These ideas remain hypotheses for now. There are lots of other theories that attempt to explain human brain size reduction. However, quite a few of them become implausible if brain shrinkage really did start as recently as 3,000 years ago. A good example is domestication. Dozens of different animals that have been domesticated, including dogs, have smaller brains than their wild ancestors. But human self-domestication is estimated to have occurred tens of thousands, or even hundreds of thousands, of years ago – long before the big brain shrink.