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Humans are tool users and through their ingenuity were able to hunt much larger animals than themselves.
Specifically, I would like to have a race of sentient beings based on octopuses. They use tools, and are able to make nets and traps for other sea-life. They can plant crops and can hunt and kill creatures much larger than themselves by team work. They never emerge from the ocean. All their activities take place under water.
What would have to happen for such creatures to evolve or is there something that would prevent their appearance?