Diverse groups of animals rely on the same genes to ensure that limbs form as they should.

Cuttlefish and their kin form tentacles by deploying the same genes that direct the growth of arms in humans and legs in spiders.

Evolutionary biologists agree that limbs evolved independently in cephalopods, such as cuttlefish and octopuses; arthropods, such as spiders; and vertebrates, such as mammals.

However, Martin Cohn and his colleagues at the University of Florida in Gainesville showed that the same genes are active during tentacle formation in cuttlefish as in leg development in vertebrates and arthropods. When the researchers blocked those genes, they found that the genes have similar functions across the various groups.

For instance, when developing cuttlefish couldn’t activate a gene that designates ‘top’ and ‘bottom’ in arthropod legs, the cephalopods formed suckers on the wrong side of their tentacles. And the gene that controls the number of fingers in humans also controls the number of rows of suckers in cuttlefish.

The results suggest that genetic programs driving appendage development have been conserved for more than 500 million years, even though the appendages themselves have not.