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.

This combination of short lives, solitude, and smarts is unique to cephalopods. And according to a recent paper by Piero Amodio from the University of Cambridge and five of his colleagues, the traits are all linked to a particular development in the octopus’s evolutionary history: Its ancestors lost their shells.

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About 530 million years ago, an ancient group of mollusks slowly modified their protective shells into buoyancy aids by filling them with gas. With this transformation, they could more easily walk along the ocean floor, and then swim over it. They were the first cephalopods. For eons, they and their descendants kept their shells.

But about 275 million years ago, everything changed. It’s possible that competition from fast, shallow-water fish forced cephalopods to become more agile, or drove them into deeper waters where buoyant shells would have been a hindrance. For these reasons, or perhaps others, the ancestors of octopuses lost their shells entirely, while the ancestors of squid and cuttlefish internalized theirs. (The white, brittle slabs that people feed pet birds are cuttlebones, the internalized shells of cuttlefish.)

Unencumbered by a shell, cephalopods became flexible in both body and mind, according to Amodio and his colleagues. They could move faster, expand into new habitats, insinuate their arms into crevices in search of prey. “This allowed them to feed on many more kinds of food, requiring more complex foraging techniques,” Amodio says. “We think this is one of the key challenges that pushed them to become smarter.”

Losing their shells also made the cephalopods exquisitely vulnerable. One scientist described their soft, unprotected bodies as the equivalent of “rump steak, swimming around.” The rest of the ocean seemingly agrees: Almost every major group of predators eats cephalopods, including dolphins, seals, fish, seabirds, and even other cephalopods. This gantlet of threats might have fueled the evolution of the cephalopods’ amazing color-changing skin, their short life spans, and their large brains. After all, intelligence can help an otherwise defenseless creature create new defenses, as Blue Planet II’s shark-defeating octopus so ably showed.

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It’s telling that the nautilus—the only living cephalopod that still has an external shell—bucks all of these trends. It lives for up to 20 years, reproducing several times during its life. It also has a much smaller brain than its shell-less relatives, and doesn’t seem to be anywhere as smart. The loss of the shell “has been linked to so many of the adaptations that make cephalopods special,” Amodio says.