Ethiopian High-Altitude Natives Are Different

 

Ethiopian high-altitude natives respond to hypobaric hypoxia differently than Andean (South America) or Tibetan highlanders.

 

In Ethiopia, a third successful pattern of human adaptation to high-altitude hypoxia is amazingly puzzling. In contrast with both the Andean “classic” (erythrocytosis with arterial hypoxemia) and the more recently identified Tibetan (normal venous hemoglobin concentration with arterial hypoxemia) patterns, the Ethiopian adaptation is very unique.

 

A field survey of 236 Ethiopian native residents at 3,530 m (11,650 feet), 14–86 years of age, without evidence of iron deficiency, hemoglobinopathy, or chronic inflammation, found an average hemoglobin concentration of 15.9 and 15.0 g/dl for males and females, respectively, and an average oxygen saturation of hemoglobin of 95.3%. Thus, Ethiopian highlanders maintain venous hemoglobin concentrations and arterial oxygen saturation within the ranges of sea level populations, despite the unavoidable, universal decrease in the ambient oxygen tension at high altitude.

 

The demonstration in the past 20 years that the “Andean man” model of high-altitude human adaptation does not generalize to natives of the Tibetan Plateau changed scientific understanding of human adaptation to high-altitude hypoxia. Comparisons of Andean and Tibetan high-altitude natives residing at the same altitudes [usually in the range of 3,500–4,000 m, or 11,600–13,200 feet, where partial pressure of inspired oxygen (P_IO₂) is 64–60% that of sea level] have revealed quantitative differences in traits associated classically with offsetting ambient hypobaric hypoxia.

 

For example, a hematocrit or hemoglobin concentration elevated over normal sea level values was long considered a hallmark of lifelong adaptation to high-altitude hypoxia; however, studies of Tibetans have demonstrated that it is not a necessary response to ambient hypoxia or arterial hypoxemia.

 

The population contrast extends to other traits as well: a comparative study reported that Andean high-altitude natives at 4,000 m had hemoglobin concentration and oxygen saturation of hemoglobin more than 1 standard deviation higher than Tibetans at the same altitude. The mean hemoglobin concentration of Tibetans was not elevated above sea level values despite very low oxygen saturation. The third major high-altitude population, natives of the Semien Plateau of Ethiopia, has not been studied for these traits.

 

These findings suggest there are three patterns of adaptation to high-altitude hypoxia among indigenous populations . Learning why the three populations differ will require two lines of future investigation. One is understanding the biological mechanisms and the underlying genetics that allow successful high-altitude adaptation with quantitatively different suites of traits for oxygen sensing, response, and delivery. The other is understanding the evolutionary processes that produced these patterns to explain how and why several successful human adaptations to high altitude evolved.