They were possibly a different species.... Perhaps even descendants of the New Humans.
- "The End of the World" establishes that Cassandra is the last of the "original" human species. That doesn't mean humans don't continue on, but through cross-breeding, etc. Their DNA is not the same as ours. This is also indicated in "Utopia" where the human-like species featured are said to have re-evolved to look like humans again.