lacked the genes and enzymes required to manufacture a long list of neurotransmitters widely seen in other animals. These missing neurotransmitters included not just the ones that Moroz had noted back in 1995 – serotonin, dopamine and nitric oxide – but also acetylcholine, octopamine, noradrenaline and others. The ctenophore also lacked genes for the receptors that allow a neuron to capture these neurotransmitters and respond to them.
Pleurobrachia was missing many common proteins called ion channels that generate electric signals that travel rapidly down a nerve. It was missing genes that guide embryonic cells through the complex transformation into mature nerve cells. And it was missing well-known genes that orchestrate the stepwise connection of those neurons into mature, functioning circuits. ‘It was much more than just the presence or absence of just a few genes,’ he says. ‘It was really a grand design.’
It meant that the nervous system of the ctenophore had evolved from the ground up, using a different set of molecules and genes than any other animal known on Earth. It was a classic case of convergence: the lineage of ctenophores had evolved a nervous system using whatever genetic starting materials were available. In a sense, it was an alien nervous system – evolved separately from the rest of the animal kingdom.
The genes involved in development and function of its muscles were also entirely different. And the ctenophore lacked several classes of general body-patterning genes that were thought to be universal to all animals. These included so-called micro-RNA genes, which help to form specialised cell types in organs, and HOX genes, which divide bodies into separate parts, be it the segmented body of a worm or lobster, or the segmented spine and finger bones of a human.
Moroz now counts nine to 12 independent evolutionary origins of the nervous system – including at least one in cnidaria (the group that includes jellyfish and anemones), three in echinoderms (the group that includes sea stars, sea lilies, urchins and sand dollars), one in arthropods (the group that includes insects, spiders and crustaceans), one in molluscs (the group that includes clams, snails, squid and octopuses), one in vertebrates – and now, at least one in ctenophores.
‘There is more than one way to make a neuron, more than one way to make a brain,’ says Moroz. In each of these evolutionary branches, a different subset of genes, proteins and molecules was blindly chosen, through random gene duplication and mutation, to take part in building a nervous system.