May 2010
Quechua woman and child in the Sacred Valley of the Andes Mountains, Peru. Quechuas is the collective term for several ethnic groups in South America who speak a Quechua language. Photo: Wikimedia commons.
What does a physical anthropologist study?
Beall: Anthropology in general is about people—all places, all times; so it is very broad. Within anthropology, there is a sub-group of physical anthropologists, also called biological anthropologists, and we are interested in the natural history and evolution of humans, including those of us living today. You may be more familiar with paleontologists, who are published on the front page of the New York Times and other popular media outlets when they find exciting new fossils; and that is an important area in physical anthropology.
Another important area is studying ongoing evolution in living people. The conceptual model used by both groups of scientists is evolution by natural selection. One of the things that those of us studying living people are interested in doing is explaining the huge range of variation that we see in the seven billion of us alive today. You look around, you see people of different sizes, of different shapes, of different skin colors, and those are traits you can see. If we move to traits we cannot see, there are even more variations. So, why are the variations there? Well, the answer is found in the theory of evolution by natural selection.
Do high altitudes present a case study for human evolution?
Beall: We know how evolution works when individuals or populations of organisms—or in this case people—are exposed to new environments. They develop different traits or different characteristics or capacities to survive and reproduce in a new environment. And so if you think in terms of designing an experiment, the biggest difference between the original environment and the new environment—the biggest stress—is likely the place where you can see evolution occurring.
Yaks carry supplies for Tibetans traveling in the mountains. The yak (Bos grunniens) is a large, long-haired bovine found throughout the Himalayan region of south Central Asia, the Tibetan Plateau, and as far north as Mongolia. Yaks have been domesticated for their milk, meat, wool, and as beasts of burden. Photo: Olmo Ling Buddhist Center.
A high altitude is a very good one because, for example, when you get to the top of Pikes Peak in the Colorado Mountains [Colorado Springs, CO], every time you take a breath of air, you are only getting about 60% of the oxygen molecules that you would get when you are in Philadelphia. That is an extreme stress because our bodies need a continuous supply of oxygen. We cannot store up oxygen like we store up calories, or say, Vitamin D. Instead, we have to have a continuous supply going to our mitochondria all the time. So, this is a problem for people at sea level who travel to Pikes Peak, where less than the usual amount of oxygen can even lead to medical problems, including altitude sickness.1
What happens to people who live at sea level when they visit high altitude places?
Beall: First, it is important to note that the partial pressure of oxygen, the amount of oxygen—let us say in a volume of air, like a lungful—is relative to how high an altitude you are. Therefore, 14,000 feet at Pikes Peak from the standpoint of stress is the same stress as 14,000 feet in Tibet. What differs is that if you are from Philadelphia, and you go to Pikes Peak, you are being acutely exposed. If you are a native of Philadelphia, or say Cleveland, Ohio, where I am from, chances are good you are not a native of high altitudes. You are in the majority of individuals, who was born and raised—and your parents were born and raised—at sea level.
All of us have a basic homeostatic method of response to cope with a decrease of oxygen. For example, let us say you get an infection in your lungs and become hypoxic; our bodies have responses to that. Let us say you are anemic, another sort of hypoxia, and you are not carrying the same amount of oxygen as you normally would at sea level—we have responses to that. We get various diseases here at sea level like sleep apnea, where people stop breathing for a while during sleep—that causes hypoxia. We have a long evolutionary history of responding to these kinds of hypoxia.
View of the city of Puno, Peru, which sits on the high-altitude Andean Antiplano, not far from Lake Titicaca. Photo: Bcasterline.
What is different when we go to high altitude is that this brief response now has to be sustained for a longer time somehow. This is where we differ enormously from Tibetan or Andean highlanders, who have been living at these high altitudes for 11,000 or more years. These people have had the opportunity to be exposed to natural selection over those 11,000 years. So, the hypothesis is, if we see biologically distinct characteristics, say of Andean highlanders, who are famous for having very big chests and very big lungs, then the hypothesis we would address is: Is this an adaptation? Does it function better than having small chests in a high altitude?
How do we know that people began settling in the high Andes and the Tibetan Plateau around 11,000 years ago?
Beall: The evidence comes from archaeology. There has been more archaeological investigation on the Andean Plateau because we have had access to the region far longer than the Tibetan Plateau. The evidence shows that from the earliest times there were people living there, rather than than the occasional group visiting. Tibet is a little more complicated because there is evidence that someone was there occasionally as far back as 35,000 years ago; but the evidence does not suggest the people were permanent residents. The 11,000 number, therefore, is the date for the time when archaeological evidence indicates that people were settled in these high altitude places for the long term.
To date, we do not know the circumstances that made people decide to live in these areas permanently.2 We do not think that they relocated suddenly. For example, we assume that people lower down the mountains were following herds of animals on their seasonal migration. They would experience temperature and altitude changes over time and gradually settle in higher location. People often ask me, “Why would anyone move there if it’s colder, drier, or windier there?” I do not have an answer yet.
What happened over time to make living possible for settlers in these high altitudes?
Beall: Starting in the 1890s, scientists started writing about the blood of people living in high altitudes. There is a great paper from 1890, where a French physician [Viault] sampled a few high-altitude natives, non-natives, and domesticated animals at 14,000 feet in Peru.3 He noticed that there was an increase in the number of red blood cells in his own blood sample. We have been studying that interesting fact ever since. Andean highlanders have been studied for decades, too. They exhibit such a distinctive suite of characteristics that it is actually referred to as the Andean Man Model—very high levels of hemoglobin, large chests, large lungs, and a special breathing rhythm.
In the 1970s, a few of us started asking simple questions:
- Is this pattern found everywhere in the world?
- Where else on Earth can we go to find high-altitude natives?
At the time, Nepal had become accessible, and we began to study Tibetans who live on the northern border between Nepal and Tibet. These Tibetans have always lived in the area—they just happen to be on the other side of the border—they were not refugees. The first thing that we discovered right away was that they could live at high levels without having high hemoglobin concentrations or large chests—they breathe very heavily. That was the first indication that the results of highlander studies would yield different results when studying populations in the Andes and those in the Tibetan Plateau.4
To get back to your question, however, about what caused, or what might be an explanation for, the two different patterns. There are two possibilities, and right now, we cannot distinguish between them. One possibility is that by chance, the genes that the colonizers took with them to higher altitudes in Tibet were different from the genes taken to altitude by Andeans. Evolution can only work with what it has. So that is one possibility: chance.
Tibetan women in full dress. Genetics seem to play a role in the survival rate of Tibetan babies. Photo: presscluboftibet.org.
Another possibility, however, is that it is possible that different mutations occurred in the last 10,000 years in these populations. We are still working on our knowledge of the genes involved. When we do more investigations, then we can really ask that question. We have recently discovered that genes play a role in the survival rate of Tibetan babies. Tibetan mothers who had copies of the allele [one of a pair or series of genes that control the same trait] for high oxygen saturation of hemoglobin had more than twice as many children that survived as mothers with a lower oxygen saturation genotype.
Do these two populations exhibit different medical problems?
Beall: Well, that is an interesting question, and I think there would be two speculations, but not necessarily firm, tested hypotheses. The Andean highlanders tend to have what we call hypoxic pulmonary vasoconstriction (hypoxic—less than the normal amount of oxygen—pulmonary lung or vaso blood vessels constriction—a narrowing of the blood vessels in the lungs). Now think about that: what is going to happen, for example, when you make a hose narrower? Your pulmonary blood pressure (pulmonary hypertension) goes up—your lung blood pressure. Your lung blood pressure is regulated somewhat separately from your systemic blood pressure. As a result, this is something that you and I—if we went to Pikes Peak—would experience immediately; that is, hypoxic pulmonary constriction and the increase in pulmonary blood pressure.
In the Andes, we see a lot of this high pulmonary blood pressure associated with hypoxic pulmonary vasoconstriction. It is so noteworthy that there is even remodeling—a different shape of the pulmonary vessels in Andean highlanders. This pulmonary hypertension has the potential for pathological consequences if it gets very bad. Tibetans on the other hand, do not show this hypoxic pulmonary vasoconstriction. We hypothesize that this is related to a molecule called nitric oxide. Nitric oxide is a vasodilator; it is something that we produce in our blood vessels, as well as other places, and it causes blood vessels to widen. We have discovered that Tibetans make, amazingly, orders of magnitude more nitric oxide than we do here at sea level. They have more than double amounts of it in their lungs than we do. Tibetans are making all this “extra” nitric oxide, causing their blood vessels to remain relaxed.
Can modern medicine learn something from people who live in high altitudes?
Beall: Yes, it can. One example is the famous Viagra® [generic = Sildenafil] study on Mt. Everest.5 Nitric oxide works on the same chemical pathway as Viagra; it causes relaxation of blood vessels. In the case of Viagra, you are interested in relaxing the enlarging blood vessels of the penis so that blood can flow. Well at high altitude, it is important to relaxing your blood vessels in general. Therefore, this led to the Viagra experiment. People were joking about it for a while. German scientists wondered if Viagra would improve physiological function at high altitude. The researchers had a placebo group and a Viagra group to test if Viagra would improve oxygen uptake and exercise capacity. And it did!
Another application is helping people deal with hypoxia. When people travel to high altitudes. they can experience acute mountain sickness.1 The symptoms of the disease vary: headaches, impaired thinking, vomiting, sleeplessness, and diminished capacity for physical activity. Some people develop life-threatening high-altitude pulmonary edema or cerebral edema. People suffering from sleep apnea or heart problems can be particularly susceptible to diseases associated with hypoxia. Studies of healthy highlanders may provide clues to new approaches and therapies for hypoxia for people everywhere—no matter how high or how low they live relative to sea level.
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