Hedron was a planet where the advanced inhabitants conducted disastrous experiments with time using a time cannon, technology only Gallifrey was meant to have. These experiments were intended to cast out genetic microbes that their immune systems had no defence against, but at the expense of spurning them upon other places and periods in time.
The Hedrons jettisoned the microbes into prehistoric Earth, which would have stopped the human race from evolving. the Sixth Doctor and Frobisher found themselves on Hedron when The Doctor’s TARDIS materialised near the operational time cannon. After discovering the effects the genes would have on humanity, they travelled back in time to recover the sphere of microbes. In this prehistoric era, an aquatic dinosaur tried to eat Frobisher. Acting on instinct, The Doctor absentmindedly threw the nearest object at hand into the dinosaur to scare it off; it was the sphere containing the genetic microbes. It broke open and irreversibly damaged prehistoric Earth.
However, on the return trip to Hedron to destroy the time cannon so the Hedrons could not inflict more harm across time, The Doctor and Frobisher learnt the Hedrons went extinct from a recording. The microbes they sent back were eventually returned when a spaceship from Earth crash-landed on Hedron containing the exact same toxic genes. The Doctor destroyed the time cannon, then left with Frobisher in his TARDIS to meet back up with Peri Brown at a Dodgers baseball game he had graciously dropped her off at. (Time Bomb)