New 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. The work, based on high-resolution imaging of bone joints from modern humans and chimpanzees as well as from fossils of extinct human species, shows that for millions of years, extinct humans had high bone density until a dramatic decrease in recent modern humans. Published this week in the Proceedings of the National Academy of Sciences, the findings reveal a higher decrease in the density of lower limbs than in that of the upper limbs, suggesting that the transformation may be linked to humans’ shift from a foraging lifestyle to a sedentary, agricultural one.
“This is the first study to show that modern humans have less dense bone in their joints than earlier human ancestors or other modern species, such as chimpanzees,” says Christopher Ruff, Ph.D., a professor of functional anatomy and evolution at the Johns Hopkins University School of Medicine. “The reason is almost certainly mechanical: A less active lifestyle and weaker muscles produce less force across joints, stimulating less bone deposition.”
Compared to our closest living relatives — chimpanzees — and 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 this unique characteristic first appeared during human evolution.
“Our study shows that modern humans have less bone density than seen in related species. 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,” says Habiba Chirchir, lead author of the paper and now a postdoctoral researcher at the Smithsonian Institution’s National Museum of Natural History. “What we want to know now is whether this is an early human characteristic that defines our species.”
To explore this question, an international team of researchers used high-resolution CT and microtomography to measure trabecular, or spongy, bone of the limb joints in modern humans and chimpanzees, and in fossil hominins attributed to Australopithecus africanus, Paranthropus robustus, Homo neanderthalensis and early Homo sapiens. A small CT scanner in Ruff’s laboratory was used to measure all of the modern bones included in the study. The team’s 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.
“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 is seen in other modern species,” says Brian Richmond, an author of the study, curator in the American Museum of Natural History’s Division of Anthropology and a research professor at The George Washington University. “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.”
This research provides an anthropological context to modern bone conditions like osteoporosis, a bone-weakening disorder that may be more prevalent in contemporary populations partly due to low levels of walking activity.
“Over the vast majority of human prehistory, our ancestors engaged in far more activity over longer distances than we do today,” says Richmond. “We cannot fully understand human health today without knowing how our bodies evolved to work in the past, so it is important to understand how our skeletons evolved within the context of those high levels of activity.”
Other authors on the paper include Tracy Kivell of the University of Kent and the Max Planck Institute for Evolutionary Anthropology, Jean-Jacques Hublin of the Max Planck Institute for Evolutionary Anthropology, Kristian Carlson of the University of the Witwatersrand and Indiana University, and Bernhard Zipfel of the University of the Witwatersrand.
This work was supported by the Wenner-Gren Foundation Wadsworth International Fellowship, the Leakey Foundation Franklin Mosher Baldwin Memorial Fellowship, the Smithsonian Institution’s Peter Buck Fellowship and the National Science Foundation (grant numbers BCS-0521835 and DGE-0801634).
—text courtesy of the American Museum of Natural History