1993; Kuzawa 2007).

1.7 Nutrition, Developmental Plasticity, and the Origins of Sexual Dimorphism

In species marked by sexual size dimorphism, the greater size of males leads to correspondingly higher nutritional requirements

1.8 Psychosocial Stress, Maturational Tempo, and Reproductive Scheduling

Although the public health and growth and development literatures have traditionally focused on the role of nutrition and hygiene as influences on growth and adult size, more recent work is showing that psychosocial stress can also influence growth rate and maturational tempo.

1.9 Summary: Phenotypic Responses to Changing Ecological Conditions

In figure 3, we summarize the pattern of life history traits that theory predicts in environments with different combinations of nutritional sufficiency and unavoidable mortality risk.

DISCUSSION OF FINDINGS

Discussion: Summary and Implications of Life History Plasticity

Organisms must manage the costs of building a body and the bodies of offspring while calibrating the scheduling of developmental and reproductive events in response to nutritional resources, the risk of mortality, and other changing or unpredictable ecological conditions (Charnov 1993; Promislow and Harvey 1990; Stearns 1992).

Did Plasticity Lead the Way in Human Evolution?

While biological anthropologists have long considered the role of phenotypic and developmental plasticity as a means of adaptation and a source of within-species variation, recent theoretical and empirical work in evolutionary biology has emphasized the role of plasticity as an influence on the pace of evolutionary change and speciation (Kuzawa 2012; WestEberhard 2003).

CONCLUSION

The extensive developmental plasticity in human life history traits reviewed here gains new importance in light of evidence that plasticity can influence the direction and pace of evolutionary change. Many of the traits that differentiate the human life history from that of other primates and great apes—including slow growth rate, early weaning, delayed maturity, high fertility, and perhaps even long life span—demonstrate phenotypic variation that traces to developmentally or behaviorally mediated plasticity in response to environmental factors such as nutrition and cues of unavoidable mortality. We have sketched some of the observations that lead us to hypothesize that this environmentally induced phenotypic variation likely preceded and ultimately facilitated genetic adaptations that gradually stabilized the life history characteristics that help define our species. We hope that this review helps stimulate interest in the broader insights that developmental plasticity may provide into the diversification and evolution of genus Homo, including the lineage that led eventually to modern Homo sapiens.

LICENSING

Source: Current Anthropology , Vol. 53, No. S6, Human Biology and the Origins of Homo (December 2012), pp. S369-S382 Published by: The University of Chicago Press on behalf of Wenner-Gren Foundation for Anthropological Research Stable URL: http://www.jstor.org/stable/10.1086/667410 Accessed: 17-09-2016 03:03 UTC