The Apex planet was one of the first developed societies in the Lyran Star Group, and thus, one of the earliest planets to ever sustain a humanoid civilization. Torn by duality and conflicts, the Apex planet nearly self-destructed in a nuclear war. Hardly anyone survived. Those who did, had to live under ground for several generations, which changed the tone of their skin into grey, and forced their eyes to grow considerably, to be able to see in the dark. (Because of their physical appearance, some people refer to them as Greys.)
"After its planetary catastrophe, the Apex planet was shifted dimensionally into another region of Time/Space. The Apexians eventually became the race of the Zeta Reticuli." (Lyssa Royal & Keith Priest, The Prism of Lyra)
Some believe that other survivors migrated to this solar system to establish colonies on Mars and Maldek, a former planet between Mars and Jupiter. They continued their wars, and Maldek exploded, thus forming the asteroid belt.

Most of this information was channeled.