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One thing that’ll limit humans in colonizing planets is the atmosphere. Currently humans primarily need oxygen to survive, and without it all life that needs oxygen to live will die. Currently, plants produce oxygen when other animals breathe out carbon dioxide, which these plants use in a process called photosynthesis. Another problem that we face with is global warming, which is caused by an excess of green house gases being produced and released in the atmosphere, with one of those major green house gases being carbon dioxide.

This leads me to the question

How could humans evolve to breathe both oxygen and carbon dioxide

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    $\begingroup$ Just to be clear, you mean what would need to change in human physiology to permit a human to obtain oxygen both from the free oxygen in an atmosphere and by extracting oxygen from gaseous carbon dioxide? I'm intentionally being specific because "evolve" is a somewhat trendy word and I could claim that we evolved this ability by creating CO2 reclamation machines that are cybernetically grafted to our lungs. (p.s., please update your question, don't just answer in comments.) $\endgroup$ – JBH Jun 10 at 2:00
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    $\begingroup$ Plants don't breathe carbon dioxide. They create sugars through photosynthesis but then break down those sugars for energy using cellular respiration just like animals do. It's just that the amount of oxygen they produce through photosynthesis exceeds the amount consumed through their respiration. Carbon dioxide in general is a really bad chemical to try and breathe. The photosynthetic pathway used by most organisms is really sloppy and pretty energy hungry because it's hard to split a double covalent bond between carbon and oxygen. $\endgroup$ – user2352714 Jun 10 at 2:43
  • $\begingroup$ Humans already breathe both oxygen and carbon dioxide: one in, the other out. $\endgroup$ – Chronocidal Jun 10 at 12:52
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    $\begingroup$ Even the worst projections of CO2 levels still leave it at a very low percentage, it's just that even 0.1% is lethal, and below that Really Bad Things still happen. A human able to tolerate a 1% level of CO2 doesn't need a fundamentally different biochemistry, they just need a better technique to extract oxygen. A planet that has an atmosphere of carbon dioxide is probably also hot enough to boil people. (e.g. Venus), or is far enough away from a star that you can't grow food. $\endgroup$ – user3757614 Jun 11 at 1:48
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    $\begingroup$ Breathing CO2 is the same as pseudosciences about using water to fuel a car. Which is the same as wanting to use ashes so you burn the ash and get firewood as an end product. CO2 after breathing (and water after burning hydrogen, or ash after burning wood) is a waste product after you have extracted energy. You can't extract energy again and turn the waste back into the original energy source. $\endgroup$ – vsz Jun 11 at 8:59
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This depends on what you mean by "breathe CO2". We breathe in a certain amount of CO2 - about 400 ppm, currently - with every breath, and exhale air with about 38,000 ppm. So humans & other animals can tolerate - indeed, depend on - having a certain low concentration of CO2 in the air they breathe.

Much higher concentrations will kill us, though. CO2 is a waste product, and needs to be removed from the body.

As for breathing it in the sense that we breathe oxygen, absolutely impossible. We obtain energy by reacting O2 with carbohydrates &c in food, producing CO2 as a waste product. There is no more energy to be extracted by reacting the CO2 with anything that could reasonably be found in an earthlike environment. We'd somehow need to emulate plants, and use some external source of energy, like sunlight, to split the CO2 into O2 and C.

But photosynthesis depends on surface area. A human needs many hundreds of square meters of photosynthetic area to supply its energy needs.

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  • $\begingroup$ Isn't the point to survive non-Earthlike environments? If you have access to plenty of hydrogen, then you can react that with CO2 to get energy. $\endgroup$ – Logan R. Kearsley Jun 11 at 0:54
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    $\begingroup$ @LoganR.Kearsley but then you can't take in oxygen, hemoglobin wont hold oxygen in a high co2 environment. $\endgroup$ – John Jun 11 at 0:59
  • $\begingroup$ @John So? If the idea is to get energy from CO2 when oxygen is unavailable... then it doesn't matter that haemoglobin won't hold oxygen that isn't there in the presence of CO2 that is. $\endgroup$ – Logan R. Kearsley Jun 11 at 1:17
  • $\begingroup$ @LoganR.Kearsley that's not how gasses work, the oxygen balance of the atmosphere will be the same across the entire planet. $\endgroup$ – John Jun 11 at 1:28
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    $\begingroup$ @Logan R. Kearsley: But the OP is asking about humans specifically. If you change the biochemistry that much, are the resulting creatures even human? Heck, would they even qualify as Eukarya? $\endgroup$ – jamesqf Jun 11 at 15:50
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Having 2 types of mitochondria

As far as it's accepted nowadays, our mitochondria, the organelles that allow us to breath and the ones which require the oxygen, were once bacteria that engaged in aerobic respiration, which were then fagocited by an ancestor euchariotic cell, developing a relationship of symbiosis with it and essentially turning into a cell organ. The reason plants can engage in photosynthesis is because, as we suspect, they also ate another bacteria which was capable of photosynthesis.

Now first, why O2? Well, oxygen is used as the final electron acceptor in our respiration cycle, being combined with hydrogen to form water and provide the energy to form ATP, so to breathe CO2 we'd also need to be able to use it as an electron acceptor. Luckily, CO2 already has that use in some bacteria which do not use oxygen. These bacteria, being known as methanogenic, combine carbon dioxide with hydrogen to form methane and water, and usually thrive in swamps and other locations in which oxygen isn't as widely available.

So one possible scenario to enable us to use both particles would require a change all the way back to when we were unicelular organisms, consuming a variation of our methanogenic bacteria, as well as the one which would become our mitochondria, could potentially allow for an entire planet of creatures which can use both oxygen and carbon dioxide for respiration. This use of CO2, however, requires more hydrogen than the use of oxygen and releases methane in addition to water, methane being considered an asphyxiating gas in high concentrations, meaning your new humans would need special metabolic processes to deal with this chemical, so I'd assume it's use to be facultative and limited to low oxygen environments in which CO2 and hydrogen rich food are abundant.

Note: although it sounds like a small change, the ability to use CO2 (a gas that's toxic in high enough quantities and is commonly responsible for decreases in blood ph in our species) means these humans WILL have a group of differences in their metabolism. So these humans, as well as the other animals, will not be just an exact copy of us and our animals with a second different group of mitochondria. The mere evolution of such a trait would show it was beneficial, hinting at the existence of certain environments in the world from which these humans came from that have low levels of oxygen but high enough levels of CO2 to allow the body to use it to produce ATP, or other different kinds of pressures that made it so this trait was selected as advantageous.

So summing up, your humans, like most other animals in your world, will have 2 different kinds of mitochondria, will be able to breathe both oxygen and carbon dioxide and will likely have some metabolic strategies to accommodate this ability as well as to deal with the methane, as it will be constantly flowing through their bloodstream until it's ditched out in the lungs. There's also a chance they might evolve to be capable of absorbing more hydrogen from food.

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  • $\begingroup$ The main issue is that if you have oxygen and hydrogen in the same place in high concentrations, it tends to go boom. (Unless they're dissolved in water of course, but even in that case they're pretty reactive and won't coexist in high concentrations for very long. It happens at hydrothermal vents, but only because the ocean is rich in oxygen and there's a constant source of concentrated hydrogen at the vent.) For this reason, it seems quite unlikely that humans would live in an environment where both forms of metabolism are needed. $\endgroup$ – Nathaniel Jun 11 at 10:29
  • $\begingroup$ In other words, if your humans need to have a methanogenic metabolism, it's probably because they live in an atmosphere made of H2 and CO2, plus water vapour, N2 etc, but no oxygen at all. $\endgroup$ – Nathaniel Jun 11 at 10:32
  • $\begingroup$ Such an individual would die from acidosis caused by the lower PH levels. Signs and symptoms that may be seen in acidosis include headaches, confusion, feeling tired, tremors, sleepiness, flapping tremor, and dysfunction of the cerebrum of the brain which may progress to coma if there is no intervention. Note that excess CO2 isn't the only way to get acidosis so those symptoms aren't caused by a lack of oxygen that would be solved by being able to metabolise CO2 $\endgroup$ – Tom J Nowell Jun 11 at 16:14
  • $\begingroup$ @Tom J Nowell I see, and what would be the factor that would result in acidosis? Since as far as I see, you said acidosis would be a problem, but you didn't say what factor other than a potential excess of carbonic acid due to the excess of CO2 (which is already naturally removed via breathing), only that it wasn't the only one. Care to elaborate what other factor (which acidic compound that couldn't be expelled from the body via breathing or other processes would this be) would result in such acidosis so that I can edit my question and fix that issue? $\endgroup$ – ProjectApex Jun 11 at 17:33
  • $\begingroup$ @ProjectApex C02 is removed by breathing in the small amounts produced by oxidizing food, because that's how we've evolved to work It wouldn't be if there was sufficient C02 in the air to use in place of oxygen. $\endgroup$ – Bloke Down The Pub Jun 11 at 20:27
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Not without radically changing our diets.

There are organisms that breathe carbon dioxide: methanogens. They rely on also having an environmental excess of hydrogen available, to reduce CO2 to water and methane.

The food molecules used by eukaryotes, including humans--things like sugars and lipids--do not have nearly a high enough fraction of hydrogen (or other strong reducing agents like magnesium) to make it worthwhile to inhale excess CO2.

On the other hand, humans could conceivably evolve (or at least be engineered) to not need to breathe anything at all, and exhale both CO2 and methane as waste products, through a combination of fermentation and facultative anaerobic respiration which converts glucose into acetic acid and then cleaves acetic acid into CO2 and methane, at the expense of needing to eat more food to make up for the lower energy yield compared to aerobic respiration.

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It makes a lot more sense to have humans evolve to breathe nothing at all.

Chemically, oxygen is like a battery. It's got a lot of energy and it wants to react with things to get rid of it. We humans (and most things you think of as "life") take advantage of this fact to fuel our needs as organisms. After you react the oxygen with things and release the energy, you get CO2, which is like an empty battery. There aren't that many things you could end up with that have less energy than CO2.

Plants breathe in empty battery CO2 for scraps and use the materials to construct sugars to store solar energy. You can think of a plant as charging up the empty batteries they breathe in with sunlight to make a different charged battery, in this case sugar molecules. They even build their bodies out of sugar, creating huge sheets of sugar molecules called cellulose which they use for their structure, but that's another story.

Breathing in CO2 is like collecting empty batteries — fairly useless unless you have another energy source to charge them up. However, you could feasibly have food that provides all the energy an organism needs without the chemical requirement of an oxygen supply. The most obvious example is gunpowder, which can actually burn both underwater and in space, releasing its energy as it does so. You could also have oxygen or other oxidizers included in the food itself. Basically any rocket fuel mix will do, as it needs to be able to burn in space.

Anything that you can set on fire without an oxygen supply is a good candidate for a "food" that humans could evolve to live off of without oxygen in the air. After all, there is some truth to the phrase "burning calories." We're a lot more like an engine than many realize.

I would play around with the "if it burns without air it works" rule of thumb. I'm looking forward to seeing what interesting ideas you come up with!

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How could humans evolve to breathe both oxygen and carbon dioxide

Okay, so they already breath oxygen. We just need to add CO2. Surprisingly, it is not too difficult in principle.

What we need is called "cellular endosymbiosis", which starts with an infection by a photosynthetic monocellular organism (probably a very simple alga). The infection will give you green skin, and the alga will release glucose and oxygen into the blood stream, taking in water and carbon dioxide.

Once the details of the symbiosis get worked out, the alga can abandon a sizeable part of its cellular machinery and dedicate itself to photosynthesis and reproduction (I just discovered from Wikipedia that this particular kind of endosymbiosis is called kleptoplasty).

We know that this is possible because it has happened in the past (that is how chloroplasts got in the plants, just as mitochondria did in humans), and it has even happened in a higher order organism such as Elysia Chlorotica.

This is not full "breathing" because the fully illuminated surface required to sustain a human metabolism is on the order of ten to twenty square meters, while a human only has between 1.5 and 2 square meters. We can imagine to greatly improve the alga efficiency and supply the human with a "energy saving" operating mode, similar to lethargy or coma. In an emergency, a human could survive almost indefinitely (provided he can also recycle their own wastes) on sunlight alone, without breathing at all.

Actually, a human would need to breath only when the additional metabolic needs overwhelmed the skin capacity and the outside air contained enough O2. Otherwise, respiration would stop and metabolism would need to slow down (this already happens with the so-called capnic reflex), so that the skin might have a chance to get rid of the metabolic CO2.

SF literature

In Beggars' Ride, a DNA modification (not organelle-based) is presented that allows humans full autotrophy - "half an hour in the sun" being sufficient for a day's metabolism (which is impossible: solar flux is at most some 1.5 kW/sq m at the Equator, so half an hour in the sun would give about 750Wh of energy; that is about 650 kilocalories, while a human metabolism requires about 2000. At USA average insolation of 1.1 kW/sq m, or 950 kCal per hour, you would need a bit more than two hours continued exposure, and that is assuming 100% efficiency; ordinary photosynthesis is about 5%).

In Heart of the Comet, humans have been supplied with artificial organelles called cyanutes that thrive on methane, cyanide compounds and sulphidric acid, and scavenge them from the bloodstream of their hosts, to allow them a safer survival on Halley's comet.

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