A team of researchers has shown that human bones have become significantly lighter and weaker since the advent of farming, when humans experienced a dramatic shift from foraging to a more sedentary lifestyle.
     The study shows the bones from hunter-gatherers of around 7,000 years ago were around 20 percent heavier than the bones of farmers from the same area more than 6,000 years later. The team has ruled out changes in diet and body size, saying a reduction in physical activity was the root cause.
     Two types of tissue form bone: the hard cortical shell which coats the outside, and the honeycomb-like trabecular mesh on the inside - the part most vulnerable to fractures.
     The researchers x-rayed samples of human thigh bones along with those from other primates, focusing on the ball which forms part of the hip joint. They examined four distinct human populations in central North America representing hunter-gatherers and sedentary agriculturalists, paying particular attention to changes in the softer core bone. According to the study, hunter-gatherers had a much higher density of actual bone.
     "Trabecular bone has much greater plasticity than other bone, changing shape and direction depending on the loads imposed on it," said co-author Colin Shaw from the University of Cambridge. "In the hunter-gatherer bones, everything was thickened." Stresses imposed on bones by hunting and gathering caused the bone mesh to grow thicker and stronger, which helped protect against age-related deterioration.
     The researchers say their findings support the notion that exercise rather than diet is the key to preventing heightened risk of fractures, as well as conditions such as osteoporosis.
     "You can absolutely morph even your bones so that they deal with stress and strain more effectively," says Shaw. "Hip fractures, for example, don't have to happen simply because you get older if you build your bone strength up earlier in life."
     Interestingly, while the foragers of 7,000 years ago had stronger bones than the farmers of 700 years ago, Shaw says neither competes with hominids from around 150,000 years ago.

Edited from Science Alert (27 December 2014)