Two recent studies show that our bones were affected by the transition from hunter-gatherers into farmers around 12,000years ago. These studies were published in the Proceedings of the National Academy of Sciences. The gradual decrease in physical activity caused by the transition saw humans become less active.
This led to a reduction in bone density, meaning that our bones got lighter and more fragile. A study called “Recent origins of low trabecular bones density in modern people” found that the bone strength of our bones began to decline around 12,000years ago, at the start of the Holocene period. This was the time when human became farmers and hunters. Habiba Chirchir (anthropologist at Smithsonian’s Human Origins Program) was co-author of the study.
She said that modern human skeletons are moving towards more light-weighted – or fragile – bodies. This trend began when agriculture was introduced to the world. Our diets changed. Our activity levels changed.” A second study, “Gracility in the modern Homo Sapiens skeleton was the result of decreased biomechanical load” concluded that the cause of joint bone weakness is the varying levels and activity levels in human societies of ancient times.
These levels decreased as we became farmers. These cross-sections of the metacarpal and head of the Femur show the decrease in trabecular bones density. This is a change from the time when humans were far left to today. The implications of these studies for modern people (Photo by Smithsonian.com). These two studies are important because modern humans’ levels of activity have declined significantly over the past years.
Tim Ryan from Penn State University is an anthropologist and co-author on the second study. He said that the lightness of our bones has a significant impact upon bone strength and stiffness. This can lead to increased incidences of osteoporosis, broken bones and other forms of bone loss. Scientists knew before these studies that humans have weaker, lighter and thinner bones than hominin predecessors.
But, no one really knew what was driving the “gracility”, or slenderness and slightness. Researchers suggested that the increased pressure on our joints caused us to become taller and leaner when we began walking straight. Others suggested it could have been due a diet change or an increase in physical activity.
Researchers have been focusing on trabecular bones, which is the soft material that forms joints at the ends of bone. Chirchir stated, “Think about the end of a chicken bones: You can cut through them and then see the meshwork of bone interwoven.” Trabecular bone density in specific bones is lower than that of our ancestors.
To determine if the differences in bone density might have been due to movement, the researchers used CT scans to examine the hands of primates including humans. Chirchir stated that an orangutan should be able to climb higher than other knuckle-walkers such as chimpanzees. A CT scan shows that air bubbles appear darkened against white background of bone. The denser bone will be, the more white the background. Scientists noticed that the scanned hands of humans looked very different to those of primates. Chirchir stated that “the human hand was very white in comparison to other primate bones”, which indicates low density and higher bubbles. She added, “So this was the striking thing.”
They set out to find out if the density of the trabecular bones in the remaining human skeleton was lower than the density found in other primates or early human ancestors. The scans were made of circular cross-sections from seven bones at the leg and arm joints. They included baboons and Bornean orangutans as well as modern humans and Neanderthals. The researchers found that humans have 50% and 75% lower trabecular bone densities than chimpanzees. However, the density of modern humans was double that that of modern humans.
When did our trabecular bones density begin to decrease? Studies showed that Homo sapiens had thick, spongy bones until the end of the Pleistocene and the beginning Holocene.
This was around the time when we began to grow our food and raise livestock instead of going hunting. Chirchir stated that “what we believe is happening is that people are becoming more passive and sedentary.” The people were becoming more involved in farming and domesticating animals. This is why we have a light skeleton. However, the study didn’t look at bone volume, thickness, and surface area. These are indicators of how strong our bones really are. Researchers did not examine bone density in modern humans foragers. The second study however did.
Colin Shaw, from the University of Cambridge, and Mr. Ryan joined forces to study trabecular bones in archeological samples from four different ancient human groups. Two were hunter-gatherers while the others practiced farming. The hip joint was a key bone that is important for walking and weight bearing. The ancient farmers had thicker hip bones, more volume and a lower hip surface than hunter-gatherers. Both farming groups showed similar results. They were not thought to have been diet-deficient as they ate a broad range of local foods.
This suggests that while diet may not be the main factor, the biomechanical stress from walking long distances and hunting could. Researchers conclude that physical activity is the main factor. However, they caution us not to draw any definitive conclusions. They suggest that we have weaker bones than our pre-farming ancestors, but this could be due to less exercise. Ryan stated that humans can develop a strong trabecular bone structure similar to non-human primates if they exercise regularly. Videos – Part I – The transition from hunter/gatherer to farmer was made gradually by our ancestors.
