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The fundamental assumption of the current work is that modern humans (Homo sapiens sapiens) are the product of gene-culture co-evolution spanning at least the past 5 to 7 million years. Current knowledge about how the evolutionary process works has been advanced by three major milestones. The first was publication of Darwin’s The Origin of Species (1859) and The Descent of Man (1871). His revolutionary concepts moved questions about human origins from the realm of philosophy and myth to the discipline of science and established natural selection as the cornerstone of evolutionary biology.

The second milestone was development of the modern science of genetics. Genes were identified early in the twentieth century as the units of heritable traits, and seminal works, such as Dobzhansky’s Genetics and the Origin of Species (1937), laid the foundation for understanding gene flow through time and space. Discovery of DNA structure in 1953 revealed the molecular mechanics of how traits are transmitted. Later advancements in DNA sequencing in the 1970s and the 2003 reconstruction of the human genome are now allowing us to probe relationships among the ancient lineages of our family tree more deeply, often with surprising results.

The third source of enlightenment on evolutionary processes was the emergence of the field of sociobiology, officially launched in 1975 by E.O. Wilson’s Sociobiology: The New Synthesis, and its sequel, On Human Nature, in 1978. These works helped to establish an interdisciplinary approach to understanding the evolution of heritable physical and social traits in all animal species, including humans. The sociobiological movement overcame initial criticisms of biological reductionism and genetic determinism and went on to spur a wealth of new research that continues to flourish decades later.

The field of sociobiology hosted lively internal debates as well, not the least of which concerned the locus of natural selective processes. One school of thought places primary emphasis on the theory of kin selection. This concept, which originated in the earlier works of biologists Hamilton (1963, 1964), Trivers (1971, 1972), and Alexander (1974), proposes that individual organisms maximize their own reproductive success or “inclusive fitness” by behaving altruistically toward close kin, weighted by the degree of genetic relatedness. The theory, also known as “Hamilton’s Rule,” was supported by a mathematical formula that calculated that altruism will develop to the extent that the benefit to the recipient times the degree of kinship to the altruist is greater than its cost.

Kin selection as the principal driver of human social evolution gained widespread acceptance among biologists, including E.O. Wilson, for about four decades. Commencing in 2010, however, a series of coauthored papers by Wilson and others challenged the mathematical and biological validity of kin selection theory as an explanation for the evolution of advanced social behavior.³ In its place has emerged the concept of multilevel selection, in which the evolutionary dynamic is seen as operating simultaneously at both the individual and the group levels. As proposed by Wilson (2012: 162), individual selection is based on competition and cooperation within groups, and promotes selfish behavior by its members, whereas group selection is based on competition and cooperation between groups, which promotes internal altruism. Wilson views human evolution as a product of these conflicting selective processes in which the interests of the individual must be balanced against the interests of the larger collective.

Multilevel evolutionary theories assume that groups that develop internal structures for cooperative endeavors have adaptive advantages that accrue to their membership. Robin Dunbar (2008), for example, proposes that individuals enter into social contracts to enhance their prospects for survival and reproductive success. He goes on to caution, however, that multilevel selection should not be confused with “group selection:”

In kin selection, the final arbiter of what happens is the gene, not the group as an entity, and hence it requires no new mechanism of evolution other than standard Darwinian processes. . . . In multilevel selection again, the unit of evolutionary cost-accounting is the gene, and not the group. Group-level processes are intended to facilitate the successful replication of the individual member’s genes, not to facilitate the successful replication of the group. The distinction is subtle, but important. (Dunbar 2008: 147)

Richard Dawkins, in The Selfish Gene (1976) and The Extended Phenotype (1982), cast the gene as the sole protagonist in the evolutionary drama, discounting the role of both individual organisms and groups in the natural selection process. He proposed that genes and their respective alleles act in their own self-interest, programming the organisms in which they reside to behave in a manner that optimizes their frequencies in the gene pool. Genes effectively hitch a ride on human “survival machines,” moving their hosts in directions that foster their own replication. In this view, adaptations represent the phenotypic effects of genes to reproduce themselves in future generations. All of this is seen as occurring beyond the conscious recognition of individuals, who are essentially temporary vehicles for gene replication. Dawkins also accounts for the role played by culture in human evolution with the parallel concept of “memes,” which are proposed as the units of cultural inheritance. Memes are crafted on the same genetic metaphor, competing with others in a meme pool. Like genes, memes have phenotypic effects, and are thought to be naturally selected by virtue of their successful replication.⁴

The significance of gene-centered theory for models on human origins is twofold. First, it proposes that sociality is (unconsciously) pursued by individuals largely on the basis of self-interest. Degrees of genetic relatedness become the floating calculus for cooperation and competition among individuals, who assemble and participate with others in a tit-for-tat world. Society thus defined becomes a collection of vying gene carriers—a procession of self-serving males and females, kin and non-kin, marching to the zero-sum drum of genomic replication. Second, some applications of gene-centered theory assume that characteristic reproductive strategies and associated phenotypic behaviors, such as dominance, aggression, or parasitism, have become imprinted into our DNA as a kind of species-specific biogram. In other words, ancient and modern humans, in their quest for self-replication, have been pre-programmed to favor certain behaviors and types of social organization to the exclusion of others.

While recognizing that the inclusive fitness of individuals rests on the replication of their genes, the present book will argue that the reproductive success of ancestral humans was not only enhanced by, but reliant on their ability to forge cooperative relationships and function effectively within social groups—communities that typically extended beyond the circle of immediate kin to include the broader membership of a breeding population. Humans are not solitary breeders, but group-bonded primates. Ancient human social groups were more than just a collection of individuals with whom to play out one’s genetic hand. The alliances and cooperative relationships on which they were based provided an internal division of labor for the acquisition and distribution of fitness-related resources that enhanced the reproductive success of all group members—a characteristic referred to by Wilson (2012: 133) as eusociality.

The process of evolution has been understood as involving the interaction of natural selection and genes that are either inherited through DNA or arise via random mutations. However, the recent discovery that an organism’s phenotype may be modified by a myriad of nongenetic factors, and that such phenotypic variants are themselves heritable, is transforming the field of evolutionary biology. The process by which this occurs, epigenesis, modifies the expression of genes without changing the underlying molecular structure of DNA. A new branch of theory, referred to as the extended evolutionary synthesis (EES), proposes that heredity is a developmental process influenced not only by genes, but by an organism’s cumulative interaction with its chemical, natural, and social environments. Epigenesis provides a source of nonrandom phenotypic variation once thought reserved for random mutations. Animal experiments have also demonstrated that epigenetic inheritance allows for the storage and transmission of learned information and provides the flexibility for organisms to modify their phenotype in response to rapid environmental change. EES proponents maintain that an organism’s niche construction (its selection and modification of its habitat and environmental resources) also affects the direction of evolution by modifying natural selective factors. In other words, the evolutionary process is more complex than simple genomic theories propose.

This perspective on the critical role played by epigenetic traits will find expression in the chapters that follow. What separated early humans from other apes was their gradual emancipation from purely hardwired responses to reproductive and subsistence challenges through a combination of epigenesis, behavioral plasticity, and cortical expansion. Instead of slavishly following an innate prescription or biogram for sociality, epigenesis provided a “soft inheritance system” that allowed humans to alter their behaviors and the structure of their social groups in response to stochastic environmental conditions. Wilson’s concept of “epigenetic rules” (2012: 193) parallels what is referred to here as social DNA. Social DNA consists of the underlying rules for characteristic human behaviors and social forms that have been selected and replicated over time by virtue of their role in enhancing reproductive success. While they provide a general framework for the human experience, the phenotypic expression of these rules is not unitary or preordained, but is sufficiently plastic to respond to external change.

One of the challenges in unraveling current conceptual models of human social origins is their tendency to meld ideas on evolutionary prime movers, reproductive strategies, sexual dominance, altruism, and social forms into a hardwired genomic platform. An effort is made in the succeeding chapters to deliberately separate these issues for closer examination and discussion.