As in the title - I've not so long ago discovered that such games (like Stellaris and Civilization) are very relaxing when there's no one around and you can just slowly expand, build economy and so on. I tried to do the same in Humankind, but the game forces at least one AI opponent to be present. Is there a way to start a new game with completely no AI opponents? I tried a private multiplayer, but there game also forces me to pick at least one AI.

There is still an option to pick just one on the easiest difficulty and then wipe it, but that's still not optimal. 

Two Thoughts (both imperfect):

1. Select just one opponent on a Huge map with several continents.  You'd almost certainly have a continent to yourself for the first 3 eras, along with perhaps a few uninhabited continents.  Also, with the enormous scale of the map, it increases the likelihood that it may take a LONG time to encounter your one opponent; and perhaps entirely unfeasible for him/her to initiate any contact, nevermind a military threat.
2. When you set up the game, under the Difficulty tab, choose Peaceful Mode.  I've never tried this, but from its description, I'd guess that it either greatly reduces or eliminates AI aggression.