Hedron (species)

The Hedrons were an advanced humanoid race, native to the planet of the same name.

When the Hedrons got ahold of a time cannon, technology only Gallifrey was meant to have, they used it cast out genetic microbes that their immune systems had no defence against by jettisoning them into prehistoric Earth. These genetic microbes would have prevented the evolution of humanity, and when the Sixth Doctor and Frobisher discovered this they travelled back to prehistoric Earth to recover the sphere of microbes. The Doctor accidentally broke open the sphere, releasing the microbes into the wild.

On the return trip to Hedron, the Doctor and Frobisher learnt from a recording that the microbes the Hedrons jettisoned with the time cannon had eventually returned to them when a spaceship from Earth crash-landed on the planet. Hedron was contaminated with the toxic genes, and the Hedrons had gone extinct. (COMIC: Time Bomb)