What the planet was before. [Issac Asimov writings]

"Why, what was it before that?"
"Empty? At least of human beings."
"That's hard to believe."
"It's true. The old records show it."
"Where did the people come from who first settled Trantor?"
"No one is certain. There are hundreds of planets which claim to have been populated in the dim mists of antiquity and whose people present fanciful tales about the nature of the first arrival of humanity. Historians tend to dismiss such things and to brood over the `Origin Question.
"What is that? I've never heard of it."
"That doesn't surprise me. It's not a popular historical problem now, I admit, but there was a time during the decay of the Empire when it roused a certain interest among intellectuals. Salvor Hardin mentions it briefly in his memoirs. It's the question of the identity and location of the one Planet from which it all started. If ,we look backward in time, humanity flows inward from the most recently established worlds to older ones, to still older ones, until all concentrates on one-the original."
Trevize thought at once of the obvious flaw in the argument. "Might there not have been a large number of originals?"
"Of course not. All human beings all over the Galaxy are of a single species. A single species cannot originate on more than one planet. Quite impossible."
"How do you know?"