Why is the octopus so smart?

Cephalopod intelligence is widely known, but scientists struggle to understand why it evolved. The New York Times' Carl Zimmer reports on one of zoology's most fascinating questions.

About 275 million years ago, the ancestor of today’s cephalopods lost the external shell. It’s not clear why, but it must have been liberating. Now the animals could start exploring places that had been off-limits to their shelled ancestors. Octopuses could slip into rocky crevices, for example, to hunt for prey. On the other hand, losing their shells left cephalopods quite vulnerable to hungry predators. This threat may have driven cephalopods to become masters of disguise and escape. They did so by evolving big brains, the ability to solve new problems, and perhaps look into the future — knowing that coconut or clam shells may come in handy, for example.
Yet intelligence is not the perfect solution for cephalopods, Mr. Amodio suggested. Sooner or later, they get eaten. Natural selection has turned them into a paradox: a short-lived, intelligent animal.
They also like MDMA.