In The End of The World, Lady Cassandra - a stretched out piece of skin witnessing the destruction of Earth in the year 5 billion, was widely regarded as the last of the human race. Her death in New Earth would seem to imply that she is the end of the line for the human race.
And yet, at the very end of time - 100 billion years in the future, the Tocalfane are explicitly called out as being regressed humans in Utopia, and clearly appear to be human. Indeed, this fact is the shocking twist in Last of the Time Lords is that the Master had persuaded them to come back to 20th Century earth and create a paradox by destroying their ancestors.
The question is - how can the Toclafane be human if they died out nearly 95 billion years earlier with Cassandra? Is there an in-universe explanation?