Life on the farm might sound awfully active, especially if you've got a job that allows you to read my blog in the middle of the day. But before the invention of agriculture, humans were hunting and gathering every morsel of food they ate -- which made for an even more active lifestyle.
That transition gave humans the energy and time they needed to build society as it is today. But according to a pair of new studies published Monday in the Proceedings of the Natural Academy of Sciences, there may have been a trade-off. Farming gave humanity a more steady source of calories, but it also gave humans weaker skeletons.
Humans bones are less dense than the great apes we share common ancestors with. Modern humans have a less dense network of spongy bone -- the stuff that makes up the inside of the bone -- in the places where joints form.
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.
They found that up until 12,000 years ago, early human skeletons were much more dense. That's around the time humans adopted agriculture.
In the second study, researchers suggest that the decreased physical activity of farm-based culture may be to blame for the shift. They compared the spongy bone of fairly recent farmers (ones who lived about 1,000 years ago) with those who had lived in the same location earlier, relying on hunting and gathering.
The density differences between these early humans (at the hip joint, specifically) seem to suggest that hunter-gatherers maintained their ape-like density because they walked more. Because the farmers studied still ate the same local foods as their foraging ancestors -- just with the added stability of farming -- the researchers don't believe that diet changes are to blame for the change.
"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," University of Cambridge's Colin Shaw, an author on the second study, said in a statement.
If sedentary life is to blame for weaker skeletons, which can lead us to more frequent fractures and problems like osteoporosis, Shaw believes that humans can take their bone strength into their own hands.
"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" Shaw said.
It would take a whole lifetime of constant activity to mimic the lifestyle of a hunter-gatherer. So even if Shaw and his co-authors are correct, it's pretty unlikely that a modern human would be able to reach peak hunter-gatherer strength — though it would be interesting to see if modern hunter-gatherers come any closer to the bone density of our ancestors. Shaw's next study will look at modern ultra-marathon runners to see how their intense activity changes their bone density.