Convergence: an animal outside standard descent (Introduction)

by David Turell , Tuesday, August 01, 2017, 19:03 (2463 days ago) @ David Turell

A strange animal appearing early in evolution, thought to be related to sponges, is not. The Ctenophore has much DNA that is part of the general pattern, but has a totally different nervous system, totally unrelated to all others and ours:

https://aeon.co/essays/what-the-ctenophore-says-about-the-evolution-of-intelligence?utm...

"This type of animal, called a ctenophore (pronounced ‘ten-o-for’ or ‘teen-o-for’), was long considered just another kind of jellyfish. But that summer at Friday Harbor, Moroz made a startling discovery: beneath this animal’s humdrum exterior was a monumental case of mistaken identity. From his very first experiments, he could see that these animals were unrelated to jellyfish. In fact, they were profoundly different from any other animal on Earth.

"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.

"The ctenophore was already known for having a relatively advanced nervous system; but these first experiments by Moroz showed that its nerves were constructed from a different set of molecular building blocks – different from any other animal – using ‘a different chemical language’, says Moroz: these animals are ‘aliens of the sea’.

"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. This separate pathway of evolution – a sort of Evolution 2.0 – has invented neurons, muscles and other specialised tissues, independently from the rest of the animal kingdom, using different starting materials.

***

"Once you repeat the experiments, says Moroz: ‘You start to realise it’s a really different animal.’ He surmised that the ctenophore was not just different from its supposed sister group, the jellyfish. It was also vastly different from any other nervous system on Earth.

***

"We all use neurotransmitters,’ he says. ‘From jellyfish to worms, to molluscs, to humans, to sea urchins, you will see a very consistent set of signalling molecules.’ But, somehow, the ctenophore had evolved a nervous system in which these roles were filled by a different, as-yet unknown set of molecules.

***

" 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.

***

"All of this pointed to a stunning conclusion: despite being more complex than sponges and placozoans – which lacked nerve cells and muscles and virtually every other specialised cell type – ctenophores were actually the earliest, oldest branch on the animal tree of life. Somehow over the subsequent 550 to 750 million years, the ctenophore had managed to evolve a nervous system and muscles similar in complexity to those of jellyfish, anemones, sea stars and many types of worms and shellfish, cobbled together from an alternative set of genes.

***

"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.

***

"Simon Conway Morris, a palaeontologist at the University of Cambridge, has stressed the importance of evolutionary convergence: that evolution tends to arrive at the same solutions over and over again, even in distant branches of the animal tree, and even when the proteins or genes used to build a similar structure are not themselves related.

***

"The phylum of ctenophores isn’t quite that exotic. It is based on the same basic chemistry that we share, but it still represents a shadow biology for animals. Ctenophores are a long-lost cousin that we didn’t even know we had.

"Because the ctenophore invented brains and muscles using a set of proteins and genes so different from any other animal that has ever been studied, it provides a unique opportunity to explore some enormous questions: how divergent can nervous systems be? Do we truly understand how life senses its surroundings and behaves?"

Comment: Convergence is a strong indicator there is a design mechanism in the evolutionary process. There may have been one or more initial starts to life but the concept of common descent needs to recognize, the chain of relationship is not so common.