Two studies suggest that during human evolution, our bones became weaker when we switched from being hunter-gatherers to farmers, about 12,000 years ago. Both studies have been published in the Proceedings of the National Academy of Sciences (PNAS).
During that transition, humans became less physically active, which resulted in a gradual reduction in bone density – our bones became lighter and weaker.
One study – “Recent origin of low trabecular bone density in modern humans” – determined that our bones started to weaken about 12,000 years ago at the beginning of the Holocene epoch, which was when humans started switching from being hunter gatherers to farmers.
Study co-author, Habiba Chirchir, an anthropologist at the Smithsonian’s Human Origins Program, said:
“Modern human skeletons have shifted quite recently towards lighter – more fragile, if you like – bodies. It started when we adopted agriculture. Our diets changed. Our levels of activity changed.”
The second study – “Gracility of the modern Homo sapiens skeleton is the result of decreased biomechanical loading” – concluded that joint bone weakness was due to varying levels of physical activity in ancient human societies, which decreased when we became farmers.
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)
Implications for modern humans
Both studies have implications for modern humans, whose levels of physical activity have decreased considerably over the past 100 years.
Tim Ryan, an anthropologist at Penn State University, a co-author of the second study, said:
“The lightly-built skeleton of modern humans has a direct and important impact on bone strength and stiffness.”
The lighter our bones the weaker they are, leading to a higher incidence of broken bones, osteoporosis and age-related bone loss.
Before these two studies, scientists already knew that modern humans had lighter, thinner and generally weaker bones than their hominin predecessors. However, nobody really understood what had been driving this “gracility” (slenderness, slightness).
Scientists had suggested that when we started walking upright, the extra pressure on joints made them go long and lean, while others argued it might have been due to a change of diet or physical activity.
More recently, researchers have focused on trabecular bone, the spongy material at the end of bones that form joints.
Chirchir said “Think of the end of a chicken bone: If you cut through it, then you see this meshwork of bone that’s interwoven.” Our trabecular bone density within specific bones are less dense that that of our ancestors.
The researchers took CT scans of the hand bones of primates, including humans, to determine whether the bone difference might be due to how the animals moved.
“We thought that if an orangutan climbs, it should have a different structure of trabecular bone than knuckle-walkers like chimpanzees,” said Chirchir.
In a CT scan, air bubbles come up dark against a white backdrop of bone i.e. the whiter the background the denser the bone is. The scientists noticed that scanned human hands looked considerably different from those of other primates.
Chirchir said “The human hand had very little white compared to the other primate hand bones,” an indication of low density and more bubbles. “So that was the striking thing,” she added.
So, they set out to determine whether the density of trabecular bone in the rest of the human skeleton might be less dense than that of other primates and early human ancestors.
They scanned circular cross-sections of seven bones in the arm and leg joints in baboons, Bornean orangutans, chimpanzees, modern humans, Neanderthals, Australopithecus africanus, Paranthropus robustus and other Australopithecines.
They found that modern humans had 50% to 75% less trabecular bone density than chimpanzees, while that of hominins was twice as high as modern humans’.
So, when did our trabecular bone density start to decline? The studies found that Homo sapiens had dense spongy bone until either the late Pleistocene or the early Holocene – about the time we started growing our food and raising livestock rather than going out hunting.
“What we think is going on is that humans were becoming less active, more sedentary. People were adopting farming, domesticating animals. That reduction in physical activity is what’s resulted in this light skeleton.”
However, this study did not look at bone surface area, thickness and volume, which can tell us how strong they are. Neither did the researchers look at bone density among modern human foragers. But the second study did.
Mr. Ryan and Colin Shaw from the University of Cambridge teamed up to examine trabecular bone in archeological specimens from four ancient human groups – two had been hunter-gatherers and the other two practiced agriculture. They focused on the hip joint, an important bone for weight-bearing during walking.
They found that hunter-gatherers (foragers) had thicker trabecular bone, greater volume and lower surface area on their hips compared to the ancient farmers. The two farming groups had similar readings and were believed not to be diet deficient because they consumed a wide range of locally available foods, suggesting that diet is probably not the key factor here, while the biomechanical stress of walking long distances and hunting is.
Studies point to physical activity as the driving factor
The researchers emphasize that their findings cannot give us a cast-iron conclusion. But they suggest that our weaker and lighter bones compared to our pre-farming ancestors are probably due to less physical activity.
Mr. Ryan said:
“Humans can have robust trabecular bone structure, similar to what is seen in living non-human primates, if they engage in appropriate levels of physical activity throughout life.”
So, perhaps the secret of strong and healthy bones, and maybe other parts of our body, is to move like early humans rather than to eat like them.
Video – From hunter-gatherer to farmer: Part 1
The two videos below, which are not connected to the two studies, show how our ancestors gradually made the switch from hunter-gatherers to farmers.
Second Video – From hunter-gatherer to farmer: Part 1