Human Evolution: Our Brains and Behavior is an ambitious, successful attempt at retelling the long and sometimes controversial story of how and why humans evolved into the highly social and cultural species we are today. Writing from his own distinct perspective, Robin Dunbar is able to extend more conventional paleoarchaeological narratives of human evolution, shedding light on the interrelated social and cognitive forces that led to the unique evolutionary trajectory of the hominin lineage.

Dunbar’s long and eclectic academic career gives him a valuable perspective from which to tell the story of human evolution. By integrating theories from paleoarchaeology, ecology, and the psychological sciences—as well as his own research on primate sociology and group living—Dunbar is able to provide a comprehensive account of the forces theorized to have driven natural selection in human ancestors.

The book begins with introductory summaries of the human evolutionary tree (p. 5) and ideas of human “uniqueness” (p. 23), which should serve to acquaint lay readers with the topic. It then outlines the trade-offs involved in nonhuman primate social living and the need for endorphin release through social grooming to bring about the cooperative, bonded relationships that are required to buffer the stresses of living in close proximity to fellow group members (pp. 37–56; Chapter 2).

Following his introduction to primate group living, Dunbar presents evidence for his “social brain hypothesis” (p. 59). The social brain hypothesis predicts a between-species relationship between brain size (specifically, neocortex size relative to the rest of the brain) and consequent social capacities: species with larger relative neocortex sizes are predicted to have greater social capacities, which allow for the maintenance of more complex social relationships and hence larger group sizes. The strong correlation observed between relative neocortex size and group size in extant primate species has allowed for the creation of a predictive equation that can be used to determine the likely group size of modern humans. This equation predicts an average human group size of 150, which has come to be known as “Dunbar’s number.”

Dunbar reviews cross-cultural evidence—ranging from studies of Twitter communities to ethnographies on hunter-gatherers—for the accuracy of this prediction, and he goes on to argue that relative neocortex sizes extrapolated from the paleoarchaeological record can be used to predict likely group sizes of extinct hominin species. Crucial to much of the discussion in the book, these predicted group sizes allow for estimates (based on extant nonhuman primates) of the amount of time these species would have needed to spend on social bonding activities in order to maintain group cohesion.

Dunbar goes on to demonstrate that combining predicted group sizes (and the requisite time needed for social bonding) with paleoclimatic data on vegetation composition and paleoarchaeological data on body size and proposed energy requirements allows for the creation of time budget models of hominin daily life (p. 84). Throughout the book, these time budget models—constructed from methods used in behavioral ecology— are used to shed light on the likely time and energy constraints faced by human ancestors and the solutions that evolved to address them.

Following this discussion of key theoretical principles, Human Evolution follows a mostly chronological layout, beginning the story of human evolution with the Australopithecines and ending it with the complex, sociocultural worlds that typify post-Neolithic societies and modern humans more generally. The book breaks human evolution into five “major evolutionary phases” (each making up one chapter): the Australopithecines (p. 95; Chapter 4), early Homo (p. 137; Chapter 5), archaic humans (p. 179; Chapter 6), modern humans (p. 215; Chapter 7), and the post-Neolithic (p. 303; Chapter 9). Dunbar also includes a chapter that showcases his own work in a critique of research to date on the evolution of human kinship, culture, and language (p. 259; Chapter 8). The generally chronological layout of Human Evolution makes the book best read as an entire piece, although individual chapters can offer thorough coverage of specific topics.

Chapters include extensive reviews of relevant theories and findings from biological anthropology, primatology, paleoarchaeology, paleoclimatology, behavioral ecology, and the psychological sciences to recreate the worlds and daily life of extinct hominin species. This research is considered within the framework of the time budget models described earlier. Dunbar uses these time budget models to calculate the amount of time particular hominin species would have needed to devote to “core activities”—such as travel, feeding, rest, and social bonding—given the environmental, physiological, and social constraints that they faced. He then reviews potential evolutionary responses to these constraints, asking how the energetic, nutritional, social, and mating requirements of hominin species could have been met. Proposed solutions are disparate and well explained, coming from research on, among other topics, mating systems, bipedalism, thermoregulation, meat consumption, fire use, social bonding, neuroanatomy, cognition, and religion. Throughout the text, Dunbar is able to narrate an intelligible timeline of human evolution while at the same time offering interesting asides on topics ranging from Neanderthal life to the neurosciences. In sum, Human Evolution is able to cover nearly all the topics found in a collegiate level introductory text on the topic, while offering a social and ecological perspective that is sometimes overlooked.

Dunbar continually emphasizes the importance of considering the social worlds of human ancestors as a supplement to more conventional “stones and bones” accounts from paleoarchaeologists, who have tended to focus on anatomy and tool use (p. 23). Social worlds do not preserve like stones and bones, and hard evidence relating to hominin sociality can be difficult to piece together. However, through using an impressively eclectic collection of research findings—many coming from his own work—Dunbar gives a coherent, empirically based account of the role of sociality in human evolution, focusing particularly on the importance of social bonds and relationships.

To take one of many potential examples, his discussion of fire use adds a social layer to a body of research that has traditionally focused on transitions in hominin subsistence patterns. In line with the book’s structure, fire use is first considered as a means of meeting increased energy and nutrient demands that came with the relatively larger bodies and brains of early Homo; cooking foods can lead to substantial increases in nutrient extraction rates (pp. 158–162). From here, Dunbar offers an informative account of the interplay between fire use and human sociality. He first considers when human ancestors would have evolved the cognitive “orders of intentionality” necessary to coordinate the maintenance of large campfires—an often overlooked but undoubtedly crucial aspect to consistent fire use (p. 165). Also discussed are the many implications of the “extended day” that campfires provide (pp. 229–232). In the newly illuminated night, campfires would have provided spaces for communal, endorphin-releasing song, dance, and feasting, which Dunbar argues were crucial to bonding the increasingly large group sizes predicted by the social brain hypothesis. By leading to a collective release of endorphins, these activities would have allowed hominins to bond with multiple group members at a given time, making group cohesion possible when social networks became too large to be bonded through the time-consuming dyadic grooming used by extant nonhuman primates.

Language is also considered in the context of fire use (pp. 232–234). Again, Dunbar offers a unique perspective, arguing that natural selection for language abilities would have been strong in dimly lit campfire scenes, where words would take precedence over more gestural forms of communication. In this context, language abilities would allow not only for the exchange of fitness relevant information about the physical world but also for increased social bonding through jokes and storytelling. Joke telling leads to group laughter that, like song, dance, and feasting, can result in social bonding through the collective release of endorphins—“grooming at a distance,” as Dunbar calls it; this is another focus of his research (p. 167). Storytelling opens the door to religion and the creation of the shared social histories that bond groups together. Dunbar’s discussion of fire use is therefore exemplary of the book as a whole: a succinct, thoughtful summary of current research from a variety of fields, plus insight from the unique perspective of his own work on social cognition and bonding.

The book is written in a style that is similar to that of other popular psychology publications (e.g., The Secret of Our Success,Reference Henrich¹How the Mind Works,Reference Pinker²Incognito Reference Eagleman³). This makes Human Evolution accessible for undergraduates and the general public alike. Students looking for an easy-to-read introductory text will find this book a valuable point from which to begin their research, although a lack of in-text citations could hamper its use as a stand-alone text for college courses. The book also lacks certain perspectives, such as those from the fields of cultural evolution and gene-culture co-evolution, that are relevant to discussions of language and religion, among other topics. Overall, Dunbar’s narrative relies heavily on his own theories and research, such as the social brain hypothesis, which have been critiqued for various reasons, although he does offer thorough rebuttals to most conflicting ideas, while still favoring his own. That said, Dunbar’s perspective is valuable, and one of the strengths of the book, giving it the social and cognitive perspectives that academic readers interested in evolutionary psychology (and Dunbar’s work in particular) will find especially valuable.

The book is recommended reading for a wide audience, including the general public as well as academics interested in the influence of sociality on evolutionary processes in humans. By drawing from disparate fields of work, Dunbar is able to tell the story of human evolution in a fresh, nonrepetitive fashion that covers most major theories—something that cannot be said for all popular science publications on the topic. As a whole, Human Evolution is a thorough introduction to the field and a testament to Dunbar’s theoretical and empirical contributions to our understanding of the factors leading to the human condition as we know it.