Tibetans compensate for low oxygen content much differently. They increase their oxygen intake by taking more breaths per minute than people who live at sea level.
"Andeans go the hematological route, Tibetans the respiratory route," Beall said.
In addition, Tibetans may have a second biological adaptation, which expands their blood vessels, allowing them to deliver oxygen throughout their bodies more effectively than sea-level people do.
Tibetans' lungs synthesize larger amounts of a gas called nitric oxide from the air they breathe. "One effect of nitric oxide is to increase the diameter of blood vessels, which suggests that Tibetans may offset low oxygen content in their blood with increased blood flow," Beall said.
A pilot study Beall conducted of Ethiopian highlanders living at 3,530 meters (11,580 feet) suggests thatunlike the Tibetans they don't breathe more rapidly than people at sea level and aren't able to more effectively synthesize nitric oxide. Nor do the Ethiopians have higher hemoglobin counts than sea-level people, as the Andeans do.
Yet despite living at elevations wih low oxygen content, "the Ethiopian highlanders were hardly hypoxic at all," Beall said. "I was genuinely surprised."
So what adaptation have the Ethiopian highlanders' bodies evolved to survive at high altitude? "Right now we have no clue how they do it," Beall said.
Tracking Prehistoric Migrations
Knowing how long the populations have been living at the top of the world is crucial to answering the evolutionary question of whether these adaptations are the result of differences in the founding populations, random genetic mutations, or the passage of time.
Archaeologists, paleontologists, and climatologists are pooling their knowledge to pinpoint when some of these early migrations to the high plateaus occurred.
Aldenderfer, the University of California, Santa Barbara, archaeologist, says cultural adaptations would have to occur first.
"The ability to survive in such harsh environments required control of fire, an expanded tool kit that included bone needles to make complicated clothing that protected the body in a significant way, and the cultural flexibility to change subsistence practices," he said.
Climatologists' changing understanding of the nature of the last ice age is contributing to archaeological efforts.
Ice-core and other evidence show that, rather than being a monolithic period lasting 100,000 years with frigid temperatures and glacial landscapes, the Ice Age included long periods of relatively mild weather.
"Through most of the 20th century it was thought that the Tibetan Plateau was covered by a monstrous ice sheet during the last glacial maximum, about 21,000 years ago," Aldenderfer said. "People couldn't live on an ice sheet. So archaeologists wouldn't even bother to look for sites from that time period."
Knowing the Tibetan Plateau more closely resembled Arctic tundra has lead to the discovery of new sites. Archaeological evidence suggests hunter-gatherers occupied the Tibetan plateau some 25,000 to 20,000 years ago. People began moving into the Andean Altiplano around 11,500 to 11,000 years ago.
What motivated prehistoric people to move into the harsh and challenging conditions presented by high altitude?
"The highlands offered an attractive option with a landscape that was open and pristine," Aldenderfer said. "People probably started out moving up and down for short terms, and then gradually settled at the higher elevations."
Changing environmental conditions also created "new opportunities and new constraints," he said.
In South America, for example, the maritime environment began transforming as temperatures warmed, glaciers retreated, and sea levels rose. Large mammals such as mammoths and mastodons gradually went extinct, as did other herbivores. Warmer temperatures allowed plants and animals to move to higher elevations, creating resource-rich patches of habitat in highland areas. Familiar coastal resources also changed as fish and shellfish habitats shifted.
Similar processes likely occurred in Tibet. Prehistoric people occupied the landscape during the interglacial process, when conditions were relatively benign and hunting was plentiful, Aldenderfer said.
"Suddenly it gets really cold. Biomass declined precipitously. It becomes very arid because of wind-flow patterns. The landscape becomes one of very patchy vegetation, rocky. And the huge herds of gazelle, antelope, and sheep wax and wane," Aldenderfer said. "What happens? Do the people adapt and tough it out? Did they abandon the highlands? Or do these early populations more or less go extinct? There's no evidence yet. But finding biological differences suggests they toughed it out and adapted."
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