Why is physical activity so good for us as we age? According to a novel new theory about exercise, evolution and aging, the answer lies, in part, in our ancestral need for grandparents. The theory, called the "Active Grandparent Hypothesis" and detailed in a recent editorial in Proceedings of the National Academy of Sciences, suggests that in the early days of our species, hunter-gatherers who lived past their childbearing years could pitch in and provide extra sustenance and succor to their grandchildren, helping those descendants survive. The theory also makes the case that it was physical activity that helped hunter-gatherers survive long enough to become grandparents " an idea that has potential relevance for us today, because it may explain why exercise is good for us in the first place. 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. 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 ... 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.