Waechter

 was a human colonist from the planet Agelaos. He was the last remaining human on the planet and he was tasked with keeping the beacon, which warned people to stay away from the planet, in check. Through the story, it is discovered that the mutants on the planet were once humans and that they were mutated by the billions of types of alien DNA being filtered through the wormhole above the planet. The only thing keeping Waechter alive was the chip in his neck which also kept him from aging. When the Doctor deactivates the chip, Waetcher begins to mutate. He eventually completes his mutation and the Doctor and Martha leave him on Agelaos with the rest of the mutated humans.