Ctenophora Rewriting 750 Million Years Of Neural Evolution

Ctenophora were long considered just a kind of jellyfish. Turns out that was a gross mistake. Indeed a Russian immigrant to the USA, Leonid Moroz, found that these animals were unrelated to jellyfish. In fact, ctenophora are so profoundly different from any other animal on Earth, that it has been discovered they are much older, and unrelated, to sponges (previously sponges were thought to be by far the oldest animals; now this is known to be wrong).

In 1995, Moroz tested the nerve cells of ctenophora for the neurotransmitters serotonin, dopamine and nitric oxide, chemical messengers considered the universal instruments of the neural language of all and any animals. He didn’t find any of them.

Ctenophora were already known for having a serious nervous system, complete with neurons; but these first experiments by Moroz showed that ctenophora nerves are built from molecular building blocks – different from any other animal – using ‘a different chemical language’! says Moroz: he calls these animals ‘aliens of the sea’.

If vertebrates had not appeared, 200 million years after ctenophora, probably confining the latter into an ecological niche, civilization may have evolved from ctenophora.

An obscure force seems to compel the apparition of complex nervous systems to evolve. It is universal – not just on Earth, but also on inhabited exoplanets. And I will show roughly what it is, and where it comes from in a companion essay (to which this one is introductory).

Jellyfishes use muscles to flap their bodies and swim. Whereas ctenophora use thousands of cilia to swim. They can be very small, but the largest are 1.5 meter long (5 feet). Jellyfishes sting, ctenophora capture prey using two sticky tentacles that secrete glue. Ctenophora ambush their prey.  

Studies of ctenophora, starting 130 years ago, showed neuron masses, and, more recently, what looked like synapses. 

Ctenophore. It looks as if the ancestors of vertebrates MUSCLED out (serious pun intended!) the ctenophora. With sheer muscle power the cilia smarts ctenophora were thrown into a niche!

Moroz finally was able to make a “transcriptome” of the DNA of ctenophora in 2007.   5,000 or 6,000 gene sequences were actively turned on in the animal’s nerve cells. His team showed that Pleurobrachia lacked the genes and enzymes required to manufacture neurotransmitters seen in other animals. These missing neurotransmitters included the ones that Moroz found to be absent back in 1995 – serotonin, dopamine and nitric oxide – but also acetylcholine, octopamine, noradrenaline, etc. Ctenophora also lacked genes for receptors that to respond to conventional neurotransmitters.

As Moroz team put it in Nature:

“The origins of neural systems remain unresolved. In contrast to other basal metazoans, ctenophores (comb jellies) have both complex nervous and mesoderm-derived muscular systems. These holoplanktonic predators also have sophisticated ciliated locomotion, behaviour and distinct development. Here we present the draft genome of ten… ctenophore transcriptomes, and show that they are remarkably distinct from other animal genomes in their content of neurogenic, immune and developmental genes. Our integrative analyses place Ctenophora as the earliest lineage within Metazoa. This hypothesis is supported by comparative analysis of multiple gene families, including the apparent absence of HOX genes, canonical microRNA machinery, and reduced immune complement in ctenophores. Although two distinct nervous systems are well recognized in ctenophores, many bilaterian neuron-specific genes and genes of ‘classical’ neurotransmitter pathways either are absent or, if present, are not expressed in neurons. Our metabolomic and physiological data are consistent with the hypothesis that ctenophore neural systems, and possibly muscle specification, evolved independently from those in other animals.”

[Nature, June 2014. The ctenophore genome and the evolutionary origins of neural systems]

Further studies have confirmed that ctenophora have evolved earlier, and completely independently of other animals.

Ctenophora lack entire classes of genes that had been 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 vertebrate.  Such genes are present in simple sponges and placozoa.

Ctenophora are the oldest type of animal known! (Moroz tried to publish a paper in 2009 which implicitly led to that conclusion; it was rejected. He then did more refined studies which led to the 2014 Nature paper.)

Moroz now counts up 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 ctenophora.

“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, different genes and proteins ensembles got elected through random gene mutations, to take part in building a nervous system. The details are completely different, yet, the big picture is the same!

And that’s no accident, as I will argue, there is an underlying Quantum force pushing towards intelligence… Thus Lamarck was right.

Moroz rejected much of what he was taught. Because his ‘initial hypothesis was exactly what was in the textbooks’, moving to the correct way of thinking about ctenophora took him 20 years.

Science is truth, but truth is not obvious. And searching for it is even more demanding.

Patrice Ayme’

Tags: Ctenophora, Evolution, Morotz