As Theodosius Dobzhansky once said, "nothing in biology makes sense except under the light of evolution". Cephalopoda is one of the eight Classes in the Phylum Mollusca. The class is divided in two extant subclasses: Nautiloidea (which contain only one species, the nautilus, a basal animal with external shell, tentacles with no suckers and eyes without cornea or lens) and Coleoidea (which contains octopuses, squids and cuttlefishes). Coleoids first appeared on the fossil record in the Devonian period, about 430 million years ago.

At that time the population of cephalopods was declining due to a competition with jawed fish. Previously, fish did not have jaws (Agnatha). This new trait made them more efficient in hunting, leaving cephalopods behind. The development of intelligence was the key for coleoid success in these new, challenging times.

It is only natural that octopod nervous system is organized differently from ours: they faced a different challenge from our ancestors, lived in a different environment and had a different starting point. Gastropods, mollusks like snails and sea hares, are the sister group to cephalopods in the molluscan phylogeny ("family tree"). We have known since the 1970's that sea hares can learn by conditioning and can be trained to avoid certain stimuli. They have been used as an animal model for memory studies since then, and they share some neural characteristics with their sister group. The cephalopod nervous system is a modified, "enhanced" version of what they've received from their molluscan heritage. We humans, on the other hand, have a chordate ancestor, with no neurological relation with mollusks. While mollusks have ventral neural strings, we have dorsal neural tubes. And that difference is really, really huge.

If the starting points are so different, why have octopuses turned out to acquire abilities only found in vertebrates? Well, that is due to a process called convergent evolution. When a group faces an evolutionary problem, it will probably be solved by some optimal solution found during a "try and error" iteration. If different groups face similar challenges, they will most likely find homologous solutions. For example, bats and birds have similar adaptations for flight. Remember, evolution is not guided or rationalized. Check this TED-Ed lesson for a quick review of evolutionary mechanisms and this one for clarification of common misconceptions about this subject.

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