[staunch]

That's probably closer to the truth but doesn't seem like the whole story. Modern environments are very different but it seems a stretch to claim they're always (or even often) better.

We're usually comparing modern populations to industrialized populations that lived nothing like ancient populations.

It seems plausible that some ancient populations might've had sufficient nutrition (particularly Egypt at various times) and lower pollutants (less lead, for example), and maybe come out net ahead in terms of average general intelligence.

Would be fun to know if anyone has come close to answering these questions, but it seems like a challenging problem.