Our ancestors, who had to hunt and gather their food before the invention of agriculture, were more physically active than we are. Their bones were much stronger, too.

A new study shows that human skeletons today are much lighter and more fragile than those of our ancient ancestors. This is mainly a result of the invention of agriculture and a drop in our level of physical activity.

In the study, published in Proceedings of the National Academy of Sciences, researchers from the University of Cambridge and Penn State University analyzed X-ray images of thigh bones from modern humans, as well as those from humans who lived thousands of years ago.

They compared these samples to bones from other primates, including orangutans. The femur, or thigh bone, is the longest and strongest bone in the human body.

According to the researchers, after people stopped hunting for food and became involved in agriculture, a more sedentary lifestyle became the norm. This sedentary lifestyle led to more delicate, lighter, and weaker bones.

Discover Other Essential Facts About Bone Health »

“Contemporary humans live in a cultural and technological milieu incompatible with our evolutionary adaptations,” said study co-author Colin Shaw, Ph.D., a researcher with the University of Cambridge’s Phenotypic Adaptability, Variation, and Evolution research group, in a press statement.

The researchers focused on the inside of the femoral head, which is the ball at the top of the femur that fits into the pelvis and forms the hip joint. The hip joint is a load-bearing joint, which means it is affected by wear and tear from everyday exercise.

The team analyzed the bones of 229 individuals from various primate species and 59 individuals from four historical human populations. They looked at the bones of those who obtained food without being physically active, and of those who were foragers, meaning that they searched and hunted for their food.

Read More: World’s Oldest Cheese Found in Tombs of 4,000-Year-Old Mummies »

While human hunters from about 7,000 years ago had strong bones, similar to those of modern orangutans, farmers six generations later had much weaker bones. In fact, the ancient hunters’ bone mass was about 20 percent greater than the bone mass of the later farmers.

“The morphological differences between the highly mobile foragers and relatively sedentary village agriculturalists clearly point to physical activity as a major determinant of bone mass in the hip joint,” the researchers concluded.

Shaw said that in the past 50 to 100 years, there has been a major and potentially dangerous shift from physical activity for survival to a more sedentary lifestyle.

The study emphasizes the importance of physical exercise for bone health, in particular skeletal loading. “Sitting in a car or in front of a desk is not what we have evolved to do,” said Shaw.

Learn to Recognize the Symptoms of Osteoporosis »