The clash between our genes and our contemporary lifestyle is making us sick. That's the premise of "It Takes a Genome," which examines how the slow pace of genetic evolution and modern environmental stresses have left us susceptible to diseases like cancer, asthma and depression. Turns out, we 21st-century humans are exposing our cells to unprecedented historical extremes—longer life spans, sugary diets, cigarette carcinogens, earlier menses for women. Our genes are "outside their comfort zone" and haven't had time to adapt to higher levels of insulin (which can lead to diabetes), strange new viruses (HIV) and more exposure to estrogen (a prime culprit behind breast cancer). The reasons for our vulnerability are complex. Most diseases are influenced not by one gene, but by dozens if not hundreds of them. Each genetic variation may increase an individual's risk by a tiny percentage, but it's the cumulative effect that tips the scale into illness. These riskier gene varieties often escape evolution's purges because they may have little impact on youthful fertility (like certain genes in Alzheimer's) or because they actually confer benefits early on and don't become a problem until after the reproductive age. Theoretically, if our species is around in another hundred thousand years, we'll have adjusted to diabetes and autism. In the meantime, we'll struggle to figure out how to best buffer our risky adolescent genome.