One feature all octopuses lack is a backbone. They’re invertebrates, part of a vast lineage of cold-blooded animals including jellyfish, crustaceans, insects and spiders.
Within the invertebrates, octopuses are molluscs. Their close living relatives include slugs and snails (the gastropods), clams, oysters and mussels (bivalves), and closest of all are the squid, vampire squid, chambered nautiluses and cuttlefish – all types of cephalopod (pronounced with either a soft or hard ‘c’, your choice).
Among molluscs and the other invertebrates, octopuses are truly remarkable. They have enormous brains and behave in complex ways that we usually only see in backboned animals, the vertebrates, including us humans.
And yet we last shared a common ancestor with molluscs at least 600 million years ago, back when complex animal life was just getting going. Probably that ancestor was a small, worm-like creature, although we can’t be sure: no fossils have been found. Scientists have estimated when they existed by reading ‘genetic clocks’. Genetic codes gradually change across generations of all living creatures and by comparing DNA from any two species you can estimate when they shared an ancestor.
Following that ancient split, animals with and without backbones parted ways, forming two distinct groups. Roughly 525 million years ago, there were tiny, eel-like creatures that were probably the first vertebrates. There was also an assortment of swimming, scuttling, shelly invertebrates. This was the beginning of two immense, independent strands of life on Earth that have existed for hundreds of millions of years.