Chapter 21 Summary

Humans evolved from arboreal, social primate ancestors with binocular vision, grasping hands, and cognitive abilities associated with social life—features that were important foundations for later human evolution.
Important evolutionary changes in the evolution of the human body include adaptations for bipedality, opposable thumbs, alteration of the vocal tract, a higher reproductive rate, a higher metabolic rate, and a longer childhood. The most important change is our very large brain, with its unparalleled cognitive abilities.
Hominins are the lineage that includes humans and that diverged from chimpanzees about 7 Mya. Fossil hominins show that humans originated in Africa. Fossils of species in the genus Homo date from about 3 Mya. H. erectus was the first hominin to leave Africa, and spread through Europe and Asia. About 600 Kya, a second wave left Africa and gave rise to Neanderthals and Denisovans. Finally, Homo sapiens spread out of Africa 60 Kya, hybridized and acquired genes from Neanderthals and Denisovans, and spread across the entire Earth by 12 Kya.
Human populations do not show much divergence across the genome: among Africans, East Asians, and Europeans, only 12 percent of the total genetic variation is caused by differences in allele frequencies among populations. Some of those differences, however, are responsible for variation in skin color, metabolism, and other traits that adapt humans to different environments. Many traits show continuous ranges of variation, and the concept of discrete races does not apply to the human species.
Two distinctly human traits are our enormous brain and our use of language. A larger brain was likely selected for by ecological factors and by social interactions in groups. Our high metabolic rate supports both the brain’s huge energy consumption and our high rate of reproduction. Speech is enabled by both the large brain and a modified vocal tract.
Other ape species, especially chimpanzee and bonobo, make and use tools in the wild, and can learn and use elements of language in captivity. This suggests that the common ancestor of humans and African apes had rudimentary capacities for language, tool making, and reasoning.
Culture enabled humans to occupy more different environments, over a broader geographic area, and to use a greater variety of food and other resources than any other species. Agriculture began about 11 Kya. It had profound impacts on our diet, social organization, and population growth; the prevalence of diseases; and the fates of countless other species. The changes caused by agriculture altered the course of human evolution, as shown by several genetic adaptations to diet and changed conditions that came with agricultural societies.
Natural selection and evolution are ongoing in human populations, even in industrialized societies. Height and cholesterol level are two of the many traits that affect fitness and that are heritable. Many traits are mismatched to our agricultural diet, which has been widespread for only several hundred generations, and to other aspects of modern life.
Culture is a pronounced human feature that has enabled our species to inhabit and dominate almost all of Earth. Cultural traits change in ways that have some similarities to genetic evolution, but there are also important differences between cultural and genetic evolution. The most important is horizontal transmission: by imitation and learning, a cultural trait can spread across a population (and today, even the entire globe) within a single generation.