The argument I've heard is that humans fill the "intelligent" ecological niche. These niches tend to be winner-take-all, so any species that got close to compe

The argument I've heard is that humans fill the "intelligent" ecological niche. These niches tend to be winner-take-all, so any species that got close to competing (Neanderthals) would slowly die-out. That said, there should be room for an intelligent species in the ocean, too.