Forager-to-farmer transition 'may have weakened our bones'

There is no biological reason why today's humans could not develop skeletons as strong as those of our hunter-gatherer ancestors or modern orangutan cousins. An analysis of prehistoric bones of foraging and agricultural populations shows the move toward farming reduced the activity through life that helps build bone strength.

The hunter-gatherers' bone mass was found to be 20% higher than that of their farming descendants.
Image credit: Timothy Ryan.

These were the findings of a new study published in the Proceedings of the National Academy of Sciences by a team of reseachers from the Pennsylvania State University, and from the Phenotypic Adaptability, Variation and Evolution Research Group at the University of Cambridge in the United Kingdom (UK).

The study shows that while our hunter-gatherer ancestors who lived around 7,000 years ago had skeletons as strong as today's orangutans, farmers living in the same area some 6,000 years later had much lighter and weaker bones that would have been more vulnerable to fracture.

The researchers found that the foragers' bone mass was around 20% higher than that of their farming descendants.

This difference is about the same as the bone loss seen in astronauts after 3 months of living in space in zero gravity.

For their study, the researchers examined X-rays of femurs from individuals who lived in four distinct archaeological populations - including sedentary farmers and mobile foragers - in the same area of the USA state of Illinois. They also examined samples from other primate species.

The part of the femur they focused on was the inside of the ball where it fits into the pelvis to form the hip joint. This is one of the most load-bearing joints in the body.

After ruling out other factors that could have an effect on reducing bone density - such as changes in body size and differences in diet - the researchers conclude that the main reason behind the decrease in bone density between our foraging ancestors and our more recent farming forefathers is reduction in physical activity.

They believe their findings support the idea that it is exercise rather than diet that protects us from conditions like osteoporosis that make us vulnerable to fractures.

Modern humans live in a cultural and technological world that is incompatible with our evolutionary heritage:

"There's seven million years of hominid evolution geared towards action and physical activity for survival, but it's only in the last say 50 to 100 years that we've been so sedentary - dangerously so."

Sitting in a car or in front of a desk is not what we have evolved to do.

Trabecular bone in hunter-gatherers was much denser

Our bones are made from two types of tissue: a hard outer shell known as "cortical" tissue, and a spongy inside known as "trabecular" tissue. Trabecular bone has a honeycomb-like structure that allows the bone to flex but also makes it vulnerable to fracture.

The researchers found that the trabecular structures of the samples they examined were very similar, except in one respect - within the honeycomb structure, the hunter-gatherer samples showed a much higher amount of bone relative to air.

The research team says compared to other types of bone, trabecular bone has much greater plasticity. It can change shape and direction depending on the load it bears. It can change structure from pin-like to almost plate-like.

In the hunter-gatherer bones, everything was thickened.

The researchers suggest the constant loading in the bone that occurs as a result of fierce exertion over the lifespan of hunter-gatherers would lead to an accumulation of minor damage to the trabecular bone.

The minor damage causes the bone to grow back stronger and thicker each time, building to a peak strength that compensates for the deterioration that comes with age.

The research team believes if you build your bone strength in early life, you could avoid hip fractures later on - because then your bone strength would never dip to the level where fractures can easily occur.

The researchers now plan to investigate how different types of load and mobility affect our bodies and bones. They are going to compare archeological records with new measures taken from today's ultra-marathon runners who cover punishing distances over varied terrains. The idea is to discover what kind of mobility helped our ancestors develop such powerful bodies.

Meanwhile, recently researchers from the Japanese Aerospace Exploration Agency are using the International Space Station to investigate roundworms in outer space to help develop new treatments for bone and muscle loss in humans living in space.

References:

1.     Gracility of the modern Homo sapiens skeleton is the result of decreased biomechanical loading, Timothy M. Ryan, Colin N. Shaw,PNAS, doi:10.1073/pnas.1418646112, published online 13 January 2015, abstract.

2.     University of Cambridge news release accessed 16 January 2015 via AlphaGalileo.