10 Darkest Alternate Timelines In Doctor Who
9. Humanity Evolves Into Haemovores (The Curse Of Fenric)
Spooky Seventh Doctor serial The Curse Of Fenric introduced the Haemovores, monstrous vampiric creatures who enjoy snacking on human blood.
These creatures are clearly quite humanoid in their overall appearance, and this is no coincidence - it's because, in one possible timeline, these Haemovores are what humanity will ultimately evolve into.
In The Curse Of Fenric, Fenric's evil plan is to poison the world with a dangerous toxin. If he is successful, then this planet-wide pollution will destroy humanity, and - in half a million years - humans will have fully transformed into Haemovores.
However, after Fenric was defeated by his Haemovore ally, this possible future didn't come to pass.
Planet-wide destruction causing a possible future where humanity turns into bloodthirsty beasts was something that NuWho episode Orphan 55 also explored, only, in that case, the cause was global warming, not a deadly toxin.