What the ‘Active Grandparent Hypothesis’ Can Tell Us About Aging Well

Early humans had to move around often to hunt for food, the thinking goes, and those who moved the most and found the most food were likeliest to survive. Over eons, this process led to the selection of genes that were optimized by plentiful physical activity. Physical activity likewise appears to jump-start various cell processes controlled by genes that help to promote health. In this way, evolution favored the most active tribespeople, who tended to live the longest and could then step in to help with the grandchildren, furthering active families’ survival.

In other words, exercise is good for us, they point out in their new paper, because long ago, the youngest and most vulnerable humans needed grandparents, and those grandparents needed to be vigorous and mobile to help keep the grandkids nourished.

Crucially, the new Active Grandparents paper also delves into what it is about physical activity that makes it still so necessary for healthy aging today. For one thing, moving around uses up energy that might otherwise be stored as fat, which, in excess, can contribute to diseases of modern living, such as Type 2 diabetes, Dr. Lieberman and his co-authors write.

Activity also sets off a cascade of effects that strengthen us. “Exercise is a kind of stress,” Dr. Lieberman told me. It slightly tears muscles and strains blood vessels and organs. In response, a large body of exercise science shows, our bodies initiate a variety of cellular mechanisms that fix the tears and strains and, in most cases, overbuild the affected parts. “It’s as if you spill coffee on the floor, clean it up, and your floor winds up cleaner than it was,” Dr. Lieberman said. This interior overreaction probably is especially important when we are older, he continued. Without exercise and the accompanying repairs, then, aging human bodies work less well. We wear down. We cannot care for the grandkids.

Fundamentally, Dr. Lieberman said, lack of exercise during aging explains why there is a difference between the human life span — how many years we live — and health span — how many of those years we remain in generally good health.

“They used to be the same thing,” Dr. Lieberman said. An inactive early human would not stay healthy and probably die early. Today, many of us can remain inactive and survive into old age, but the chances are we will not remain fully healthy if we do. Our genetic inheritance and history as humans require exercise and movement, Dr. Lieberman said. “Retirement is not the time to slow down.”

This idea that we can, should and even must stay active as we age, thanks to human evolution, is at the heart of the Active Grandparents Hypothesis. Beguiling as the hypothesis is, however, it is just a theory and almost impossible to test.