According to information revealed by the leader of the Nebulan effort on Earth, Fumio Sudo, many years ago Space Hunter M was populated by an advanced race very similar to humanity. Eventually, the race polluted the planet so much that they became extinct, while insect-like creatures similar to cockroaches were the only species on the planet to survive. Over time, these insects evolved into the dominant race on the planet, developing extremely advanced technology and weaponry. With conditions on their home planet becoming more and more adverse, the Nebulans intended to take over Earth to use it as their new homeworld. However, in order to survive on Earth and hide their true appearance, the Nebulans needed to use the bodies of deceased humans as 'flesh suits.' Thus, the Nebulans decided to exterminate humanity, using World Children's Land as a disguise for their base of operations.
After Gigan and King Ghidorah were forced to retreat, and the aliens on Earth were killed, the invasion attempt was halted, but the Nebulans on Space Hunter M still survived, even loaning Gigan to their allies the Seatopians a year later.