This chapter reviews what can be gleaned about human sexuality from the evolutionary and ethnographic record. Ancestral human sexuality leaves neither fossil nor archaeological evidence, but inferences about how humans mated, consorted, parented, formed partnerships, and aggregated into families can be drawn from two large and growing bodies of work, both discussed in this chapter. The first are anatomical and biological indicators of ancestral mating patterns inferred from fossil evidence as well as observations from nonhuman primates. The second is ethnographic research across an array of contemporary human societies, which highlights variation in mating, marriage, and family structure. Together, biological indicators and cross-cultural patterns shed light on the legacy, constraints, and possibilities carried forward into the diverse and variable expression of human sexuality today. Humans have a deep ancestry in a social structure of males and females living in social groups together, although how humans organize themselves is structurally different from anything observed in our closest relatives. Not only do families form around long-term pairbonds in all societies, but there is also a great deal of flexibility in who constitutes the pairbond, the families that surround them, and in the prevalence of extra-pair relationships.

Human offspring require intensive and extended parental care. While the needs of offspring have been argued to drive paternal investment, the evolution of male care and its patterning across cultures defy any simple story. Because human paternal investment, while often substantial in relation to other animals, is facultative rather than obligatory, there is considerable cross-cultural variability in how and how much fathers invest in their children. While cooperative and flexible parenting strategies are present across human societies, male investment is heavily influenced by life history tradeoffs. Specifically, when fitness payoffs towards time and energy allocated to mating effort outweigh parenting effort, men often invest less in their children. Moreover, although men appear to be physiological responsive to childcare responsibilities (through testosterone adjustments), paternal loss often has little effect on child survival. Rather than signifying the unimportance of fathers, it highlights the remarkable flexibility in the human family in terms of how mothers are assisted in raising multiple dependent offspring at the same time. Although some form of pair-bonding is observed cross-culturally, with mothers as the primary infant caregiver, extended kin and alloparental support are also important for offspring success. I review paternal investment from across the animal kingdom, discuss the evolution of paternal care in humans, and describe variability across individuals and groups in the ways and amounts men invest.