Octopuses are the superstars of the invertebrates because of their astonishing intelligence. They’re considered mollusks, like clams or snails. Mollusks probably first appeared about 550 MYA and remained relatively simple, at least in the organization of their nervous systems, for hundreds of millions of years. One branch, the cephalopods, eventually evolved a complex brain and sophisticated behavior and may have reached something close to the modern form of an octopus around 300 MYA.
Octopuses, squid, and cuttlefish are true aliens with respect to us. No other intelligent animal is as far from us on the tree of life. They show us that big-brained smartness is not a one-off event, because it evolved independently at least twice—first among the vertebrates and then again among the invertebrates.