I’ve been reading and thinking a lot lately about the process of aging. Many scientists who study it argue — quite convincingly — that it’s the most important scientific topic of our time. In his 1997 bestseller Time of Our Lives, biological gerontologist Tom Kirkwood writes that the science of human aging is “one of the last great mysteries of the living world.”

Over the past century, Kirkwood notes, developed countries have used preventative and offensive tactics to slash infant mortality, smoking, and accident rates, and to conquer most infections. In the 1880s, the top causes of death were respiratory diseases (like tuberculosis and influenza) and digestive diseases (like cholera and typhoid), and life expectancy was around 46 years.