For thousands of years, Native American hunter-gatherers trekked on strenuous ventures for food. And for those same thousands of years, dense skeletons supported their movements. But about 6,000 years later with the advent of agriculture the bones and joints of Native Americans became less rigid and more fragile. Similar transitions occurred across the world as populations shifted from foraging to farming, according to two new papers published Monday in the Proceedings of the National Academies of Sciences.

“What our studies show is that modern people today have much less bone density than we should,” said Richmond, one of the studies' co-authors. His team studies spongy bone, a type of bone found at the joints. One day in 2010 when he and his colleague Habiba Chirchir were comparing the bone structure of a human’s middle finger with that of a chimpanzee, they noticed that the human’s was much more porous than the primate’s. His team made a similar finding when they compared foot bones between the two, which Richmond found surprising. He expected humans to have denser foot bones because they bear all of their weight on them.

“So now we asked, do modern human skeletons have unusually low density in our joints? And if so, why?” said Richmond. The team set out to find if there was a recent evolutionary change in skeletal strength. They compared the density in modern human joints from industrial time and early farming era, with that of primates and ancient human hunter-gatherers. For their analysis they performed high-resolution imaging of joints in eight different locations: the humerus, ulna, radius, the metacarpal, femur, the metatarsal and two places in the tibia. They found that modern human joints only have three-fourths to one-third the bone density that modern chimpanzees and orangutans (and ancient humans) have.

While Richmond was conducting a broad analysis of bone density, two anthropologists, Colin Shaw from the University of Cambridge in the U.K. and Timothy Ryan from Penn State, did a narrow study looking at hip joints in foragers from 7,000 years ago and farmers 700 years ago. Using a high-resolution CT scanner, the team found that the foragers had 20 percent more bone mass than the farmers—the equivalent to how much an astronaut loses after spending three months weightless in space. But both groups’ bones were far less dense than those of humans from 150,000 years ago, just like in Richmond's study.

“The fact is, we're human, we can be as strong as an orangutan—we're just not, because we are not challenging our bones with enough loading, predisposing us to have weaker bones so that, as we age, situations arise where bones are breaking when, previously, they would not have” Shaw said in a statement.