Evolutionary narrative for bipedal troglodytes

Question

It would be easy to assume conventional evolutionary heritage and have humans move into caves later, but in this otherwise earth-like world, the surface was never habitable. Given the somewhat axiomatic scientific consensus that bipedal adaptation is a culmination of surviving the geological transition to a grassland habitat (where it allegedly facilitated long treks and enabled vision over the grass), I'm not sure if I can realistically supplant man's arboreal primate lineage to caves. Clearly, vision is not a priority in the dark and trees are not common in caves.

While there are a few glaring dietary and physiological riddles I couldn't resolve, I still felt there was enough trait overlap to merit an attempt. Namely, the traits of climbing and skills to hunt insects seemed reasonable enough to assume a plausible primate-like troglophauna.

Question

If we accept primate-like troglophauna as an immutable part of my world, what is a plausible evolutionary narrative for the emergence of bipedalism?

Answer 1

Tree root tunnels:

If you had cave formations with lots of vertical shafts, climbing would be a premium ability, like with trees. Easily moving up and down these would be a big advantage. So simian isn't problem, full-biped gets tricky. Perhaps there is some food source requiring short simians to need to reach up & pick stuff off high ceilings? They'd be competing with animals that could cling to walls and ceilings.

Maybe your surface IS habitable, but not to animals (plants that feed on gamma radiation? Chemical deposition from space?) The "trees" can cut rock to establish their root systems. Plants have deep roots to reach buried water sources, and the left-over tunnels from roots are vertical shafts. Vital chemical and gas exchange reactions are done deep underground and away from harsh surface conditions.

Here's where horizontal connecting shafts come in. Maybe there's no weather, possibly the surface is a vacuum, and some of the same plants drill sideways shafts to spread. Fruits lure animals like simians to pick them, and then follow the shafts left by dead "trees", leave seeds in new locations, and give rise to new plants. Picking fruit from tunnels helps with food supply issues. your troglodyte simians might also feed on the creatures that feed on the fruits. The sideways tunnels would need to be made of a certain height to induce organisms to be tall. This could mean other species with long arms, long necks, etc.

It's just a thin ecosystem, but you're talking an odd world with no regular surface life. Fill out additional roles for more plants, fungiforms, and animals.