In previous chapters, we explored the genetic bases for biological variation within and between human populations. We discussed how, as a species, humans are remarkably genetically uniform compared with our closest primate relatives. We’ve also placed these discussions within an evolutionary framework, emphasizing the roles of natural selection and genetic drift in human evolution. With this foundation, we can turn our attention to some of the many challenges we have faced through our evolutionary journey and consider some of the ways we’ve met these challenges as a species, as populations, and as individuals. Early humans migrated out of Africa some 200,000 to 100,000 ya, and we now permanently inhabit the entire planet except for the oceans, the highest mountain peaks, and Antarctica. But as human populations spread over the earth, they had to cope with variations in ultraviolet (UV) radiation, altitude, temperature, humidity, diet, and infectious diseases. All of these factors, plus the fact that populations were separated from one another by enormous distances, have combined to produce many forms of adaptation in our species