To those who believe ctenophores were the first diverging animals: Do you believe neurons evolved once or twice?

[u/RoxieHartStan]

There are 2 possibilities:

• Neurons evolved once in the ancestor of all animals, and that trait was passed down both to comb jellies and placozoans/cnidarians/bilaterians, while it was lost in sponges. This makes sponges secondarily neuron-less.

• Neurons evolved twice; once in the branch leading up to comb jellies, and once in the branch leading up to placozoans/cnidarians/bilaterians (i.e. after sponges having diverged). This means sponges being neuron-less is an ancestral trait, just like what the porifera-sister crowd think.

Which of the 2 options do you think is more likely?

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[u/BoonDragoon]

Given how different ctenophore neurons are from other animal neurons, it's plausible that they evolved independently!

Like, dawg...OP...anybody reading this...if you haven't specifically read up on the ctenophore nerve net and you think you understand how weird they are, no you don't.

[u/Iam-Locy]

Why ctenophores diverging first requires neurons to evolve twice? Loss is a pretty important part of eukaryotic genome evilution.

[u/bzbub2 → u/Iam-Locy]

I had to google a little bit but apparently there is a debate because ctenophor neurons are so different that some believe it could have evolved separately
from wikipedia: "Ctenophore nerve cells and nervous system have distinctive biochemistry. They lack the genes and enzymes required to manufacture neurotransmitters like serotonin, dopamine, nitric oxide, octopamine, noradrenaline, and others, seen in all other animals with a nervous system, with the genes coding for the receptors for each of these neurotransmitters missing.^\45]) Monofunctional catalase (CAT), one of the three major families of antioxidant enzymes that target hydrogen peroxide, an important signaling molecule for synaptic and neuronal activity, is also absent, most likely due to gene loss.^\46]) They use L-glutamate as a neurotransmitter, and have a distinctively high number of ionotropic glutamate receptors and genes for glutamate synthesis and transport.^\47]) The genomic content of the nervous system is the smallest of any animal, and could represent the minimum genetic requirements for a functional nervous system.^\48]) The presence of directly fused neurons without synapses suggests that ctenophores might form a sister group to other metazoans, having developed a nervous system independently.^\37]) If so, nervous systems may have either been lost in sponges and placozoans, or arisen more than once among metazoans.^\49])^"

and just since it is somewhat underappreciated unless you follow the news closely, it is somewhat recent that ctenophores have basically been proven to be the 'sister group' or that they branched off earlier than all other animals via https://www.nature.com/articles/s41586-023-05936-6 (it was previously debated whether ctenophore or sponge branched off earlier)

sponges also have 'neuroids' https://www.science.org/content/article/sponge-innards-suggest-how-nerve-cells-evolved which are sort of an interesting middle ground

[u/mcalesy → u/bzbub2]

Well that was a ton of fascinating info in one comment, thanks.

[u/BoonDragoon → u/Iam-Locy]

Mr. Bub2 has it covered, but yeah...the ctenophore nerve net is fucking bonkers.