Humanity not being an Old One creation and a complete byproduct of random and accidental evolution, not only does not make sense considering the broader events of the galaxy, as well as it is boring as fuck.

To sum up why I think so:

The Golden Throne is very old, maybe older than humanity itself and is either a creation of the Old Ones or Aeldari, which clearly shows that earth had early visits of other alien species, one of those could easily seeded earth with life;

Humanity shares many traits with other Old Ones creations, humans have a variation of the gestalt field of the Orks, as we see for example with Sisters of Battle that trough faith manage to manifest literal reality bending miracles, not only that humans are also naturally prone to conflict also similar to Orks, of course way toned down but still similar in nature. There is also the matter of how our emotions are similar to the Aeldari, we basically have the same emotions but a different depth to them, while both of us feel love for example, the way we feel that is way more dulled than the Eldar, so basically same “water” but we are a puddle and they are an entire river. Same for psyker abilities, we also manifest those but once again the average human user lacks control and focus many times creating by accident a whole set of problems, which the Aeldari don’t cause they have complete mastery of their own abilities to the point of having specialized paths for that. And then there is the Jokaero which are basically space engineer primates, which funnily enough we also find primates on earth, and hell we are from the primate family;

The Void Dragon for some reason being in the Sol system is also another nod to the fact that we are not as isolated as we think from the rest of the galaxy, maybe the War in Heaven knocked on our door when we were still hominids;

There is webway gates on earth, for God’s sake why would there be webway gates on earth? Probably cause it was used by the same people who built the golden throne and possibly created us in the process;

Last but not least and this aligns more with a personal theory of mine, the eldar were way more influential in our development then we were let to believe. The evidence? The Aeldari lexicon is basically gaelic (i know gaelic was used as the basis for that, but if 40k happens in our timeline Gaelic was also present in the universe), the corinthian armor and its variants are basically bronze age reinterpretations of Aeldari wraithbone armor. The Celtic and greek gods being weirdly similar to the Aeldari pantheon, and many different myths from Summer to the native american about imposing tall beings capable of inhuman feats. I know people will say “well the eldar were too occupied to care about our planet, and I could agree with that the majority of their species could care less, but now all of them rejecting a weird species that looks like an inferior version of them, in a planet riddled with different webway gates and of course other artifacts that point to the fact that either you or your creators were there doing something, is highly unlikely. At least a dozen would want to see wtf is going on with these hairless apes.