Haemovore time stream
In one timestream which was a possible future relative to 20th century Earth, the Haemovores evolved from humans, having been mutated by the chemical slime of a polluted Earth. This time stream was accessible to time travellers. (PROSE: Goth Opera, TV: The Curse of Fenric)
Behind the scenes
History of Mankind according to Doctor Who claimed that the Haemovores were a part of the familiar, linear timeline of the Doctor's universe, placing their existence on a polluted Earth abandoned by humanity between the years 5000 and 10,000. It is further stated that the Haemovores went extinct as the planet's ecological balance was restored, leaving it to be reclaimed by humanity.