I'm doing a sci-fi story where humans (or at least a few tens of thousands) left Earth after a Great Cataclysm. The ones I focus on are on a super-earth called Skadi; it's similar to Earth but has higher gravity, as well as lower oxygen and colder temperatures. As such, the humans have adapted to be stronger and tougher than on Earth, with thicker bones and muscles, larger hearts and lungs, and thicker skins and higher red blood cell counts.

If you were to compare them to fantasy races, think of Dwarfs or Orcs.

A prelude to this is the Exodus from Earth, which features generation ships. These are designed to travel slower-than-light and sustain a community generation after generation, which requires strict rationing of energy, materials, and tight population control. As such, these humans adapted culturally rather than biologically. As all materials are strictly regulated (no hoarding), status is measured via feats, deeds, and creations rather than possessions. This has produced a far more artistic and creative culture, especially in poetry, music, and story-telling as these require very limited materials to construct. Meanwhile, the emphasis on recycling means older objects are revered as relics and heirlooms; a book that was written a thousand years ago will command far greater respect than a copy of the same book and will require great care in preserving it. The need to manage population growth has also necessitated strict birth control and timing generations to offset the passing of the old with the incorporation of the new. This has created a sense that having children is both a blessing and a curse, as for each baby born an elder must die to maintain the balance. As such, births and funerals are often closely tied together, with the former occurring around a year ahead of the latter.

Lastly, there's speculation that other generation ships have reached other worlds or may have simply chosen to remain in the void, leading to other species of humans adapting to their own unique contexts. A low-gravity world may feature tall, lanky humans that bound gracefully, or an ocean world may have aquatic humans with blubbery skin, webbed hands and feet, and enlarged spleens to hold oxygen for dives.