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I am building a science fiction universe, and I am currently thinking of many different unique sentient alien species that would populate the galaxy.

I have thought of a rocky planet with Earth level gravity which has a highly elliptical orbit that causes its tempetures to have massive fluctuations between hot and cold. These extremes make life inhospitable on the surface. However underneath the surface there are large pockets of space that are filled with water up to a certain point and then open air. Imagine massive deep lakes with large amounts of land surrounding them.

The only input of energy into these areas are very large numbers of hydrothermal vents similar to what we see at the bottom of the Earth's oceans. This planet has a very active core causing high amounts of energy output through these vents. My main goal is to have a sentient, humanoid creature that creates a decently complex society within the many pockets.

Edit: I have added ventilation shafts that serve to dissipate heat out of the pockets and assumed that there is a balance between the amount of heat produced by the hydrothermal vents and the heat dissipated out of the pockets.

The issue though is the lack of sunlight, and lack of an oxygen based atmosphere. Assuming that the pocket remains at a stable temperature and does not blow up, what kind of ecosystem would arise from this environment, and what gases would these creatures breathe to live?

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  • $\begingroup$ sentience isn't the end stage of civilization. Like any trait it needs a reason to evolve. Human hunted in groups, favoring communication. You need to figure out why your species needs intelligence. $\endgroup$ – Mormacil Apr 15 '17 at 9:00
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The ecosystem that evolves under these circumstances would be very different from what we have on earth.

Consider though: land animals have many unusual aspects which make sense when you consider their ancestors were not always land animals. We came from the sea.

So: start with circumstances that give you the humanoids you want (basically humans) and then move it to present circumstances you want and have your humanoids adapt. You could start with an earth like planet with an orbit that is perturbed - maybe gradually, maybe suddenly such that the end result is what you have. Your Morlock humanoids are descended from ancestors who lived in a very different situation. I like this too because if the previous state was not that long ago (50,000 years?) maybe they could retain wistful legends about the Top Time and how it used to be - sort of like the australian aborigine legends of the Dream Time.

Re getting them some oxygen: you could have vast stores of carbonate rocks, laid down during previous epochs (exactly as on earth). You invent a microbe that lives on these rocks, splitting off the oxygen as waste and retaining the reduced carbon for its own synthetic needs. The energy for this comes from whatever chemical energy is locally available: probably hydrogen coming up thru the rocks. Something like this could be the basis for the food chain. Your humanoids could tend them by plowing / roughing up the carbonate rocks to increase accessible surface area.

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    $\begingroup$ That is a very interesting way of going about the problem. Them breathing oxygen would also solve the issues of these aliens utility and adaptability within the rest of the galaxy (cause the goal is for them to be a tributary race of a more powerful master race). I will try to see how I could maybe work that together with the hydrothermal vents if possible. $\endgroup$ – ntchapin Apr 16 '17 at 22:17
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You have a huge problem, there would be no incentive for animals to move on to land. Plants moved to land because they could survive there and animals followed them to eat them. But life around these vents is limited to the vicinity of the vents and everything basically feeds off bacteria that are nourished by the vented chemicals rather than sunlight. Without this bacteria, nothing survives near vents.

Your other major problem is that these are enclosed pools, eventually they will heat up because there is no other place for the heat to go and you'll end up trying to have life living in boiling water and an atmosphere of incredible pressure due to steam trying to expand. I don't think it's going to be stable enough to develop an ecosystem that would support evolving intelligent humanoids.

Another thing is these vents are adding to the volume of the pool constantly, depending on output this would add up. But since evolution as far as we know takes a lonnnng time, a fraction of a percent of added volume would soon add up, so your land area around the pool would might be flooded long before anything evolved, then whatever evolved might be confined to underwater activity if/when the pool meets the ceiling.

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  • $\begingroup$ I see. If small vents, small relative to the pocket, are distributed throughout the ceilings this could alleviate the pressure problem and allow for a heat sink correct? $\endgroup$ – ntchapin Apr 15 '17 at 14:26
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    $\begingroup$ Perhaps, but if you put a pot of water on a fire and leave the lid off, it will still boil, just takes longer, so you'd need a happy equilibrium between the amount of heat the vents disperse and the size of your body of water and the opening allowing heatsink. Once that is solved, you have lots of other issues, but at least it would be less likely to explode violently. $\endgroup$ – Kilisi Apr 15 '17 at 16:37

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