Evolving a nervous system is easy-peasy
When scientists began examining the ctenophore nervous system in the late 1800s, what they saw through their microscopes seemed ordinary. A thick tangle of neurons sat near the animal’s south pole, a diffuse network of nerves spread throughout its body, and a handful of thick nerve bundles extended to each tentacle and to each of its eight bands of cilia... By stimulating the right nerves, researchers could even prompt its cilia to rotate in different patterns – causing it to swim forward or back.
In short, the ctenophore’s nerves seemed to look and act just like those of any other animal. So biologists assumed that they were the same. This view of ctenophores played into a larger narrative on the evolution of all animals – one that would also turn out to be wrong.
Moroz’s transcriptome and genomic DNA sequences showed that the ctenophore also lacked many other genes, known from the rest of the animal kingdom, that are crucial for building and operating nervous systems... 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.
Ctenophores provide an extreme, striking example of what is probably a general pattern: just as eyes, wings and fins evolved many times over the course of animal evolution, so too have nerve cells. 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.
https://aeon.co/essays/what-the-ctenophore-says-about-the-evolution-of-intelligence
The best thing about these types of discoveries is that they give more and more defined characteristics for alien life.
ReplyDeleteWe know that alien life is most likely based on carbon molecules and stores its energy in carbohydrates. It will have a fish-like shape in a fluid environment and now we know that it is likely to have nerves.