So far, multi-generational adaptation to 0g appears to be a non-starter. That environment really screws with gestation. In space station experiments, rodent embryos don’t implant in the uterine wall, blastulas/gastrulas don’t unfold correctly, and defects like brain voids appear if gestation gets that far. Given that all Earth life evolved in a 1g environment, it’s not surprising that a 1g signal is important to various gestational processes. (There’s also experiments showing that infant rodents don’t develop normally in a 0g environment, either.)
It’s unclear yet if a low-g environment, like 0.17g at the Moon or 0.38g at Mars, provides enough of a signal to keep mammalian gestation on track. We just have not run the experiments in those environments. But I’m doubtful. Given our evolutionary history, I would guess that either a near-1g planet (like Venus at 0.9g) or a large centrifugal space station that approaches 1g would be needed for safe gestation in space.
The other roadblock is cosmic radiation, exposure to which over multiple years will induce cancers and shorten lifetimes by decades. Unless settlers are comfortable condemning themselves and future generations to lives underground, a thick atmosphere is required to avoid the omnipresent irradiation. This again points away from smaller bodies like the Moon and Mars and more towards Venus- and Earth-sized planets. Or, again, large, well-shielded space stations.
FWIW...
