Scientists are discovering that compared with our early human ancestors and other primate relatives, modern humans have skeletons that are actually more lightweight and weaker.

The theory? Because we got lazy.

Biological anthropologist Habiba Chirchir and her colleagues at the Smithsonian’s National Museum of Natural History studied the bones of different primates including humans. They were surprised when they looked the inside of the bones near the joints. This spongy area of bone is considerably less dense in modern humans than early humans and primates.

“So the next step was, what about the fossil record? When did this feature evolve?” Chirchir asked.

They thought they would find that less dense bones were going to show up a couple of million years ago. This is the time around when Homo erectus, an early generation human, left Africa. The speculation was that having lighter bones would have made it a lot easier to travel long distances.

After examining a lot of early human fossils, however, she realized they were on the wrong track. “This was absolutely surprising to us,” she says. “The change is occurring much later in our history.”

The lightweight, less dense bones don’t appear until about 12,000 years ago. This is when the agricultural revolution was beginning and humans started being less physically active. They were leaving their nomadic hunter-gatherer life behind and settling down to pursue agriculture.

In these cross-sections of the head of the femur and metacarpal, one can see the decline in trabecular bone density, starting with chimpanzees (far left) to humans today (far right). (Photo: Smithsonian.com)

From Smithsonian Magazine:

The team scanned circular cross-sections of seven bones in the upper and lower limb joints in chimpanzees, Bornean orangutans and baboons. They also scanned the same bones in modern and early modern humans as well as Neanderthals, Paranthropus robustus, Australopithecus africanus and other Australopithecines. They then measured the amount of white bone in the scans against the total area to find trabecular bone density. Crunching the numbers confirmed their visual suspicions. Modern humans had 50 to 75 percent less dense trabecular bone than chimpanzees, and some hominins had bones that were twice as dense compared to those in modern humans.

Another report on the work appeared Monday in the Proceedings of the National Academy of Sciences, along with a study from a different research group that came to much the same conclusion.

The second group of researchers looked at the bones of people in more recent history who lived in farming villages nearly 1,000 years ago and compared them with the bones of people who had lived nearby, earlier, as foragers.

The bones of the foragers were noticeably stronger and denser. Their bone strength was comparable to nonhuman primates. The bones of people from the farming communities far weaker.

“We see a similar shift, and we attribute it to lack of mobility and more sedentary populations,” says Timothy Ryan, an associate professor of anthropology at Penn State University, and co-author of the second study. “Definitely physical activity and mobility is a critical component in building strong bones.”

Evidence is showing us that a sedentary lifestyle adds to modern problems, such as fractures and osteoporosis. Colin Shaw, who co-authored with Ryan, believes that humans can take this information and use it to their advantage.

In a statement, Shaw said, “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.”

He continues, “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.”

If we don’t use it, we lose it.