If it was all environment, then why do we see the same general order of cognitive ability across societies? Societies dominated by different groups and that social dominance doesnt produce different results or even mean that group actually has the highest IQ (asians in Western Nations for instance). At some point the disadvantaged would not be so disadvantaged, at some point the environmental disparities would be less and we'd see some sign that environmental determinism matches observed reality. And the variation we do see is usually due to selection effects or regional differences.
 
I say this because we do see IQ parity when conditions improved...among whites. Eeast Germany caught up to West Germany in a decade when the big environmental barriers fell with the wall. We do not see this in other places despite the $$$ going into making it so. We instead see ever more abstract systems of oppression and deprivation to explain away the failure.
 
no special reason or observation to make us default to innate equality. In fact, when the various permutations of man did meet other groups he assumed all sorts of differences were innate to some degree, including a vague concept of intelligence: ability to accumulate knowledge, mental quickness, and inventiveness. Now this was primitive man and we surely know better but...why? And why since WWII? We know a moral reason to change our minds from that time but no new knowledge which has lasted, so Boas is out (though Nazi Germany was hardly unique for practicing racialism or race realism, except that Aryanism was denounced by other racialists and race realist societies as quackery and isnt the forebear of modern race realism)