Around 9,000 BC the Altairan homeworld surface became too toxic for its inhabitants and so they went underground to rebuild their civilization. Because of their few numbers and the demands of maintaining the underground installation, the Altairans transferred their consciousness to android bodies which would live forever but be dependent on a local power source for energy. Some died during the transfer process and of those who survived, one group ventured through the Stargate with portable power sources never to return, other groups ventured to the toxic surface to face certain death, and the rest remained to maintain the facility.
In the end, Harlan was the last Altarian remaining on the planet. Upon their arrival, Harlan managed to create copies of SG-1 using the Altair database so that they could help him maintain the facility, which had severely degraded. (SG1: "Tin Man")