We can look at the genetic changes and timeframe of them, and how certain mutations have spread - novel changes in our species are not that frequent on the historical timescale. People 3000 years ago are the same with respect to their biology, so they have to be "just as smart"; hominids 300,000 years ago are different in some aspects so for them we can't really know how "smart" they were, perhaps they were as smart, perhaps they were not.