When humans settled down as farmers instead of hunter-gatherers, something else went down as well, researchers say -- our bone density, leaving our skeletons just a bit more fragile.

While no one argues that farming isn't physically strenuous, the former lifestyle -- a constant search for sustenance that had humans on the go and foraging most of the time -- was even more so, the researchers say.

A transition to settled agriculture allowed society as we know it today to develop, by giving humanity a stable and reliable source of food energy, but it came at a cost, two new studies report.

Our bones are not as dense as those of our ancestors 1,000 years ago or more, especially in the joints in our skeletons, researchers found.

In the two studies, samples of human bones from different time periods in the past were X-rayed, focusing on a sponge-like internal structure of bones known as trabecular bone, which gives them additional strength.

Trabecular bone can exhibit changes in its structure and shape when loads are applied on it, so the more we exercise the denser and stronger our bones become.

The X-ray comparison showed early hunter-gatherers had trabecular bone that was much more dense than what is found in modern humans.

The density in 7,000-year-old hunter-gatherer bones was 20 percent greater than in 700-year old farmer bones, the researchers found.

"Trabecular bone has much greater plasticity than other bone, changing shape and direction depending on the loads imposed on it; it can change structure from being pin or rod-like to much thicker, almost plate-like. In the hunter-gatherer bones, everything was thickened," says researcher Colin Shaw on the University of Cambridge in England, an author of one of the studies.

People today who experience even the most intense physical activity are unlikely to be subjecting their bones to sufficiently frequent and intense stress to obtain the increased bone strength seen in the 'peak point' of traditional hunter-gatherers, he says.

"Contemporary humans live in a cultural and technological milieu incompatible with our evolutionary adaptations," he says. "There's seven million years of hominid evolution geared towards action and physical activity for survival, but it's only in the last say 50 to 100 years that we've been so sedentary -- dangerously so."

"Sitting in a car or in front of a desk is not what we have evolved to do."