Modern human skeleton evolved for sedentary lifestyle

Chuck Bednar for – Your Universe Online
Humans evolved their current lightweight modern skeletons relatively recently, no earlier than the start of the Holocene about 12,000 years ago and even later in some populations, researchers from the American Museum of Natural History (AMNH) report in a new study.
The study, which appears in this week’s edition of the journal Proceedings of the National Academy of Sciences (PNAS), involved high-resolution imaging of bone joints from both modern humans and chimpanzees, as well as from the fossils of extinct human species.
That imaging revealed that ancient humans had high bone density for millions of years before a dramatic decrease took place in recent modern humans. Furthermore, the authors said that there was a higher decrease in the lower limbs than in the upper ones – a discovery which suggests that the cause of the transformation may be linked to the adoption of a more sedentary lifestyle.
“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,” explained study author Brian Richmond, a curator in the AMNH Division of Anthropology and a research professor at George Washington University.
When compared to both our extinct ancestors and our closest living relatives, the chimpanzees, modern humans have a larger body size, lower-limb joint surfaces and a relatively lightweight and spongy skeleton. Previously, however, experts were not aware that our bone joints were far less dense than those of other animals, or at what evolutionary point this trait 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,” explained lead author Habiba Chirchir, 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 solve the mystery, Chirchir and Richmond (both of whom were with George Washington University when they started the study) and an international team of colleagues used high-resolution computed tomography and microtomography to spongy or trabecular bone of the limb joints in modern humans, chimpanzees and in several groups of ancient human predecessors, including Australopithecus africanus, Paranthropus robustus and Homo neanderthalensis.
The researchers found that the only group with low trabecular density throughout their limb joints was the recent modern humans, and that the decrease was particularly evident in lower joints such as the hip, knee and ankle. They concluded that the appearance of this physiological change may have resulted from the transition from a nomadic lifestyle to a more settled one.
“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,” Richmond said. “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. 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.”
The study was supported by the Wenner-Gren Foundation Wadsworth Fellowship, The Leakey Foundation Baldwin Fellowship, Smithsonian’s Peter Buck Postdoctoral Fellowship, and a grant from the National Science Foundation (NSF). Richmond and his colleagues added that their findings provide an anthropological context to osteoporosis and other related bone disorders.
“Over the vast majority of human prehistory, our ancestors engaged in far more activity over longer distances than we do today,” he explained. “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.”
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