Chimera were the creations of the Silurian Mortakk. Created on an Earth where the Doctor was able to stop the Brigadier from blowing up the Silurians at Wenley Moor, they represented a possible genetic future for the Earth. Mortakk alleged that they would be more adaptable than either homo sapiens or homo reptilia alone, since they could "exist underwater, in blinding heat, in freezing cold".
Mortakk's plan, however, was not simply to introduce a new species alongside humans, Silurians and Sea Devils. Instead, he planned to convert individuals then alive on the surface of the Earth through the use of a mutagenic gas. The gas, however, only had a 20% success rate, so most of the planet's sentient population would be wiped out in the process.