Modern Tibetans’ fortitude for high altitudes has been traced back to a mixing of two ancestral populations. A recent genetic study has found that the low hemoglobin levels that help Tibetans survive in a low oxygen environment came from a mixing of high and lowland populations.
The highland population would have migrated to higher altitudes around 30,000 years ago. Their genome would have adapted to their new low oxygen environment, making them genetically similar to modern Nepalese Sherpa populations.
This population then interbred with a population similar to the modern day Han Chinese, resulting in the ancestors of what the group we now call Tibetans.
The Tibetan Plateau reaches up to more than 14,800 feet above sea level and has an area about four times the size of France, making it the world’s largest and highest plateau. That kind of height carries a serious risk of hypoxia (dangerously low levels of oxygen in the body) for most people, which can do some serious damage to your lungs and brain and most anything else really. Tibetans don’t have this problem though.
According to National Geographic, Tibetans’ resilience comes from their low hemoglobin levels and breathing patterns. Most people, when at high altitudes, produce more hemoglobin, since it carries oxygen through the bloodstream to the body’s tissue. So you would expect a group that lives comfortably at high altitudes to have unusually high levels, right? The problem being that high hemoglobin has been linked to hypertension and chronic mountain sickness. So chances are good developing higher levels would be fairly detrimental in the long run.
Instead, studies have found that Tibetans have a genetic makeup changing how oxygen is processed and leading to lower hemoglobin levels. Tibetans’ lungs produce more nitric oxide from oxygen, which leads to wider blood vessels. This, combined with more breaths taken per minute than the average person, helps Tibetans to deal with altitude without the risks of high hemoglobin levels.
Interestingly, according to another National Geographic piece, adaptations to three of the highest populated altitudes in the world all seem to have different mechanisms. In Tibet they alter breathing, lower hemoglobin levels, and widen blood vessels. On the Andean plateau, which rises over 13,000 feet above sea level as well, people have adapted a means of carrying more oxygen per blood cell. In Ethiopia, where highlanders live at around 11,500 feet above sea level, researchers have no idea how natives avoid hypoxia. They don’t take more breaths, produce more nitric oxide, or have altered oxygen levels in their blood.
Understanding how the Tibetans’ low hemoglobin levels and nitric oxide productions came about through mixing of populations can help researchers to better understand how populations adapt to unique environments. Given that humans have shown they’re capable of adapting in multiple ways to the same stressor, understanding the history of those changes is just as important as understanding the mechanisms that underlie them.