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The Heights We Go To

The links among extreme environments, genetics, and the human ability to adapt

Monday, August 14, 2017

Heights Tibet Mountains Plateaus


High-altitude landscapes are some of the most inhospitable places on Earth. They are cold, dry, and oxygen-poor. They were the last places humans settled—yet people did it and they survived. But how? For archaeologist Mark Aldenderfer of the University of California, Merced, a fundamental trait of humanity is our ability to adapt, especially to extreme environments. From the Himalayas to the Andes to the Ethiopian Plateau, people have evolved in ways that allow them to live at high altitude. “They’ve all converged on a solution,” he says. “They’ve all found a way to live at high elevation.”


Aldenderfer has assembled a team of experts in disciplines ranging from bioarchaeology to ethnography, paleoclimatology, geochronology, genetics, evolutionary medicine, and even mountaineering to augment archaeological research in order to understand what has clearly come about by way of evolution. He says, “You need the story of the people who lived at that time to tell you how these things actually worked. It’s about the whole process by which people adapt culturally, biologically, and genetically.”


Aldenderfer began his research in the 1970s as a graduate student on the Ethiopian Plateau, one of the highest in the world, where he studied the remains of the Iron Age trading empire of Aksum. Political turmoil cut his research short, and it became impossible to go back. A short while later, he took a research position high in the Peruvian Andes, studying early hunter-gatherers. Aldenderfer realized he had stumbled into a niche of untapped archaeological research on early human adaptation to high-elevation environments.


Working at such heights can be excruciating. A throbbing head, aching lungs, sleeplessness, fatigue, wheezing, coughing, confusion, and rapid pulse are all associated with hypoxia, a condition where tissues can’t get enough oxygen. It occurs when people accustomed to living at lower altitudes climb above 8,000 feet, and it can be dangerous and lead to pulmonary edema, stroke, and even death. Women who aren’t adapted to high elevations can have a much harder time bearing children and risk having low-birth-weight babies. Nevertheless, people have successfully settled at these altitudes for millennia—perhaps 7,000 years or more. The question is, how does such an adaptation come about? Jacqueline Eng, Western Michigan University biological anthropologist and osteology expert on Aldenderfer’s team, explains, “When we’re challenged by the environment, if some individuals have genetic traits that enable them to survive and reproduce more successfully than others who lack that trait, then those with the beneficial traits live and pass those traits along until it becomes more common in populations owing to the advantage it confers.”


It can be observed that, compared with people who live at sea level, Tibetans breathe more frequently and take in more oxygen, and they have expanded blood vessels that enhance the delivery of oxygen throughout the body. Andeans have higher levels of hemoglobin, the protein responsible for transporting oxygen in the bloodstream, so their blood cells carry more oxygen than those of lowlanders. Studies of highland Ethiopians, too, indicate genetic adaptations to low-oxygen environments. Something must confer these adaptations. Eng says there are several telltale alleles found in high-altitude populations where hypoxia is a major challenge. An allele is one of multiple versions of the same gene that determine various physiological traits—blood type in humans, for example, or the distinct color of a rose. Alleles can be thought of as recipes for the same gene—think of the difference between spaghetti with meatballs and spaghetti with ground beef. In evolutionary terms, random mutations create new recipes for alleles that may eventually come to exist throughout a population.

High Altitude Genetics