Gene Bombs being used during the Xorda's attack on Earth
The Gene Bomb is a weapon used by the Xorda, which causes the de-evolution of a species on a planet. The Gene Bombs were used during the Xorda's attack on Earth which resulted in the de-evolution of the human race into the four-fingered Overlander species (although it was apparently supposed to de-evolve humans much further back, to pre-intelligence). It also resulted in the rapid evolution of the animal kingdom, blending their DNA with that of humans to form the Mobian species. Additionally, the Gene Bombs mixed with various beryl deposits beneath the planet's surface, resulting in an environmental cataclysm known in Echidna theology as the first Day of Fury, as well as the creation of the Chaos Emeralds (StH: #124, #125, #148)