But why did they become intelligent in the first place? Why did this one group of mollusks, among an otherwise slow and dim-witted dynasty of snails, slugs, clams, oysters, and mussels, evolve into creatures that are famed for their big brains? These are hard questions to answer, especially because cephalopods aren’t just weirdly intelligent; they’re also very weird for intelligent animals.

Members of the animal kingdom’s intelligentsia tend to be sociable; indeed, the need to remember and manage a complex network of relationships might have helped drive the evolution of their brains. Smart animals also tend to be long-lived, since a large brain both takes a long time to grow and helps an animal avoid danger. Apes, elephants, whales and dolphins, crows and other corvids, parrots: They all share these traits.

Cephalopods do not. With rare exceptions, most of them are solitary animals that aren’t above cannibalizing one another when they meet. Even those that swim in groups, like some squid, don’t form the kinds of deep social bonds that chimps or dolphins do. Cephalopods also tend to live fast and die young. Most have life spans shorter than two years, and many die after their first bout of sex and reproduction.

Somewhat relevant video. Also, if you ever have a chance to watch a nature documentary on cephalopods, I highly recommend it. It's one thing to know how beautiful and intelligent these things are, but to actually see them in action can bring a whole new level of appreciation.