In the earlier days of complex, multicellular life on Earth, animals started without any spines or brains. They only had a web of neurons scattered throughout their body. Throughout millions of years, nonetheless, that system somehow evolved.
Tunicates, or ‘sea squirts,’ are the nearest living relatives of vertebrates, and they don’t have an authentic head.
Their central nervous system is rather made up of clots of neurons in the anterior and posterior parts of their body, with a dorsal thread connecting them both. As adults, these animals look like thick sponge-like chunks, with no prominent head or tail. However, as tadpole-like larvae, their cerebrum is easier to spot.
“Tunicates are like an evolutionary prototype for vertebrates,” clarifies zoologist Ute Rothbächer from the University of Innsbruck in Austria. “Our common ancestor was probably very similar to a tunicate larva.”
Research has established that HMX genes, which encode for a pair of neurons in a tunicate tadpole’s tail, are associated with the genes that encode for lumps of neurons in a lamprey’s head.
“HMX has been shown to be a central gene that has been conserved across evolution,” declares zoologist Alessandro Pennati, who is also from the University of Innsbruck.
“It has retained its original function and structure and was probably found in this form in the common ancestor of vertebrates and tunicates,” he added.