Updated| When the first humans left Africa around 100,000 years ago, they got shorter.

The evolutionary shift helped them cope with the colder conditionsa more compact body size helped protect them from frostbite, whileand shorter limbs would be less breakable when they fellbut it also appears to have come with a downside: arthritis.

In a study published in Nature Genetics on Monday, scientists at Stanford University, California, have shown how variants within the GDF5 gene, which are related to reduced growth, was repeatedly favored by our ancestors as they migrated out of Africa and across the continents.

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But GDF5 has also been linked with osteoarthritis,a degenerative joint disease that affects an estimated 27 million Americans. Risk increases with ageit is sometimes referred to as wear and tear arthritisbut it also has a strong genetic component.

Previous research has shown how mutations in part of the GDF5 gene cause malformation in bone structure in mice. In humans, it has been associated with a shortness and joint problems, and two changes in particular are linked with a heightened risk of osteoarthritis.

In the latest research, the scientists find GDF5 provided an evolutionary boost for our ancestors, with arthritis apparently a byproduct of it."The gene we are studying shows strong signatures of positive selection in many human populations," senior author David Kingsley said in a statement

"It's possible that climbing around in cold environments was enough of a risk factor to select for a protective variant even if it brought along an increase likelihood of an age-related disease like arthritis, which typically doesn't develop until late in life."

A display of a series of skeltons showing the evolution of humans at the Peabody Museum, New Haven, Connecticut, circa 1935. Study finds humans became shorter when they first left Africa 100,000 years ago. Hulton Archive/Getty Images

To better understand GDF5, the team studied the DNA sequences that might affect how the gene is expressedspecifically those that are known as promoters and enhancers. From this they found a previously unidentified region they called GROW1.

When they looked for GROW1 in the 1,000 Genomes Project databasea huge database of genetic sequences of human populations around the worldthe team found a single change that is very common in European and Asian populations, but is hardly ever seen in Africans. The team then introduced this change to mice and found it led to reduced activity in the growth of bones.

They then looked at the change to the genetic variant over the course of human evolution, and found it had been repeatedly favored after Homo sapiens left Africa between 50,000 and 100,000 years ago. The team says the benefits of being shorter in colder conditions probably outweighed the risk of developing osteoarthritis in later life.

Because evolutionary fitness requires successful reproduction, alleles that confer benefits at young or reproductive ages may be positively selected in populations, even if they have some deleterious consequences in post-reproductive ages, they wrote.

Researchers believe this change could help explain why osteoarthritis is rarely seen in Africa, but is more common in other populations.Concluding, Kingsley said: "Because it's been positively selected, this gene variant is present in billions of people. So even though it only increases each person's risk by less than twofold, it's likely responsible for millions of cases of arthritis around the globe.

"This study highlights the intersection between evolution and medicine in really interesting ways, and could help researchers learn more about the molecular causes of arthritis."

Lots of factors are involved in how human height evolved. Jamie/Flickr

Christopher Ruff, Director of the Center for Functional Anatomy andEvolution at Johns Hopkins University School ofMedicine, says that while the findings are fascinating and provocative, there are some caveats that should be considered.

For example, the first anatomically modern humans found outside of Africa (in the Middle East, and later in Europe and then East Asia) were all quite tallheight only declined after the Upper Paleolithic (20,000+ years later), he tells Newsweek.Therefore, while the gene coding for shorter stature may have been present in these populations, it certainly wasn't selected for until much later.

"Populations living in Europe today are among the tallest in the world, in some cases exceeding even the very tall Upper Paleolithic people. This has been interpreted as a response to improved living conditions over the last 150 years, with very recent populations reaching their full genetic potential. This implies, however, that the genetic potential is there, i.e., that they are not predestined to be short.

Finally, modern African populations vary greatly in height (even excluding Pygmies), he continues. Some of this is doubtless due to environmental conditions, but it also suggests other genetic factors at work.So, I think the evolutionary basis of stature variation is likely to be quite complex.

This story has been updated to include quotes from Christopher Ruff.

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