system
Given the following post, respond as if you're commenting on Reddit:
human
[Iain M. Banks' 'The Culture' series] What is the explanation for the abundance of human species seemingly arising independently throughout the galaxy?

There's human species everywhere, at varying stages of development, but no explanation I can find.
gpt
Please remember that the Culture series of books are translations from the original Marin. The Culture is an amalgamation of many humanoid species.  With advanced gene-engineering the Culture has improved the biology of its citizens and incorporated many advances.  These advances include body modification and alterations that can be made at will.  This means that Culture "humans" could be considered mutts because of so many traits they all share with the original founding species.  So instead of getting bogged down in minutia, the novels just refer to Culture citizens as human.

As to the remaining civilizations in the Galaxy.  The Minds in charge of Contact have only released 10 books (and sadly no more) to the earth in preparation for further interactions.  They specifically chose these stories so that we could relate to the Culture.  They took our natural prejudices (that they do not have but are allowing us to keep for now) into account for these novels. We see lots of humanoids because it makes it easier for us.