Much like how certain cultures have adapted to live at extremely high altitudes, what would be the evolutionary effects if humans today were taken to a planet with atmospheric pressure not unlike an early Earth? Think something oxygen-rich, about 4 bars of atmospheric pressure. The catch is that gravity is only 80% of Earth's gravity (think Venus' gravity compared to Earth).

I understand that humans living in a lower-gravity, oxygen-rich world might adapt to become taller and more spindly on average compared to humans today, but I don't know what effects atmospheric pressure would have on that growth -- would they become taller but 'stronger' (i.e., denser bones) compared to people from Earth? I'm no scientist and so my attempts at researching this have confused me at best! Any ideas you could suggest would be very much appreciated.

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    $\begingroup$ Casually, it seems that high energy output would be significantly simpler given a significantly higher partial pressure of oxygen. As long as the cells themselves can handle the pressure, the lungs and diaphragm have air pressure on both sides and won't necessarily see a gain or loss due to the pressure. I imagine humanoids could be easy going and slow moving due to easy food production, or intensely strong and fast due to increased metabolic rates and reduced strain. It really depends on what they have to outlast - evolution finds the winner. $\endgroup$
    – user8827
    Oct 18, 2015 at 4:27

2 Answers 2


Weight 80% Earth normal: Minor adaptions, less muscle mass needed, perhaps a little taller, maybe thinner bones. Heart has been mentioned, a smaller heart would likely suffice as the muscle demand is smaller. Hemoglobin can only carry so much oxygen so the amount of blood transport should be very similar ignoring the reduced oxygen demand of smaller muscles, blood viscosity would be unchanged, pressure required would be a bit less, so maybe circulation gets a little easier for 2 reasons.

Atmospheric pressure 400% Earth normal: While this heavier atmosphere requires a greater pumping effort, it is also more oxygen rich so perhaps the amount of air pumped could be reduced. Since you pretty much assume a redesign based on optimal changes in a question like this you should expect a change to the response to carbon dioxide. Humans do not have oxygen sensors, they have CO2 sensors. The oxygen starvation sensation is actually a response to elevated CO2 levels. This is why inert gases are dangerous, you don't notice that oxygen is being reduced when displaced by the inert gas. So, either adjust the CO2 sensitivity response or waste energy pumping a lot of air just to keep CO2 level down. Not sure how much tolerance to increasd CO2 is really possible, so this may not really provide a useful optimization if you have to dump CO2 just as much as we do.

Oxygen partial pressure 400% of Earth normal: No problem if you don't mind dying and a lot of painful and debilitating symptoms resulting from oxygen toxicity. before you actually die. Well, that and some other problems too.

In your scenario, you are breathing 0.84 bars worth of oxygen instead of the 0.21 bars we do. Within 90 minutes, many people would be having serious health problems (largely reversible if the source of the problem is corrected). But the long term effects are deeply wired into our biology because of the prevalence of free oxygen radicals, the hydroxyl ion would kill you as it damages vital system like kidneys, lungs, and every other organ since it degrades the lipids in every cell wall. Even bones are damaged by oxygen toxicity.

The body has coping mechanisms for our oxygen levels, but there must be fundamental changes for your scenario. It is commonly held that the oxygenation event a.k.a. oxygen catastrophe is responsible for a major extinction due to oxygen build up over a period a few few thousand to a few ten thousand years. I.e. you really would not have time to evolve sufficient coping mechanisms. Lack of animal life meant that free oxygen was not being removed quickly enough by non-biotic mechanisms to keep many anaerobic species viable. Now assuming humans arrived a few hundred million years after this, the oxygen coping mechanisms would necessarily be present in all animal life (otherwise you die before reproducing).

You may not notice these changes (as they occur at the cellular level) but they would be significant and the body would have to spend a lot of resources on oxidation prevention and cleanup. Cleaning up oxidation damage is already expensive, this would be a dramatic change in resource usage. Probably more redundancy mechanisms as well as more aggressive cleanup methods would be needed. A higher rate of apoptosis would also be a probable outcome. Higher animal forms may not have the margin of error needed to keep functioning -- i.e., sum of resources required for respiration, circulation, digestion, calculation, etc. could exceed the available resources given the added burden if the apoptosis burden increase is large. We operate on a much thinner margin than most people realize. In stressful times, the margins are even thinner. Pregnancy and lactation could be push child bearing into a very marginal outcome zone. Remember that childbirth used to be very dangerous for both the baby and the mother and will be again if would lose modern medicine. Even now, child-bearing is the 6th most common death among women 20-34 in the US.

Perhaps men could share the burden by providing the milk giving the women a change to recover from pregnancy or start another pregnancy before the child is weaned. Lots of social adjustment, but think of this perhaps men could not go topless on the beach instead of women.

Forest fires would also be a huge problem, once started they would be essentially impossible to put out unless you redesign the plant kingdom to be less flammable you will have mega fires.

Plants contains an enzyme RuBisCO that responds strongly to high oxygen levels and happens to be critical to photosynthesis. High oxygen levels can shut down photosynthesis. So you probably need to redesign plants even ignore the fire problem. RuBisCO is and very important and prevalent protein in plants. Designing around the RuBisCO issue may be very difficult.

What about 4 bars atmosphere but only 0.3 bars oxygen?

This is still a pretty rich oxygen environment, forest fires will still be an issue. 0.25 bars O2 is a better choice.

So, you increase the inert gas mix correspondingly. N2 is considered an inert gas in this sense and it would presumably be very likely essentially the remainder of the atmosphere except for some argon, H20, CO2 and trace gases.

For us humans, this presents the problem of nitrogen narcosis. For 3.75 bars of N2 you only get relatively mild narcosis. I could not find any long-term exposure studies and since the clinical basis of inert gas narcosis is murky, I would prefer not to predict the effect of long-term exposure. Since artificial environments are required to induce narcosis I don't think there is much incentive to study long-term exposure.

Since this is only mild level narcosis exposure, I would except that the adaptation would either be mild or non-existent. Maybe a little more computational power in the brain to compensate or scavenging mechanisms in nerve fibers or such-like.

Reduced oxygen levels in hyperbaric environments is how we avoid oxygen toxicity. Such environments are often helium enriched since helium does not cause inert gas narcosis. It is hard to justify a rocky planet having high level of helium in the atmosphere as it is prone to reaching escape velocity. Your lower gravity would accelerate the helium loss, so you are probably stuck with the narcosis unless you could justify a large increase in neon levels. Neon either does not cause narcosis or it is simply considerably smaller effect compared to nitrogen. Xenon is actually worse than nitrogen, you can use an oxygen/xenon atmosphere to replace anesthesia. Xenon is too expensive for this to be practical and you would sound like a Go'auld too.

I can't think of a good way to adapt our lungs, etc. to negate or strongly mitigate the oxygen toxicity problems along the lines commented upon by Ville Niemi. But supposing I'm missing something and it is reasonable, you still need to deal with the other issues with high O2 levels. So overall, 0.25 bars of O2 is probably the best solution - At least it does not cause any major problems that I can think of. Since it is also necessary to solve the other problems from high O2 levels, it seems a much more useful change than trying to make adaptive changes to compensate for oxygen toxicity in humans.

Of course, we also reintroduce the pumping burden problem in full. Since people in good health can already handle this extra burden (though reports of fatigue and soreness are common at first), I would have to say this is not a big problem. A more muscular diaphragm seems pretty easy compared to other changes.

  • $\begingroup$ I think higher animals are to some extent protected by oxygen by the skin. Assuming, as you should, that breathing is adjusted to keep the level of oxygen inside the body the same, only exposed areas would need adjustment. The eyes, the mucous membranes in the mouth and nose, and the lungs. The lungs could be partially protected by simply breathing shallower - the air in the lungs would have less oxygen and more carbon dioxide. This would require the lungs to have extra space. Protecting these exposed surfaces should be doable. They probably have a protection mechanism to upgrade already. $\endgroup$ Oct 19, 2015 at 1:27
  • $\begingroup$ Sorry, but the oxygen exchange mechanism is the lungs is simple diffusion making it impossible to alter the equivalent O2 pressures within the body. Any other oxygen exchange design I can conceive of would be prohibitively expensive. $\endgroup$ Oct 19, 2015 at 1:47
  • $\begingroup$ Simple diffusion does not mean the two sides need to be in equilibrium, in fact it doesn't really work if they are. The oxygen level in lungs will be lower than in the atmosphere and the difference depends on your breathing. You can try this by simply stopping breathing and verifying that the oxygen level of your blood does drop. The oxygen level of the blood will be lower than in the lungs and the difference depends on the rate of diffusion. Many diseases that alter the composition of the mucus in the lungs cause issues with sufficient oxygen. $\endgroup$ Oct 19, 2015 at 2:00
  • $\begingroup$ Additionally since the lungs and the rest of the body do IIRC have separate circulation joined at the heart, you could add a "mix control" system there. I doubt there is any reason to. Sounds like something likely to fail with lethal consequences to me. But you could have one... PS: oops, forgot to +1 you... $\endgroup$ Oct 19, 2015 at 2:03
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    $\begingroup$ Updated answer per 300 millibar O2 adjustment. $\endgroup$ Oct 19, 2015 at 4:31

A lower gravity would rather imply a lesser atmospheric pressure, not greater, unless the planet has a huge atmosphere going thousands of miles above the surface.

Greater atmospheric pressure would imply a much more powerful heart, as you would need to pump blood with more force to help it reach the extremities when you have a huge pressure all around your body. This is the most significant anatomical change.

Another difference would be in the lungs. Since the atmosphere is oxygen-rich and high-pressure, probably a small lung-set would do. Maybe a single lung instead of two would suffice. Breathing rate might be slower than us (Earthly creatures).

Due to excessive air drag, the muscles would probably be slightly stronger than us, to not be blown away by gusts of wind. Also, the eyes might have a protective covering on them (read Ophthalmosaurus for detail).

Interestingly, instead of eyes, ears could become the most acute sensory organs. Under high pressure, sound tends to travel farther and faster, allowing for a sound-oriented creature. Not saying or implying that vision would become useless, but hearing would be much more enhanced than here on Earth.

I cannot say how bones or skin would be affected.

  • $\begingroup$ While the blood pressure would be higher, the extra pressure would be supplied for free by an external force - the higher atmospheric pressure. There would be an obvious correspondence for the two effects. Extra work for heart would come from overcoming differences in blood pressure in different body parts and that is related to gravity which would be reduced. // While noting the effects on sound is a good catch, the denser air would still be more transparent for light and less for sound than water and fish still rely on sight unless adapted to low visibility or darkness. Like bats are. $\endgroup$ Oct 18, 2015 at 6:29
  • $\begingroup$ Good point about fish. But the canids (dogs, wolves, bears, foxes, jackals) have several genera which live more by smell and hearing than sight, and they are perfect hunters. A denser atmosphere would also imply slightly less visibility as the bluishness of the sky would be deeper for a denser atmosphere. $\endgroup$ Oct 18, 2015 at 8:08
  • $\begingroup$ Would denser air carry scent better? Scent is superior to sight and hearing for finding food and detecting predators in a wide variety of circumstances. Superior hearing is typical of species that live in the undergrowth or hunt such species. The plants hide the prey from sight and scent is usually not precise enough to hit the fast moving prey. So it is find by scent, target by hearing, and then finally bite by sight when so close that the target is visible. So it is still a low visibility adaptation, just a specialized one. Why would bluer sky reduce visibility on the ground? $\endgroup$ Oct 18, 2015 at 8:57
  • $\begingroup$ Transparency issues. $\endgroup$ Oct 18, 2015 at 10:52
  • $\begingroup$ "Greater atmospheric pressure would imply a much more powerful heart, as you would need to pump blood with more force to help it reach the extremities when you have a huge pressure all around your body. This is the most significant anatomical change." -- not quite true, as already pointed out, the internal and external pressure would cancel out. Consider professional divers, who sustain 4 atmospheres routinely, and the heart is not a problem (breathing and gas narcosis is). $\endgroup$ Oct 18, 2015 at 11:15

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