History and specifics
More than 1,000 years ago, the inhabitants grew sterile. They built Avatar's DNA-harvesting facility to help their species survive, but it failed and all of the people died off a thousand years previous (circa the late 13th century).
For the next 1,000 years, the facility on Gobi III was maintained by Avatar and robot nannies. It acquired DNA samples of various species and splicing them together with DNA from the original inhabitants, birthing and raising infants, then at some point selling them as merchandise to agrarian and industrial planets in the sector.
In 2268, when the USS Pasteur surveyed the Gobi system, the vessel was destroyed by a particle beam from Gobi III. Survivors made their way to the planet, but they were killed and their DNA harvested.
In 2269, the USS Enterprise tracked wreckage from the Pasteur and orbited Gobi III. Avatar's facility was later shut down and all of the children spawned there were repatriated by the Federation. (TOS - Year Four comic: "Issue 6")