Cataracts, arthritis, wrinkled skin-- most of us assume these symptoms of aging are annoyances that serve no greater purpose. But a new study suggests they may be part of the body’s natural defense against cancer. How? It all has to do with senescent cells.

Once upon a time, scientists believed that human cells-- if supplied all the nutrients they need-- would continue multiplying forever. But in 1961, biologists Leonard Hayflick and Paul Moorehead discovered this is not the case. They tested fetal human cells in a lab, giving the cells everything they needed. Despite this, the cells stopped dividing after about 50 times. Cells that stopped growing like this became known as “senescent cells.”

Senescent cells are the culprits for many of the symptoms of old age. Cataracts, arthritis, and wrinkled skin can all be traced back to them. In fact, when scientists rid mice of senescent cells, they found that the mice became rejuvenated. So problem solved, right? We just get rid of senescent cells and we’ll all stay young forever.

But it turns out things aren’t that simple. Senescent cells were then discovered in embryos, which got scientists to thinking senescence must serve some purpose in development. What they learned is, when cells multiply their DNA slowly gets damaged. And when the DNA becomes too damaged, that’s when those cells turn senescent. It they didn’t, the cells would run the risk of multiplying uncontrollably and turning into a cancer.

Basically, if we didn’t have all the negative side effects of aging we’d have the even worse side effects of cancer. It’s not a perfect system, but it’s the best one we have. And now that scientists know more about it, hopefully they can work toward better treatments for aging.