How is The Culture's universe populated by humans if current human culture on earth exists contemporaneously with it?

Question

Does Iain M. Banks explain anywhere how in the world of the Culture the galaxy came to be populated by humans and humanoid races? ... given that it can't be that the human race became a space-faring civilization and colonized the galaxy long before the time of the Culture novels.

This can't be the case because we know how the time portrayed in the novels maps to earth years. The Culture stories take place from 1331 CE to 2375 CE according to Wikipedia. In The State of the Art, Diziet Sma actually goes to earth in 1977.

Answer 1

There seems to be a heavy amount of convergent evolution towards a humanoid form in the Culture universe, but the people in the stories are indeed biologically different from Earth humans. This is spelled out for example in the novella "The State of the Art" from the story collection by the same name, in which several members of the Culture visit Earth in 1977, and after the ship's Mind absorbs a great deal of information about human society from orbit, some humanoid members of the ship undergo surgical modification so they can travel to the surface and not be seen as alien. From p. 105:

A fraction of this avalanche of data (it felt like a lot but it was actually pifflingly small, the ship assured us) was stuffed into the heads of those of us sufficiently close in physique to pass for human on Earth, after a little alteration (I got a couple of extra toes, a joint removed from each finger and a rather generalized ear, nose and cheekbone job. The ship insisted on teaching me to walk differently as well)

p. 115 mentioned another humanoid crew member named Li whose physique was too different to travel to the surface:

Li was one of those who just wouldn't have passed for Earth-human without a vast amount of physical alteration (hairy, and the wrong shape; imagine Quasimodo crossed with an ape)

Another character named Linter wants to stay on Earth, and to that end he asks the ship to "give me a set of guts more like the locals" (p. 154). Before that happened, another character had worried not just that he might be killed in some accident with primitive technologies like trucks and planes, but also "What if you're only injured and they take you to hospital? You'll never get out again; they'll take one look at your guts or your blood and they'll know you're an alien. You'll have the military all over you. They'll dissect you." (p. 131) So presumably his internal organs also differed from a human's in obvious ways.

In addition, the novel Matter includes this comment in chapter 10 about a character from a less developed society who joins the Culture:

She learned about pan-humanity, about the great diasporic welter of human-like, human-ish and humanoid species scattered throughout so much of the galaxy ... There were bogglingly large numbers just of these pan-humans (of which, of course, she was one), but they still formed less than a single per cent of all the aggregated life-mass of the greater galaxy.

And in chapter 5 of the novel Surface Detail, a character from a humanoid species called the Sichultians meets a strange man at a party:

The man looking at her appeared terribly old. He was either a much-altered Sichultian or a pan-human alien; the human type had proved to be one of the galaxy’s more repetitively common life-forms.

This answer also mentions a line from Excession about an alien planet that is said to be "one of the twenty or so planets that could fairly claim to have been one of the home worlds of the Culture".

Answer 2

It's quite clear that "human" as used in the Culture novels does not refer to homo sapiens, but rather it is a shortened form of "pan-human" which the Culture uses to refer to intelligent beings with a roughly human body plan. (Bilaterally symmetric, two arms, two legs, two eyes, head separated from torso, etc.)

There is a huge variety of what falls under the heading of "human" within the Culture:

Lenipobra was the CAT's best excuse for a medic and was rarely seen without a small screenbook which contained one of the more up-to-date pan-human medical textbooks. He proudly showed this to Horza, including a few of the moving pages, one of which showed in vivid colour the basic techniques for treating deep laser burns in the most common forms of digestive tracts.

_{Consider Phlebas, "4. Temple of Light"}

In fact the description of the crew of the Clear Air Turbulence goes to some lengths to describe the variations in what is considered "human."

• Yalson is slim, with dark skin and fair hair, and is lightly furred over her entire body;
• the Bratsilakins are heavy-set and thickly furred;
• Dorolow has strange ears and a very high voice (which she says is deep);
• Aviger has great joint mobility;
• Gow and kee-Alsorofus have grey skins and solid black eyes;
• Lenipobra has extremely long arms and frequently goes about on all fours.

A snippet of the conversation between the drone Jase and Fal 'Ngeestra points to additional structural variations:

"Assuming that we are going to win the war..." Jase said thoughtfully, "... it could lengthen the proceedings by a handful of months."

"And how many's that supposed to be?" Fal said.

"Somewhere between three and seven, I suppose. It depends whose hand you're using."

_{Consider Phlebas, "state of play: one"}

Even Horza, the Changer, is considered "human" when we see from Balveda's viewpoint:

Xoxarle seemed to have hoped some pan-human compassion would make Horza stop and save her, and so give the Idiran a few precious extra moments to make his escape; but the Idiran had made the same mistake about Horza that his whole species had made about the Culture. They were not that soft after all; humans could be just as hard and determined and merciless as any Idiran, given the right encouragement...

_{Consider Phlebas, "13. The Command System: Terminus"}

Note that there is no single "human species:"

[...]the Culture, that seemingly disunited, anarchic, hedonistic, decadent melange of more or less human species, forever hiving off or absorbing different groups of people[...]

_{Consider Phlebas, "2. The Hand of God 137"}

Jase felt pleasure at the girl's words (if not the snort), but at the same time detected in them a tinge of that mixture of contempt and patronising smugness the Culture found it so difficult not to exhibit when surveying the mistakes of less advanced societies, even though the source civilisations of its own mongrel past had been no less fallible.

_{Consider Phlebas, "state of play: one"}

Pan-humanity doesn't even meet the biological definition of "species" since most random pairings aren't genetically compatible (without technological assistance):

[...]theirs - they both knew - an almost inevitably barren cross-matching of species and cultures thousands of light-years apart[...]

_{Note: speaking of Horza and Yalson}

Answer 3

The term “human” here doesn’t mean terran homo sap. In the Culture the term is used to cover all manner of roughly humanoid species. The Culture is largely run by vast AI Minds, and to them we’re all nearly indistinguishable meat pops.

Banks discusses it here.

Answer 4

He doesn't explain it because you're wrong. The galaxy of the Culture is never stated to be populated by humans; the organic citizens of the Culture are described as humanoid, but that absolutely does not imply they are human.

The use of the phrase "human" within the Culture and any writings around it, including Banks' own commentaries, is almost entirely used as a literary device of familiarity to allow the reader to more easily understand and empathise with the organic citizens of the Culture depicted in these writings. It would be difficult for a reader of a Culture novel (a human) to become invested in the actions of a non-human sentient being that experiences life entirely differently to us, for example.