Why do all animals, including human beings, age? One popular theory for how species-typical rates of aging emerge is that individuals are selected by nature so that they can keep their health long enough to reproduce and get the next generation up to reproductive snuff. If a body invests a lot of energy in repairing itself, it will reduce the amount of energy it can devote to reproduction. Thus, natural selection favors reproduction over individual longevity. "Understanding the nature and extent of biological constraints on the rate of ageing and other aspects of age-specific mortality patterns is critical for identifying possible targets of intervention to extend human lifespans," the researchers note. Colchero optimistically adds: "Not all is lost. Medical science has advanced at an unprecedented pace, so maybe science might succeed in achieving what evolution could not: to reduce the rate of ageing."