Octopuses hail from a humble neighborhood in the taxonomic
tree: the mollusks, which include snails, clams and other
dimwitted animals. Yet octopuses are clever critters, known
to use tools in the wild, play in captivity and even watch what’s
happening outside their aquarium.
Scientists have long wondered how they evolved such smarts
independently of mammals and birds. The sequencing of the octopus
genome, published in August, offers a surprising answer.
The genome shows that octopuses have the same small repertoire
of neurotransmission genes as lower mollusks. This limited set of
nervous-system building blocks should have restricted their ability to
develop advanced intelligence, says Daniel Rokhsar, an evolutionary
biologist at the Okinawa Institute of Science and Technology Graduate
University in Japan.
But Rokhsar and his collaborators found two vastly expanded gene
families in octopuses: protocadherins, which steer neurons to connect
into complex circuits during development, and C2H2 zinc finger
transcription factors, which turn other genes on and off with exquisite
precision. This could have enabled the genome to be expressed “in a
more complicated way than in a clam or a snail,” says Rokhsar. These
two gene families likely allowed the octopus to build unique neural
circuits serving functions such as memory, navigation and planning.