Aldenderfer, Baker, Beall, & (...gasp!) Merriweather:
Recently I spotted a National Geographic news item about humans and high-altitude. The central point of the article is that different adaptations to life at high altitudes seem to have evolved in each of the populations found in the Andes, Himalayas, and in the high Semien plateau of Ethiopia. However, the way this article was written made me wonder about critical thinking.
The article starts with the most well-known adaptation against hypoxia, found in Andean populations, where compensation for the low oxygen concentration in the air is made up for by an increased concentration of haemoglobin in the blood. An Andean can take up more oxygen at the same breathing rate than someone at sea-level.
In the Tibetan case, the article mentions that the populations have a higher average breathing rate, and increased blood flow.
For the Ethiopians, there is no evidence for increased haemoglobin concentrations, for higher breathing rates, or for increased blood flow. The scientist involved, Cynthia Beall of Case Western, is quoted as saying "Right now we have no clue how they do it."
And that immediately made me go back to the beginning of the article. Picture a table, with the three populations down one side, and the adaptations along the other:
|Increased [Haemoglobin]||Increased Breathing Rate||Increased Blood Flow|
The point being that one wants an entry in each square - there is no way of knowing from the article whether Andeans do or do not have increased blood flow, and one can only infer that Tibetans do not have increased haemoglobin concentrations.
It is implied that these are evolved characteristics developed over the tens of thousands of years that the areas have been populated. Usually, similar environmental pressures on separate populations will lead to similar adaptations - parallel evolution. However, in this case the interest is in the fact that different adaptations to hypoxic conditions appear to have occurred.
The article then leaves us hanging, and wanders off into the (just as interesting) territory of migrations and social adaptations to environmental pressures.
The National Geographic article was certainly not intended to be technical, and the scientists involved are well-recognized, so I have to conclude that the TNG author, Hillary Mayell, either did not quite get to cover all the points the researchers had made, or deliberately left something out in order not to confuse the lay reader.
Here are some things that occurred to me that had been left out of the TNG article:
1) Among the medical and most of the athletic community it is well known that training at high altitudes raises the red blood cell count (RBC) and therefore the haemoglobin concentration in the blood - which makes me think this is a short-term physiological response, rather than a longer-term genetic response. The Ethiopian case is therefore quite interesting if in fact their RBC is not elevated. Does the RBC increase, or is it haemoglobin concentration in each cell?
2) The fact that lung capacity increases is not mentioned. Perhaps because this occurs with all three populations? It is fairly well known among the female community that moving to live at high altitudes leads to an increased bra size - not because of any change in cup size, but because their lung volume changes, expanding more against their ribcages. Men don't wear bras and so don't notice this effect unless they are doing measurements of lung displacement, or measuring exhaled vs. inhaled chest sizes over time. Moving from my place of birth in the Andes at 8,600 feet to Toronto, essentially at sea level, I lost several inches off my own chest, even while working out more.
3) Each of these areas has been populated for different lengths of time: the Simien for about 50,000 years, Tibet for about 25,000, and the Andes for about 15,000. Could it be that these differences reflect a series of changes, and that we could expect the Tibetans to eventually display Simien characteristics, and the Andeans to progress to Tibetan and then Simien?
Further reading was necessary. I went and found several nice sources that should round out your knowledge about hypobaric hypoxia and human evolutionary responses.
A general press release from CWRU that is more detailed, but still fairly non-technical.
A set of RealAudio interview segments with Cynthia Beall, still non-technical. Interesting.
The real deal: the Proceedings of the National Academy of Science article by Beall, Decker, Brittenham, Kushner, Gebremedhin, and Strohl (abstract, if the previous link failed). Nice and technical for those who like to get their hands dirty.
...and you should all be able to breathe easier now.