Xenophanes

If cattle and horses, or lions, had hands, or were able to draw with their feet and produce the works which men do, horses would draw the forms of gods like horses, and cattle like cattle, and they would make the gods' bodies the same shape as their own.

The historical novelist has to consider what has actually happened, while the SF writer is dealing in possibilities, but they are both in the business of imagining a world unlike our own and yet connected to it.