The Ancient People are lifeforms who dwelled in the solar system around 1.5 million years ago.
On Brittle Hollow four drawings can be found which depict the story of how they arrived. They came from another solar system, following an unknown signal. Their vessel crashed into Dark Bramble and they subsequently sent out three escape pods, one of which did not leave Dark Bramble because it crashed and its inhabitants were eaten by Anglerfish. The second one landed on Brittle Hollow and the inhabitants built a Hanging City. The last one landed on the Hourglass Twins. The inhabitants of this pod also build a city inside of Twin A.
The Ancients possessed very advanced technology. Aside from interstellar travel and teleportation devices they had also the ability to simulate tornadoes and build hologram devices, light sources capable of working for 1.5 million years, and a pretty big telescope! It is not clear if they took the gravity crystals from the Nomad or brought them in their vessel since the escape pods seem to contain similar crystals. They built a myriad of wonders on Twin B: it is full of probe launchers and things that look like antennas. Around the core is a ring on which there are five towers. Three of them teleport you to different locations. One to Timber Hearth, one to Giant's Deep and one to Brittle Hollow. Each of those places has a "receiver" which suggests that the Ancients have also been to these planets. In the core of Twin B (you can get there by being teleported from the two left towers) is a device which has anti-gravity. On Giant's Deep it is referred to as "The Conduit of Possibilities". It is known on the forums as The Timeloop Device because we think it is what causes you to always get to live the same 20 minutes again upon death.
The Ancients speculated that the Unknown Signal they followed originated from the Quantum Moon's fifth location.
It remains unknown why the Ancients seem to have disappeared. There is no indication as to whether they left the solar system or died out.