The shorter stature may have helped these prehistoric humans retain heat and stave off frostbite in their extremities, the authors said. It also may have reduced their risk of life-threatening bone fractures when slipping on icy surfaces. But the same gene puts humans at greater risk for arthritis in the modern era as they live well beyond their reproductive years.
The study looked at variants of the GDF5 gene, which was first linked to skeletal growth in the early 1990s, and is known to be involved in bone growth and joint formation. The researchers wanted to understand how the DNA sequences around it might affect the gene’s expression, focusing on one region they named GROW1.
After analyzing the sequence of GROW1 in the 1,000 Genomes Project database, a collection of sequences from human populations around the globe, the researchers identified a change in one nucleotide, the basic building block of DNA. The change is prevalent in Europeans and Asians but rare in Africans. To see if that mutation was incidental or actually caused shorter stature, they tested the nucleotide change in mice and found it decreased the length of their long bones, much as it is thought to do in humans.
That mutation of the regulatory region analyzed in the study is present in more than 50 percent of the population in Europe and Asia. In some Asian populations, it’s up to 90 percent, Dr. Kingsley said. Even if the variant plays only a small role in increasing arthritis risk, the sheer number of people who possess it means it can have a significant effect.
“The very abundance of the change means it could contribute to a lot of cases of arthritis,” Dr. Kingsley said.
A similar evolutionary paradox can be seen with sickle cell anemia, a condition in which a low number of red blood cells makes it difficult to carry adequate oxygen throughout the body, Dr. Kingsley said. A genetic variant causes a high rate of the disease in African populations. But that variant was favored because it also confers protection against malaria.
“The genome is complex and our evolutionary history is complex,” said Terence D. Capellini, one of the lead authors on the study and an associate professor in the department of human evolutionary biology at Harvard University. “Because of that complexity, relationships emerge between different aspects of our biology that may seem paradoxical. As we reveal this history of our genome and how it affects our biology, we begin to understand the connections.”
As with many aspects of evolutionary research, it’s easier to figure out what traits were favored than it is to explain why. While shorter stature may have been a protection against the cold and icy terrain, it’s hard to be certain, said George Perry, associate professor of anthropology and biology at Pennsylvania State University, who is not affiliated with the study.
“We’re not going to know that without some combination of a time machine and an experiment we can’t do,” he said.
But detailed studies like this can help further our understanding of complex evolutionary processes and the potential consequences for modern medicine and human health, Dr. Perry said.