Evolution: a different view (Introduction)

by David Turell @, Tuesday, April 28, 2015, 15:15 (1726 days ago)

Do we really picture evolution as it really happened, or do we classify animals by prejudiced belief? DNA research is upending evolutionary cladistics relationships.


"Haddock says that ctenophores are now becoming trendy, in part because they appear to be the sister group of all other animals—that is, the earliest clade to have branched off from the main trunk of animaldom, hundreds of millions of years ago. For centuries, that honor had fallen to the sponges, but in 2008 Dunn, a Brown University biologist, compared genes from twenty-nine animals belonging to several phyla and concluded that ctenophores, not sponges, were the first to diverge. “That really raised some eyebrows,” he said. “Some people took that to mean that our analysis was intrinsically flawed.” But more and more evidence has since piled up in support of that view, including the first complete ctenophore genomes.

"This revised tree, with ctenophores on the earliest branch, complicates several once tidy stories about the evolution of animal traits, notably the nervous system. Sponges lack neurons entirely, but their genes seem to allow for chemical signalling of some kind. Ctenophores have nervous systems but lack the genes that other animals use to build neurons and neurotransmitters. If sponges are the earlier of the two clades, the story unfolds neatly: they had the genetic building blocks for a nervous system, which ctenophores elaborated and bilaterians went to town on. But this narrative shatters if ctenophores branched off first. It could mean that they evolved nervous systems independently from all other animals, including us. Meanwhile, sponges either never developed true nerves or started off with nerves and lost them (after all, what need does a sedentary filter feeder have for such an extravagance?).

"This is a much tougher reality to accept. The idea of one group of supposedly primitive animals going off-script and inventing a different nervous system, and then a second group actually losing theirs, is practically unconscionable. “There's a tendency to think that, in the evolutionary lottery, humans lucked out,” Dunn told me. “We have cool articulated skeletons with complex muscles, and brains that we're very proud of.” But the belief in an orderly, stepwise progression—sponge to ctenophore to bilaterian—is “complete rubbish,” he said. “It's deeply flawed, but it's there, inserting its imprint in how we talk and think.'”

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