Social DNA. M. Kay Martin
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DNA—that provided not one, but a menu of standard options for the organization of reproductive and socioeconomic life. What emerges from this discussion is a model of Paleolithic society that challenges prevailing theory on issues such as ancient diet, group size, encephalization, labor division, mating behaviors, and variable systems of kin affiliation. This discussion is undertaken from the perspective of multilevel selection, which addresses how the flexible organization of group life has structured the distribution of fitness-related resources through time.
Chapter 1 begins this journey at the beginning, namely, with conception and an exploration of how differing perspectives on human reproductive biology have influenced past and current theory about male and female natures and their strategies for inclusive fitness. A salient issue is whether the sexes are viewed as pursuing their fitness through cooperative partnerships, or at one another’s expense.
Chapter 2 considers how our mammalian origins and multilevel selective pressures may have shaped the evolution of proto-human family groups. Prevailing androcentric models of primeval families are critically evaluated. These theories are then contrasted with alternative perspectives based on the matricentric family, cooperative breeding, and the optimization of male and female fitness within the framework of multimale-multifemale groups.
Chapter 3 addresses the emergence of our genus in the late Pliocene, and considers recent data on the paleoecological conditions thought to have played a role in the evolution and geographic radiation of Homo erectus. Contrasting theories on the relative importance of terrestrial and aquatic fauna are examined, and an alternative evolutionary scenario offered that links dietary, morphological, and life history changes in early humans with fundamental shifts in female subsistence and reproductive strategies.
Chapter 4 explores more fully the potential range of dietary protein available to hominins throughout the Pleistocene, and the extent to which assumed reliance on the hunting of mammalian herbivores has influenced the reconstruction of Paleolithic economic and social life. In short, diverse biomes translate into diverse adaptations. Dietary breadth is regarded as a key issue for paleontological questions involving all hominin lineages, including the geographic expansions of Homo erectus and anatomically modern humans, and the ultimate fate of Neanderthals.
Chapter 5 pauses to consider signature traits that have characterized the hominin experience through time—the essential qualities that define and bind together both ancient and modern humans. Featured topics include opportunistic omnivory, spatiotemporal awareness, mating patterns, behavioral plasticity, intelligence, social demography, and changes in energetics. This chapter serves as a reminder of the qualities that made hominins unique among primates, and that separate us from other ancient and contemporary nonhuman apes.
Chapter 6 returns to the role played by kinship in structuring human adaptations through time. This chapter explores how kinship has been historically portrayed in anthropological theory and how notions about ancient subsistence patterns, innate dominance, inclusive fitness, and biobehavioral traits shared with contemporary apes have biased our perception of Paleolithic social life. Prevailing theories are reviewed and critiqued, and new genomic evidence is introduced that sheds light on the potential nature of early hominin social groups.
Finally, Chapter 7 addresses the long-standing debate among anthropologists and sociobiologists on the antecedents of matrilineal, patrilineal, and bilateral kinship systems. Kinship is examined as a technology for human niche construction that has allowed humans to manage the two basic elements of fitness—food and sex—by structuring their mating relationships and their social groups in a manner that optimized the recovery of energy and other fitness-related resources in a given ecological setting. The author’s perspective on factors that select for uterine and agnatic organization is explored by noting their distinct architectures for structuring reproductive, labor, and political groups in relation to available resources. The chapter explores both the origins and resiliency of matrilineal and patrilineal systems and how these variable strategies for niche construction have responded to change in the post-Neolithic era.
This book will be guided by the initial assumptions presented here. Its story on the origins and nature of human sociality blends mainstream theory and empirical data with some nuanced plot twists. To the extent that its conclusions challenge popular notions about our evolutionary past, the reader is reminded that this endeavor naturally summons a number of theanthropic questions, the answers to which no one really knows for sure.
PERSPECTIVES ON ANISOGAMY
The casual reader may wonder why a book devoted to the topic of human social origins would begin with a discussion of human reproductive physiology. The simple answer is that all past and present theories about human nature and social evolution make certain assumptions about the significance of male and female reproductive behaviors and how they have helped to shape the biocultural trajectory of our species. In considering these theories, therefore, it is essential to gain an understanding of where their proponents stand on the fundamental issue of male and female reproductive strategies and their effect on human social life over time. This chapter will explore the basics of human anisogamy and discuss how its significance is perceived in both historic and contemporary schools of thought.
Size Matters
Anisogamy, simply defined, is sexual reproduction involving the fusion of gametes of different size or form. In humans, a large, slow-moving egg cell (ovum) is fertilized by a tiny, highly mobile sperm cell (spermatozoon). Egg cells, which are over eighty-five thousand times larger than sperm cells, contain DNA, mitochondria, nutrients, and the resources essential to support new life. Females are born with about 2 million egg cells, only four to five hundred of which will ripen (one each month) over a fertile lifespan, with the remainder degenerating over time. Sperm cells also contain DNA and mitochondria but no nutrients, and are instead optimized for mobility. Gamete production in males is continuous, with about 100 million sperm cells generated daily, or an estimated 2 trillion over a lifetime.
Anisogamy, by its very nature, creates disparate but complementary reproductive roles and strategies. Females produce a small number of nutrient-rich eggs that emit pheromones to attract male sperm for fertilization. Males, in contrast, produce huge numbers of sperm cells that must vie with one another in their typically ill-fated race to unite with an available egg. Since the demand for eggs is greater than the supply, ova enjoy much better odds of passing along their DNA to the next generation than do individual spermatozoa. The greater reproductive investments required of females by gestation and lactation also have the effect of delaying ovulation, thereby further restricting the supply of eggs potentially available to sperm for fertilization. These factors combine to make females the primary limiting resource in male reproductive success.
The traditional textbook version of how the fusion of ova and spermatozoa occurs typically begins with the egg being swept up into the fallopian tube and drifting toward the uterus. Sperm cells released in the vagina move through the female genital tract and race toward the large and essentially dormant egg. At the climax of this intense and somewhat perilous journey, the first among the thousands of sperm to arrive, successfully attack, and forcefully breach the egg’s zona or protective barrier wins, and fertilization occurs. This scenario thus paints female gametes as the passive recipients of male competition and penetration, and male gametes as active contestants and penetrators of the coveted target.
Perception of Conception
In 1991, Emily Martin (no