JOIN USModern humans evolved a more fragile skeleton about 12,000 years ago, probably driven by a shift from hunting to agriculture, a new study has found.
The research shows that modern human skeletons evolved into their lightly built form only relatively recently - after the start of the Holocene about 12,000 years ago and even more recently in some human populations.
It suggests that the dramatic decrease in bone density seen in recent modern humans may be linked to our shift from a foraging lifestyle to a sedentary agricultural one.
"Despite centuries of research on the human skeleton, this is the first study to show that human skeletons have substantially lower density in joints throughout the skeleton, even in ancient farmers who actively worked the land," said Brian Richmond, curator in the American Museum of Natural History in New York, and research professor at George Washington University.
Compared to our closest living relatives - chimpanzees - as well as to our extinct human ancestors, humans are unique in having an enlarged body size and lower-limb joint surfaces in combination with a relatively lightweight skeleton.
But until now, scientists did not know that human bone joints are significantly less dense compared with those of other animals, or when during human evolution this unique characteristic first appeared.
"Our study shows that modern humans have less bone density than seen in related species, and it doesn't matter if we look at bones from people who lived in an industrial society or agriculturalist populations that had a more active life. They both have much less bone density," said Habiba Chirchir, lead author of the paper from the Smithsonian Institution's National Museum of Natural History.
An international team of researchers used high-resolution computed tomography and microtomography to measure trabecular, or spongy, bone of the limb joints in modern humans and chimpanzees, as well as in a variety of fossil hominins.
Their results show that only recent modern humans have low trabecular density throughout limb joints, and that the decrease is especially pronounced in the lower joints - those in the hip, knee, and ankle - rather than the upper joints in the shoulder, elbow, and hand.
The appearance of this anatomical change late in our evolutionary history may have been a result of the transition from a nomadic to a more settled lifestyle, researchers said.
"Much to our surprise, throughout our deep past, we see that our human ancestors and relatives, who lived in natural settings, had very dense bone. And even early members of our species, going back 20,000 years or so, had bone that was about as dense as seen in other modern species," Richmond said.
"But this density drastically drops off in more recent times, when we started to use agricultural tools to grow food and settle in one place," Richmond added.
The research was published in the journal PNAS.