In reality it was like this: Earth was colonized by the Zycronites, who developed the ability to travel from one space dimension to another at a period several millennia after the epoch of which we speak. They arrived here eight thousand years ago. They brought a lot of plant seeds with them, which is why we have apples and oranges, not to mention bananas--one look at a banana and you can tell it came from outer space. They also brought animals--horses and dogs and goats and so on. They were the builders of Atlantis. Then they blew themselves up through being too clever. We're descended from the stragglers.