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The TBD are a fantasy humanity offshoot that is fully adapted to life a few kilometers beneath the surface, settling in cracked earth near vents and hot spots.

Air quality ranges from stale to toxic, including sulphur vapor and vitriolic fumes that condense on the walls. Water is available in reservoirs further from the vents, and occasionally in condensation. Fungus-like life feeds on the upwelling chemistry, and there exists an ecosystem of mycanovores and carnivores around the fungus.

Light is available in the blue flames of burning silver deposits and orange glow exposed hot rock. The amount of light is equivalent to moonlight. A little like the lava lakes of Erta Ale and the surface sulfur mines of Sicily, but underground and provided enough air by some undefined source.

What evolutionary adaptations would help the TBD to thrive in this situation?

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Enhanced sense of smell. Preferably with a way to not immediately breath in the poisonous vapors. Also essential for information purposes: touch, sound, and sight will not provided sufficient information about the edibility of what one has in hand. Taste occurs a little late in the puzzle but might also help here.

Orientable Ears. Human hearing is good, but caves echo. Being able to alter ear shape and orientation would allow for specific sounds to be narrowed in on.

Small frames and overall lighter body structure. Smaller fissures can be traversed if you are smaller, and lighter builds are generally more flexible. This gives more options in moving around, and that can be life-saving when poisonous gas is around.

Chelating. Either by a behavioural method such as ingesting it, or by internal body processes. Its simply unavoidable that a person would not absorb at least some toxic chemicals, as such individuals who make them inert before they cause havoc on the more sensitive tissues will live longer.

Reflective Eyes. Not so much an evolution as retrovolution. Humans would need to obtain some more of the old night vision adaptations loost during the transition to day time living. This list also include larger pupils, and larger eyes.

Second eyelid. It might even be beneficial to re-evolve the second eyelids from the deep lizard past. This will provide some measure of protection and also means of eye-wetting without closing the eye. In such low-light levels one cannot say exactly when the elusive photon will reach the eye, best to just keep them open.

A Higher internal body-temperature tolerance. Either a fixed higher level locking them into living with a permanently higher body temperature (making them positively ravenous away from heat), or as an adaptive stress response (such as sweating) that can be more or less harmfully maintained for lengthy durations. Humans are already quite good at this, they can ancestrally hunt by forcing the hunted animal to experience heat-exhaustion first. But there are always ways to improve.

Loss of hair. Simply put who needs all that grooming. More seriously rock is more thermally stable and a simple fat lining is superior in such a stable setting. Need only look at naked mole rats. Also hair hampers the ability to cool actively, one of the reasons why humans can run so many creatures into the ground.

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Higher heat tolerance and a method for managing short-term exposure to intolerable heat would be critical. Perhaps an enhancement of our existing perspiration system but with a larger insulated interior fluid reserve. Perhaps something completely new and novel such as a heat-reflecting outer dermis or a second circulatory system dedicated to transporting heat to a centralized excreting organ which concentrates that heat in a thick scalding fluid which can be vacated from the body similar to how we urinate.

In addition to reflecting heat, the outer dermis should also facilitated an ablative defense with multi-layer hardened dead-cell calluses covering all exposed skin, which flake away when exposed to more than a safe threshold of contact-transmitted heat.

A secondary metabolic energy system, preferably an anaerobic one, would be useful for the times when the breathable oxygen is tainted by airborne poisons. A simpler version of this could implemented by leaving our single metabolic energy system alone and developing larger lung capacity, perhaps with a mechanism for filling a pressurized reserve of breathable air within a close-able third lung.

Those features would help a species to survive in your hellish environment, but to thrive they need to be able to see in the dark. Since the spectrum and presence of light will vary greatly from cave to cave, I would suggest that your creatures bring their light with them in the form of bio-luminescence.

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