If humans were to live in an earth-like planet with an atmosphere with a significantly higher concentration of $\mathsf {CO}_2$ (Higher than what can be tolerated by normal human for a long period of time, but otherwise with the same concentration of oxygen), what sort of modifications would they need in order to survive, assuming that they have the technology to alter DNA and their own bodies, but with no external breathing apparatus?
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$\begingroup$ You should probably define "significantly". I'd also be tempted to stick in a "science-based" tag, but I'll leave that up to you. $\endgroup$– Starfish PrimeJun 3, 2019 at 18:26
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$\begingroup$ The only thing I can think on is a filter on alveoles, as CO2 become toxic over 2000 ppms ("From 2000 to 5000 ppm, CO2 concentration starts to cause problems (headaches, insomnia, nausea). It is a dirty air.") $\endgroup$– user61971Jun 3, 2019 at 18:36
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$\begingroup$ You absolutely must say what is the target partial pressure of carbon dioxide. The maximum partial pressure of carbon dioxide tolerated by humans with no ill effects whatsoever is about 0.0015 atm, more than 3 times as much as the current atmospheric level. Adaptation to 0.002 atm partial pressure of carbon dioxide is most likely easy, adaptation to 0.02 atm partial pressure of carbon dioxide is probably feasible, while adaptation to 0.2 atm partial pressure of carbon dioxide is likely not feasible. $\endgroup$– AlexPJun 3, 2019 at 19:04
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$\begingroup$ Welcome to Worldbuilding.SE BHMI. Please check out our tour and help center. Thanks for an interesting question. $\endgroup$– CynJun 3, 2019 at 22:27
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2 Answers
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The kidneys will deal with high CO2
The high CO2 levels you describe would produce chronic respiratory acidosis.
https://www.ncbi.nlm.nih.gov/books/NBK482430/
The primary disturbance of elevated arterial PCO2 is the decreased ratio of arterial bicarbonate to arterial PCO2, which leads to a lowering of the pH. In the presence of alveolar hypoventilation, 2 features commonly are seen are respiratory acidosis and hypercapnia. To compensate for the disturbance in the balance between carbon dioxide and bicarbonate (HCO3-), the kidneys begin to excrete more acid in the forms of hydrogen and ammonium and reabsorb more base in the form of bicarbonate. This compensation helps to normalize the pH.[1]
Usually people who have this issue are not moving enough air when they breathe (alveolar hypoventilation). They have symptoms from the underlying problem causing it, or from low oxygen levels - examples include obesity hypoventilation, emphysema, etc.
Your folks would be moving loads of air, breathing as hard as they could because high CO2 is what triggers the need to breathe (and why you can hold your breath longer after hyperventilating and blowing off CO2). I think eventually that trigger gets numb because people with emphysema are not breathing as hard as they can despite having high CO2 levels. The lungs are not going to help those folks or on your world.
The kidneys step up to maintain pH by excreting more acid and retaining bicarbonate which moves the blood back towards alkaline. I think that if pressed normal kidneys can do more and more of that, presumably with some upper limit. Over evolutionary time having kidneys which could compensate better would probably confer better genetic fitness.
A short term fix might be to eat more bicarbonate; that helps people with problems on the kidney side where their kidneys don't make enough. If your needs are beyond what your kidneys can produce, you can eat more. I think calcium also has to do with generation of bicarbonate (sort of getting int he weeds here) and they could eat extra calcium too.
Sweat offers a route to dump excess acid from the blood. I don't think that actually happens with humans but it would not be that outlandish a mutation to take place. Your people could have very acidic sweat. Feces offers another fairly voluminous space to offload acid. Sour poop.
Unfortunately this sort of thing is not that cool for a story because the adaptation turns on altered salts in the urine and blood pH as opposed to growing a tentacled hump or something. I have not yet seen acid-base physiology turned into gripping high science fiction but I am so ready.
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3$\begingroup$ +1 for assuming 'tentacled bumps' are all that sci-fi writers want! :D $\endgroup$– bioJun 3, 2019 at 20:56
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$\begingroup$ Xenomorphs dont count as "acid-base physiology"? $\endgroup$ Jun 4, 2019 at 2:07
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If you're prepared to have a good fiddle with your own DNA, then there are some species out in the world that might serve as a useful template, or at least inspiration.
Crocodiles have some interesting adaptations that cause oxyhaemoglobin in their blood to give up its oxygen more readily in the presence of higher levels of biocarbonate ions in the bloodstream. This means they can more effectively make use of the oxygen that's already in their blood whilst holding their breath, but it also means that there's more spare haemoglobin available to bind that that CO2 as carbaminohemoglobin. CO2 bound in this way, instead of as bicarbonate ions, does not contribute to respiratory acidosis and as such reduces the problem of hypercapnia. The Martian settlers in Kim Stanley Robinson's Mars trilogy used a crocodile-derived genetic modification to allow them to better tolerate higher levels of CO2 in the martian atmosphere during terraforming.
The other animal isn't quite as... marketable as a crocodile. Yep, its everyone's favourite burrowing mammal, the naked mole rat.
Whilst they're some of the least photogenic mammals on earth, they can survive in atmospheres of up to 80% CO2 and as low as 5% O2 for hours at a time, which is no mean feat. Their haemoglobin has a higher affinity for oxygen, but they also seem to have a number of interesting (and poorly understood) mechanisms for coping with acidosis which has also apparently contributed towards a high resistance to pain. What's more, they seem extremely resistant to cancers and are very long lived for rodents. I don't have any more details of exactly how they do all this (and I suspect a lot of the important trick are currently poorly understood, if understood at all) but hopefully this will give you a good starting point for further investigation.
Now I think of it, all of this tolerance of terrible atmospheres, the lack of need for light or circadian rhythms and the high resistance to cancers suggests that an uplifted molerat would probably make an excellent interplanetary or interstellar colonist. They do make a brief appearance in Charles Stross Singularity Sky, though these beneficial attributes are not mentioned there.
answered Jun 4, 2019 at 19:19