This series of novels revolves around a galactic civilization called the Culture, which is mostly made up of various species of humanoids and sentient machines. Though there are various species of humanoids, most of them are described as being similar to humans on Earth. I thought at first that the first novel I read in this series was set many thousands of years in the future, but I found out in the end that it is set around the 13th century AD (there are excerpts from a history published for Earth in the 22nd century that use our dating system at the end of the book).
So, how is there an 8000+ year old civilization of essentially human beings scattered across our galaxy long before mankind left Earth? I see a few possible explanations.
My favorite idea is that humans left Earth sometime in our prehistory, spread rapidly, and speciated. This fits in with what I know of the setting so far, as many civilizations interfere with primitive races on other planets - maybe someone took some cavemen as pets and they got out of control.
Another possibility, which I’m not sure fits with the setting as I know it so far, is time travel - maybe humans from Earth travel back in time sometime in our future, and start over in our prehistory.
Then there’s the possible explanation that parallel evolution lead to humanoids evolving elsewhere in the galaxy, but I don’t think this fits with the setting, which is fairly ‘hard’ SF. This Star Trek-ish explanation doesn’t seem to fit, especially as the book has several believably alien alien species.
So, does this ever get explained? I know there is one novella where an agent from the Culture comes to modern-day Earth, but the collection it is in was checked out when I last went to the library - maybe it explains it. I know there are some fans of the series on this board (that’s why I decided to check it out), and I was hoping someone could explain it to me.