W hen Gertrude Stein famously quipped that “we are always the same age inside,” she certainly wasn’t referring to the conglomerate of cells, carefully organized into tissues, that form a human body. We all understand that despite our best efforts to preserve youth, our material bodies inevitably age and fail us. Yet trying to understand whether all our body’s cells age at the same rate is not a trivial question in biology. Some of our tissues are built to last and degenerate slowly, like the soft spongy tissues that form the brain, while others, like a red blood cell, have a much shorter lifespan. It’s long been a central tenet of biology that aging results from the random accumulation of damage to specific cells over time.
But in the past decade, Steve Horvath, while a professor in human genetics and biostatistics at the University of California, Los Angeles, honed the thesis that aging in every tissue can be predicted by a single mathematical formula. In a 2022 paper, Horvath, together with an international group of scientists, identified this formula in 185 mammalian species.1 “I think it’s unbelievable that this is even possible,” Horvath told me. “But one formula can measure age in all species and all tissues.”
The formula implies that there is a universal clock to aging. As the years tick by, the risk of mortality increases. Tissues degenerate by accumulating wear and tear in the form of molecular damage, while the aging program depletes the body’s reserves of young stem cells to replace them. Some see this as evolution, a cruel mistress, having programmed us to die. But to Horvath, aging is unintentional. “I don’t want to say evolution selected a program that makes us die. I think it’s just that Mother Nature never selected against it,” he says.