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What the Ctenophore says about the evolution of intelligence (aeon.co)
110 points by Symmetry 27 days ago | hide | past | web | favorite | 18 comments



One of the ways that science has demonstrated the ancient roots of serotonin is by giving octopuses MDMA[0], resulting in the normally wary creatures getting all cuddly.

[0] https://www.nature.com/articles/d41586-018-06746-x


Wow that sounds ridiculous but true


I can't make sense of this:

"Moroz reached this conclusion by testing the nerve cells of ctenophores for the neurotransmitters serotonin, dopamine and nitric oxide, chemical messengers considered the universal neural language of all animals. But try as he might, he could not find these molecules. The implications were profound."

But plants make and use serotonin and dopamine

Phytoserotonin also plays a role in the following aspects of plant function: Growth regulation Xylem sap exudation Flowering Ion permeability Plant morphogenesis Regulation of ripening

from https://www.news-medical.net/health/Serotonin-in-Plants.aspx

and

The functions of plant catecholamines [seems to be a ref to dopamine] have not been clearly established, but there is evidence that they play a role in the response to stressors such as bacterial infection, act as growth-promoting factors in some situations, and modify the way that sugars are metabolized. The receptors that mediate these actions have not yet been identified, nor have the intracellular mechanisms that they activate

from https://en.wikipedia.org/wiki/Prolactostatin#Plants and

Plants and animals diverged ~1.7 billion years ago, which is plausibly why they share these neurotransmitters, yet the article seems to say we diverged from ctenophores ~ 0.5 billion years ago "If Moroz is right, then the ctenophore represents an evolutionary experiment of stunning proportions, one that has been running for more than half a billion years"

So unless serotonon and dopamine evolved in plants and animals after the split between plants and animals, (or somehow ctenophores lost these after they split from us) how is this possible?


It looks like you are correct in your assumption that serotonin evolved in the common ancestors of plants and animals, instead of being convergent evolution. [1],[2]

So it would seem as if the ctenophores lost the ability to produce these chemicals, and went on to develop a nervous system that didn't require them.

But the article is really about the evolution of the nervous system, and so the pertinent question is whether (a) the evolution of the nervous system was more recent than 0.5B years ago, in which case the two nervous systems evolved independently, or whether (b) the nervous system evolved earlier than that, and then the evolution of the ctenophores adapted its nervous system to not require serotonin and dopamine.

I think the article is suggesting (a).

1. https://www.sciencedirect.com/science/article/pii/S156973391... 2. https://en.wikipedia.org/wiki/Serotonin#Comparative_biology_...


In evolution, the current uses of a given structure or molecule is not necessarily an indication of what it originally evolved for, and in this case, whatever plants have been doing with serotonin and dopamine, it is not as neurotransmitters. When animals began to evolve neural systems, presumably these already-present molecules proved useful as neurotransmitters. The fact that ctenophores use a different set of neurotransmitters is taken by Moroz as strong evidence that neural systems evolved more than once - and furthermore, according to the article, "Moroz now counts nine to 12 independent evolutionary origins of the nervous system" on the basis of other differences. In this view, ctenophores are the last representatives of one of those alternatives.

I wonder if Moroz is harming his case by stretching it too thinly -- small differences do not necessarily require independent origins -- but I am not a biologist.


Evidently ctenophores don't use serotonin anymore, and have found some other molecule they like better for what they are doing. There is nothing special about the molecules we use, they just happen to fit the receptors we use, so switching would require changing both, which is too hard to bother with.

If you haven't watched youtube videos of ctenophores eating one another yet, you are in for a treat.


Thanks!

> youtube videos of ctenophores eating one another yet, you are in for a treat

https://www.youtube.com/watch?v=MmoChWQ6xCk



This article is deeply unsatisfying. Too much fluff for a very basic point: ctenophores are different kinds of organisms than we’ve come to expect. That’s it. “Different” appears 24 times in the article, almost every single instance of which is just, “ctenophores are different.”

There’s not much more information in this article than that. I was hoping for some compelling science writing. But this feels like a tiny story (for now) stretched way too far. In a year or five when there are some interesting hypotheses about WHY they’re different, then we’ll see.


I think you're missing the key point (and perhaps the article doesn't explain it clearly): if the nervous system is completely different from other animals, then that implies (maybe) that the nervous system did not just evolve once, and then branch from there, but evolved multiple times, each time slightly differently but with similar results.

This is called "convergent evolution," and is fairly common (though not necessarily common for big systems like the nervous system), but what it implies is that there is something fundamental about the structure of the nervous system that leads it to being evolved again and again, even if in slightly different versions.

Contrast that to other things that are evolved, let's say endoskeletons (bones). These seem fundamental to us, but (as far as I know) they only evolved once, and it would be perfectly reasonable to imagine a world where they never evolved, and all the animals had exoskeletons.

Indeed, if endoskeletons really only evolved once, then we might really have no reason to expect them in any alien species. But if something like the nervous system evolved multiple times, then we expect that to be a solution that evolution finds again and again.


Yeah, I understand all that going in to the article. I have, let's call it, an advanced beginner's understanding of neuroscience & evolutionary biology.

It was just an extremely long article, in my personal opinion, for "this is different."


I was hoping that the article would detail what those not-dopamine, not-serotonine neurotransmitters were and what they did.


Yeah, exactly the info I was looking for. "They're different" is like half the story. How do they work?


The article makes the case that they're the outgroup to the entire rest of Animalia. I don't really know what you'd consider a tiny story, but that's certainly not a tiny claim!


Maybe we’ll eventually conclude they aren’t even Animalia?

If life came to earth on meteors, there might be life from different corners of the universe on earth.


He did sequence the genome which has shown that ctenophore lacked too many nerve-related genes that are shared in whole animal kingdom, and other genes were very different too, the article explains how...isn't that satisfying answer WHY it is different?


Ciliates, such as paramecium, are another interesting group with a completely different approach to life. They are usually described as "single celled", and by implication simple, but the cell in this case is much more complex than one of ours.

Animals (and for that matter, plants) develop from a single-celled zygote, into a multicellular organism, which eventually dies, after producing gametes to merge into zygotes to continue the cycle.

Ciliates have an analogous, but totally different, approach. A newly "born" ciliate has a "micronucleus", similar to one of ours, from which it makes multiple copies of its DNA to form a "macronucleus", which does all the work. Perhaps gene expression in the macronucleus even changes in a way that learns about its environment. But eventually, the ciliate mates, or goes through a sort of self-conjugation event, dividing into more than one cell, each of which gets a new micronucleus formed by recombination. These cells discard the macronucleus, forming a new one from the new micronucleus.

It's sort of like a multicellular organism dying, since whatever the old macronucleus learned will be forgotten. (Perhaps that's the point, if the environment has changed, requiring a new approach.)


I think a possible caveat is the Ctenophore had to continue to reproduce in an environment where other organisms possessed a central nervous system and intelligence. Although I don't think it is likely, it could be the case that evolving to that level of complexity is unlikely but once one successful organism in the ecosystem possesses it, it exerts evolutionary pressure on other organisms to move towards the greater complexity as well.




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