In 1961, two biologists named Leonard Hayflick and Paul Moorehead discovered that old age is built into our cells. At the time, many scientists believed that if healthy human cells were put in a flask with a steady supply of nutrients, they would multiply forever. But when Dr. Hayflick and Dr. Moorehead reared fetal human cells, that’s not what they found. Time and again, their cells would divide about 50 times and then simply stop.

Cells that stop growing this way came to be known as senescent. For years after Dr. Hayflick and Dr. Moorehead’s discovery, many scientists suspected that cells became senescent only in the unnatural confines of a lab. But researchers then discovered that cells stop growing in the human body.

In fact, it turned out, senescent cells are involved in many of the ravages of old age. Wrinkled skin, cataracts and arthritic joints are rife with senescent cells. When researchers rid mice of senescent cells, the animals become rejuvenated.

Given all this research, the last place you would expect to find senescent cells would be at the very start of life. But now three teams of scientists are reporting doing just that. For the first time, they have found senescent cells in embryos, and they have offered evidence that senescence is crucial to proper development.