According to a new study, the ends of human bones became much lighter and less dense around 12,000 years ago -- a loose demarcation that corresponds with modern man's transition from an existence reliant on hunting and gathering to one dedicated to agriculture.

In other words, scientists conclude, as humans settled down to farm -- abandoning their mobile ways and assuming a more sedentary disposition -- their bones weakened.

The difference in bone structure was first noticed by Habiba Chirchir and her fellow researchers at the at the Smithsonian's National Museum of Natural History, who were analyzing bones of both primates and humans. Chirchir and her colleagues noticed that the ends of human bones, the portions near joints made of what's called trabecular bone, were less dense and almost spongelike when compared to primate bones.

Chirchir initially estimated that humans developed less weighty bones when they first made their exodus out of Africa. Lighter, less dense bones would make traveling easier, she hypothesized. But further research showed that less hardy bone ends didn't arrive until 12,000 years ago -- much later than she and her colleagues anticipated.

"Our study shows that modern humans have less bone density than seen in related species, and it doesn't matter if we look at bones from people who lived in an industrial society or agriculturalist populations that had a more active life," Habiba Chirchir explained in a recent press release. "They both have much less bone density."

In the new study, published this week in PNAS, Chirchir and her colleagues show that trabecular bone is less strong and less dense among longtime farming communities than among foraging populations.

"Despite centuries of research on the human skeleton, this is the first study to show that human skeletons have substantially lower density in joints throughout the skeleton, even in ancient farmers who actively worked the land," said co-author Brian Richmond, a researcher at the George Washington University.