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And Fdgsfg, that's 100.000 years for a change in skin colour, versus significantly less to create an entire new species.

The potential difference in intelligence between neanderthals and homo sapiens is nothing compared to the difference between humans and ogryns.

Furthermore, I am not saying that intelligence would win out, only that for such a massive difference to occur, it would have to be an advantage in itself (though that still wouldn't help according to me, since I still say ten thousand years isn't nearly enough for changes that drastic).

You're entitled to your opinion, of course, but unless I see a source directly supporting your claim, I'm going with what I see as a far more reasonable explanation - Chaos did it.

Really, like I said earlier, from an evolutionary perspective, ~40,000 years is still not much. But from an in-universe perspective, it doesn't matter, because apparently, at the very least on these planets and regions, evolution charged straight ahead, possibly with a blind fold. So it doesn't really matter what your opinion is on it, because we know that abhumans (bar those created artificially, such as the Afriel Strain) evolved on specific worlds under specific circumstances, and those circumstances are even mentioned in the abhuman backgrounds.

Evolution can be fun when you engineer scenarios that Earth can't, like heavy-gravity, toxic climates, etc. Also, some of the Abhumans, or at least their predecessors, could've been around on Earth well before the DAoT, the Strife, the Great Crusade, etc. meaning that they had more time to evolve. Lastly, the genetors of the Mechanicus aren't Chaos (well, not ALL of them), and I wouldn't put it past the AdMech to have done some "acceleration" to the evolution of something like Ogryns. They want something that can work in harsh, extraterrestrial environments, and so, like Space Marines, they take the machine that is Man, and modify it with enhancements to make Ogryns as we have them.

Also, comic-book logic must apply, as this is fiction. In 10,000 years, with some xenos help (the Celestials), Marvel comics mutants on Earth went from regular people to Omega-level Telepaths, human howitzers, and even more bizarre "how does your DNA apparently let you do THAT?" cheese. 40K doesn't seem to be stretching this too much worse with Orgyns, Ratlings, and what have you. The Old Ones might've modified something that infuenced some Abhuman evolution, somehow, or some ancient AdMech might've found something, and used it. Hell, Ork spores could've had an impact, since Ogryns are basically Human Orks, IMO. And yes, Chaos might've played a part, in any of many possible ways. but it is in no way more supported, or provable, at this time.

It's somewhat difficult to believe that intelligence and creativity would actually be maladaptive in any high pressure survival context. Mental flexibility and the ready ability to devise and improvise new tools and solutions to problems in a perilous environment are generally always going to be desirable to some extent. What's more plausible is that the growth regimes and metabolic alterations associated with the dramatic increases in muscle mass, bone density, etc. and other distinguishing properties of Ogryns were deleterious to the development of higher brain functions- the survival and reproductive utility of the former proved sufficiently greater than the latter in the relevant brutal alien environment (at least up to the point that the Ogryns reached), hence the end evolutionary result.