It's also been said that ancient humans were more intelligent than modern ones. In fact, both the argument for human intelligence and the wolf-dog argument have put forward the idea that being domesticated lowers intelligence (in the case of humans, it can be said we domesticated ourselves). I don't really think this is a simple hypothesis to investigate at all given the complexity of investigating intelligence.

Some references:

News Article on Human Intelligence--News article discussing this hypothesis.

Gerald Crabtree--This is the researcher I've seen quoted a lot lately on the idea that ancient humans were more intelligent than modern ones. From the article above, and looking at his published work, it sounds like this is just a hypothesis he wants to test, rather than something that he has thoroughly investigated.

News Article on Dog-Wolf intelligence--This news article has some discussion of an experiment trying to determine differences between wolf-dog intelligence.