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.

Life history theory has revolutionized the study of the evolved life course both across and within species. Humans are no exception. Life history theory provides nuance to claims of human uniqueness in the tree of life, including among mammals, and among primates in particular. This chapter first explains how life history theory delivers optimal trait characteristics given trade-offs that are defined by costs and benefits of different “solutions” to fitness-relevant problems. It reviews the current understanding of the evolution of the human life course, and the chapter evaluates explanations of long postreproductive life spans in human populations. It then provides a framework for modeling how selection pressures in different socioecological settings could shape the flexible expression of demographic, physiological, and even psychological traits in our species. Special emphasis is placed on the role that exogenous mortality and environmental unpredictability (and their cues) play on shaping a number of behavioral and physiological traits related to survival. Claims certainly outpace definitive tests, but after a half century, since life history theory was first introduced to the human life sciences, promising new directions are expanding the breadth of topics covered beyond demography, using multiple methods and spanning diverse disciplines.

Human mothers commonly receive help from others to support their children, an unusual reproductive system known as cooperative breeding. Because cooperative breeding is not a trait shared with other great apes, its emergence in the human lineage marks a significant departure in reproduction and parenting, which has far-reaching consequences for the life history, sociality, and demographic success of our species. This chapter first defines cooperative breeding and establishes those characteristics that distinguish humans from other cooperative breeders. To unravel the evolutionary puzzle of cooperative breeding, the benefits of helping to mothers and offspring, why helpers help, who helps, and what helpers do are then reviewed. The discussion then turns to the question of why humans become cooperative breeders. Because humans provide food, shelter, and protection not just to infants but also to juveniles, humans expand the range of helping behaviors beyond those observed in other cooperative breeders, which has implications for many other aspects of sociality. Cooperative breeding has much to offer as a framework for conceptualizing the cooperative nature of humans and to explain our derived life history of early weaning, high fertility, and the long developmental period during which juveniles benefit from both receiving and giving help.

Improved clarity of Neanderthal ways of life brought about by advancements in analysing the fossil and archaeological records, accompanied by increased willingness to accept complex Neanderthal cognition, makes it appropriate to begin to understand their sexual behavior. In this chapter, we briefly review current understandings about Neanderthals based on anatomy, genetics, and behavior evidenced from the archaeological record. We then integrate this with broad behavioral ecology and evolutionary sexual selection concepts to consider potential selection pressures on Neanderthals’ sexual and reproductive behaviors. Large adult brain size, rapid infant brain growth, and protracted offspring development, similar to Homo sapiens, were supported by adaptations in social organization, mating and parental effort. It is likely that male provisioning and investment in offspring strengthened reproductive pair bonds, improved infant survival, and impacted mate choice in both sexes. Systematic collaborative subsistence strategies were probably matched by a heavy reliance on kin and other trusted adults within the cooperative breeding group, reducing the energy burden on reproducing females, and enabling shorter lactation and reduced interbirth intervals. Neanderthals’ wide ecological tolerances and behavioral flexibility suggest that they also adjusted their sexual and reproductive behavior according to environmental circumstances. Small group size, local-to-regional social networks and potentially seasonal breeding enabled populations to adapt to fluctuating energy availability. During harsher climatic phases, limited access to mating opportunities may have favored social monogamy, with genetic isolation and inbreeding more likely. When conditions were milder (during interglacials, in warmer regions or seasons) with more plentiful resources, group sizes and social networks may have permitted polygyny. Finally we explore the behavioral implications of genetic evidence that Neanderthals interbred with other hominins including H. sapiens. This suggests that differences in physical appearance and social structures did not prevent copulation or raising hybrid infants, although sterility and lower fitness of the latter may have limited the spread of genes between species.

Cooperative interactions are widespread in the animal kingdom. Their occurrence can be explained by mutually non-exclusive benefits increasing an individual's (1) indirect fitness by cooperating with kin, and (2) direct fitness by mutually or reciprocally cooperating with others. Many cooperative behaviors require well-developed neuroendocrine mechanisms regulating their quantity and quality. Fishes offer great opportunities to increase our insight into ultimate and proximate questions of cooperation. Their social systems range from solitary- and pair-living to lose fission–fusion groups and highly complex societies. Cooperative interactions are an essential part of the behavioural repertoire of most fish species, occurring in a variety of social situations like predator inspection, foraging, mating, or brood care. Such interactions take place among related and unrelated individuals and even between members of different species. This fascinating diversity allows investigating all crucial factors mediating cooperation, e.g., by studying behavioural interactions within and between species, by applying comparative approaches between taxonomic groups and by using state-of-the-art genetic and neuroendocrine technologies to resolve the underlying mechanisms. This chapter provides an overview of the mechanisms and functions of cooperative behaviour in fishes, with the overall aim to illuminate the evolution of cooperative behaviour in general.