Evolutionary biologists reasoned that natural selection produced these adaptations as each population settled at a high elevation. People with mutations that let them withstand low oxygen levels would be more likely to survive and have healthy children. Several teams of scientists have traveled to the mountains to gather DNA samples from people and search for traces of that evolution.

In 2010, Dr. Nielsen and his colleagues found variants of certain genes that were much more common in Tibetans than in the Han, the major ethnic group in China, who have lived for thousands of years at lower altitudes. By a wide margin, the winner was a gene called EPAS1. People with different variants turned out to have different levels of hemoglobin, suggesting that the gene was important to adapting to life at high altitudes.

Recently, Dr. Nielsen and another group of colleagues published a study on people who live in the highlands of Ethiopia. They found no evidence that EPAS1 had evolved there as it did in Tibet. Instead, a different gene, BHLHE41, appeared to have experienced natural selection.

Two other teams of scientists have recently searched for high-altitude genes in Ethiopians, and neither put BHLHE41 on their list. It’s possible that Dr. Nielsen’s method is more sensitive than the others, but that remains to be proved. “It’s going to take a while to sort through the discrepancies in Ethiopia,” said Dr. Di Rienzo, a co-author of one of the other studies.

It will be intriguing to see that unfold. BHLHE41 and EPAS1 turn out to have something in common: they work together in a network of genes that lets us cope with low oxygen levels. Even at sea level, low oxygen can threaten our bodies from time to time. Exercise can strip it from our muscles, while inflammation can eliminate it from wounds. The oxygen-sensing pathway triggers defenses to protect our bodies from damage.

Dr. Nielsen’s study suggests that evolution has stumbled across a way to retool this pathway to help people live at high altitudes. But it turns out there’s more than one way to retool a pathway. Though evolution has some creative freedom, it seems to stick to a few themes.