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What would the implications be for a humanoid species to excrete only gaseous waste from themselves instead of solid or liquid waste?

Public bathroom facilities would not be needed, but how would this effect the original evolution of the species, and assuming it evolved to a current day human race what would be different?

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    $\begingroup$ It would be a stinky world. Probably hot, too if their biology was anything like Earth biology. They be putting tons of methane into the air. $\endgroup$ – Howard Miller Oct 17 '15 at 2:39
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    $\begingroup$ Methane and other digestive gases are primarily created as a byproduct of digestive bacteria in the gut. A species that excreted only gas would likely have a much different digestive mechanism thus the gas may be of much different composition. Perhaps even helium $\endgroup$ – Johnny Oct 17 '15 at 6:06
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    $\begingroup$ Helium? Since helium is inert, it plays no role in biological activity. If you are hydrocarbon based like us, you emissions need to include carbon and hydrogen, as well as some oxygen, nitrogen, and traces of sulfur, potassium, etc. just like we emit. $\endgroup$ – Gary Walker Oct 17 '15 at 8:18
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    $\begingroup$ Plus, helium isnt even a component of any creature's diet. How is it going to be produced and excreted at all? $\endgroup$ – Youstay Igo Oct 17 '15 at 10:17
  • $\begingroup$ Geeze, tough crowd, apparently there are no Farscape fans here. I even provided a link to the helium expelling creature in question. $\endgroup$ – Johnny Oct 18 '15 at 4:38
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I think the only answer is "they wouldn't be human." It's only reasonable to excrete gas if you either consume an incredibly small amount of matter, or limit yourself to consuming only gasses.

The density of air is 1.225 kg/m^3. This means, at 1/3kg of food per day, we'd have to exude about a quarter cubic meter (~250L) of gas in the ultra-idealistic case. So, the first thing this goes to show is that the concept of bathrooms vanishes entirely, because you must be constantly exuding waste.

However, there's a problem. That's assuming you just turn the food straight into gas. Some foods seem to be easy to do that to (I'm looking at you, evil microbes in my gut making me bloat), but a lot of things are simply not easy to break down into gas. You may need to bring in 4-5 times as much gas to combine with these foodstuffs to turn them into gas, raising the amount of gaseous output dramatically. It also starts to become very energy inefficient. How would you like to consume a morsel of food, only to find that it's going to take more energy to expel it than you got from the food?

Now, perspective. 250L is a lot. Its about equivalent to how much we exhale in 35 minutes. If I have to multiply this by 4-5x, you're starting to see large volumes of gas movement dedicated to this effort. Also, much of that air volume is things like nitrogen, that are really had to use in a chemical process. Consider oxygen as a key reagent we might need, at 21% of the atmosphere. Now we have a need for 5x more gas flow, because we're only using 1/5th of it. Now 3 hours of your daily breathing has to go just to the mass dedicated to gasifying waste, in an ideal case, ignoring the need to burn fuel to do so.

You'd certainly watch what you eat.

Also consider that we have other "excretements" as well, when you start getting down to the dirty side of it. We exude skin cells on a constant basis. You might be able to be sneaky and just piggy-back on that process.

If I were to find a creature that could do this, and ignored the challenge of explaining why it was evolutionary beneficial to limit onesself in such a way, it would be a massive flyer, with a high bypass airflow past its lungs, which would double as a stomach. Just like baleen wales, they'd captrue the little bit of matter they can't get from the air by snaring smaller bugs, and using their bodies. Its waste could be limited to cells shedding off the edges.

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This depends on what gas they are excreting. If their metabolism is perfect or near-perfect then the gases would contain completely oxidized products such as sulphur dioxide, carbon dioxide, phosphorous pentoxide etc. It happens most of these gases have a strong greenhouse effect. In the absence of a cycle reconverting those gases into solid products, we (humans) are going to make Earth a planetary hell like Venus. Furthermore, the diet would be very very different and so would be the digestive system. I am expecting a predominantly herbivorous diet with a herbivore-mammalian-like digestive system (with multi-chambered stomach and huge appendix etc).

It would not change the evolutionary history much anyway. I don't think shit has had a major role in human evolution. Environmental effects would be immense and are already discussed above.

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Human waste didn't really become a large issue until we had large cities. At which point sanitation became an important part of city planning. Humans have not really evolved much in the last 2000 years where sanitation makes a difference in the health and survivability of individuals to procreation age.

The big things would be what kind of biology has evolved with these people that handle turning the human waste back into biological mater? Right now everything recycles and much of it pretty rapidly.

One of the big things would be scent would be much more important to these creatures not living in cities, since it is likely that many other animals are going to have similar digestive systems and being able to recognize their scents will have to replace spotting their spore.

In Cities, scent will be down played, since it will stink even more, or cities will be spread out a lot more allowing for better 'ventilation'. Quite likely 'bathrooms' will be 'gardens' of plants that utilize the 'fertilizer' the most efficiently, and give off pleasant smells.

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The evolutionary implications will be huge compared to H. Sapiens because so much of our body is devoted to handling wastes. Our bowels and urinary tract are specifically built to handle wastes.

Lacking a solid and liquid waste system may be a disadvantage because if the body is only able to eliminate gaseous waste then what happens when it needs to flush a solid or liquid contaminant or toxin?

Gas only wastes are unlikely since eventually, every terrestrial creature is going to eat dirt or something similar that simply will not, under any circumstances digest. Or, it will digest but at temperatures far in excess of what the creature can generate. That dirt has to go somewhere.

Likewise, what will you do with excess solvent, such as water or ammonia? Or is the organism completely contained and doesn't need to consume a solvent except in very limited quantities? How does sweating work? Does this organism cool itself?

While it's a simple thing to say "let's make a human that excretes gas", the implications involved in the operation of such a metabolism rapidly pushes a creature far away from human living habits.

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