In 1962 a human geneticist named James Neel proposed a hypothesis to solve a vexing evolutionary puzzle. What is now called type 2 diabetes—which he thought was caused by a single variant of some unidentified gene—can cause debilitating symptoms, including blindness, heart disease and kidney failure. It can also affect people in their reproductive years. In ancient humans, when no treatments were available, those features could have kept afflicted individuals from finding a mate, having children and passing the disease-causing gene down to future generations. In other words, natural selection should have eliminated the gene and, thus, the disease.