Revisiting convergence: best example (Introduction)

by David Turell , Saturday, December 02, 2017, 20:38 (2335 days ago) @ David Turell

Ctenophores have strange neurons. Not like any others:

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

"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.

"This animal, the ctenophore, provides clues to how evolution might have gone if not for the advent of vertebrates, mammals and humans, who came to dominate the ecosystems of Earth. It sheds light on a profound debate that has raged for decades: when it comes to the present-day face of life on Earth, how much of it happened by pure accident, and how much was inevitable from the start?

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"Unlike the jellyfish, which uses muscles to flap its body and swim, the ctenophore uses thousands of cilia to swim. And unlike the jellyfish with its stinging tentacles, the ctenophore hunts using two sticky tentacles that secrete glue, an adaptation with no parallel in the rest of the animal kingdom.

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"So he fetched a net and fished a dozen or so ctenophores, a species called Pleurobrachia bachei, from the water. He froze them and shipped them to his lab in Florida. Within three weeks, he had a partial ‘transcriptome’ of the ctenophore – some 5,000 or 6,000 gene sequences that were actively turned on in the animal’s nerve cells. The results were startling.

"First, they showed that Pleurobrachia lacked the genes and enzymes required to manufacture a long list of neurotransmitters widely seen in other animals. These missing neurotransmitters included not just the ones that Moroz had noted back in 1995 – serotonin, dopamine and nitric oxide – but also acetylcholine, octopamine, noradrenaline and others. The ctenophore also lacked genes for the receptors that allow a neuron to capture these neurotransmitters and respond to them.

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"The ctenophore was turning out to be unique from other animals in far more than just its nervous system. The genes involved in development and function of its muscles were also entirely different. And the ctenophore lacked several classes of general body-patterning genes that were 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 human. These gene classes were present in simpleton sponges and placozoa – yet absent in ctenophores.

"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.

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"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.

"‘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, a different subset of genes, proteins and molecules was blindly chosen, through random gene duplication and mutation, to take part in building a nervous system.

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"The late Harvard palaeontologist Stephen Jay Gould argued in his book Wonderful Life (1989) that accidents matter: that the evolutionary history of animals has been shaped by decimation as much as by innovation. He pointed out that the Cambrian world 570 million years ago contained more groups of animals, called phyla, than exist today. Those diverse branches in the early animal tree were steadily pruned by mass extinctions. Those extinctions fuelled evolution by opening ecological niches that surviving animal groups could diversify into – providing opportunity for innovation.

"At the same time, 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.

Comment: The process of evolution is designed to innovate complexity. Convergence proves it!