I once read a theory proposing that primates got their minds from birds due to sharing trees with avians, and that viruses may have transferred genetic material horizontally from birds to primates.

It showed a chart of brain weight relative to body weight, and birds had a much higher ratio than any other animal group. Primates were on the same curve, which wasn't shared with any other mammalian group. Primates were literally on the avian curve.

But I don't remember where I saw that. I searched for it on Google and found nothing.