If Cassandra was the last human in the events of 'The end of the world', how is that possible that humans were exist a few years later during the events of 'New Earth' and 'Gridlock'

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.