And probably not because of any genetic difference from you or me but because they walked and ran many miles every day, often loaded down with game animals, foraged plants, firewood, their infant children, and everything else they had to transport.
The evidence comes from leg bones, which have a strikingly robust cross section in Stone Age people. The oval shape and larger cross-sectional area relative to length makes bones more resistant to breaking and less vulnerable to osteoporosis in old age. In a recent and very thorough scientific comparison, Christopher Ruff of Johns Hopkins University School of Medicine and colleagues measured the strength of tibia, femur, and humerus bones from more than 1,800 human skeletal remains from sites across Europe, the oldest from 33,000 years ago and the youngest from the 20th century. Arm bone differences were negligble but in the leg bones they found a sharp decline in front-to-back bending strength starting around 7,000 years ago, and continuing until about 2,000 years ago – coinciding with the transition from the hunter-gatherer way of life to farming.
That fits with a lot of earlier reports. The unexpected result in the new study was finding that leg bone strength remained about the same during the past 2,000 years, even after the mass urbanization of human populations, the mechanization of labor, and the rise of desk jobs. That’s a telling insight on the problem of osteoporosis and its link to physical activity. The authors say that:
This finding is consistent with recently reported results based on limb bone trabecular structure, in which significant differences in bone density were found between Holocene foragers and farmers but not between farmers and a modern industrial sample. Our results for a much more comprehensive Holocene sample indicate that moderate variation in activity level, i.e., between the Iron/Roman period and 20th century, does not affect relative bone strength significantly. The results of the present study suggest that only very vigorous exercise is a sufficient stimulus for increasing bone strength, as a possible protective mechanism for age-related bone loss.
The evidence from bones suggests that Stone Age humans probably roamed more widely than modern hunter gatherers, such as the Ache, among whom the men regularly cover ten miles per day hunting and burn five to six times more calories engaged in physical activity than the average man in an industrialized society. A 2013 study comparing the strength of tibial bones concluded that Stone Age humans and Neandertals may have routinely outdistanced even highly trained modern cross-country athletes who train by running 80 to 100 miles per week.
Gradual decline in mobility with the adoption of food production in Europe by Christopher B. Ruff et al. PNAS (2015)
Extreme mobility in the Late Pleistocene? Comparing limb biomechanics among fossil Homo, varsity athletes and Holocene foragers by Colin N. Shaw and Jay T. Stock, J Human Evolution (2013)
Organic Fitness: Physical Activity Consistent with our Hunter-Gatherer Heritage. James H. O’Keefe, Robert Vogel, Carl J. Lavie, and Loren Cordain. The Physician and Sportsmedicine (2010)
An evolutionary perspective on human physical activity: implications for health. S. Boyd Eaton and Stanley B. Eaton III. Comparative Biochemistry and Physiology (2003)
Etching: Twee worstelaars, Dirck Volckertsz Coornhert (1552) via The Rijksmuseum