If our atmosphere was much thinner (half as thin as we have now) how would animals and humans evolve?

Well, the first effect would of course be for anything that flies, like birds or bats. Half the air density means flying gets much harder. So I'd expect flying animals to be smaller in general, but with larger wings.

If it is only the nitrogen that is less dense, this would probably be the main effect on biology. However, let's assume that the partial pressure of all gases (eps. oxygen and carbon dioxide) is reduced.

We can see that effect by going to the mountains. Note that at the top of Mount Everest the pressure is only 1/3, and the partial pressure of oxygen falls up faster because oxygen is heavier than nitrogen. Only on the very top of Mount Everest (above ca. 8000 meters) the oxygen pressure gets too low. So life at half the pressure definitively would still be possible. However, it would also mean that the "death zone" would be at lesser heights.

One obvious adaption to lower oxygen levels is a higher concentration of haemoglobin /the substance that transports oxygen). Indeed, this effect is why athletes like to train in the mountains, because the human body can do this change already; with half the air pressure as norm, life would have been evolved to have such larger haemoglobin levels by default. Also I can assume that lungs would tend to be larger, just in order to get more oxygen per breath.

Another effect of half the oxygen would be less ozone in the ozone layer, because the ozone there is created from oxygen. That means there would be more UV radiation on the surface of earth, and thus life would have to adapt for that. For example, since black skin colour is an adaptation to strong UV, I can imagine that people would have evolved to be black not only near the equator, but also in the temperate zones.

The next class of gases is greenhouse gases, like CO2. Half the greenhouse gases would, of course, mean colder temperatures. Again, you'll find that in the mountains, and of course also closer to the arctic regions.

So in summary, if you want to see the effect of a thinner atmosphere on life, all you have to do is to look at life in the mountains. The only differences of the thin-air earth would be that this would already be the situation at sea level, and that there would be more UV light.

posted almost 10 years ago

celtschk‭

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