Translucent swimmers, comb jellies come in a variety of forms. New genetic data suggest that these relatively complex animals may have evolved before, not after, sponges.
[Credit: Clockwise from top left: L.L. Moroz & M. Citarella/Univ. of Florida; Dimijian Greg/Getty Images; Ingo Arndt/Minden Pictures/Corbis; Boris Pamikov/Shutterstock; Dimijian Greg/Getty Images; Casey Dunn/Brown Univ.]
COMPLEXITY MAY EVOLVE MORE OFTEN THAN WE THINK
[Selections from: ”Evolutionary enigmas Comb jelly genetics suggest a radical redrawing of the tree of life” by Amy Maxmen in Science News May 2, 2013 - Print edition: May 18, 2013; Vol.183 #10 (p. 20)]
Go to article …
Comb jellies [phylum Ctenophora] are gelatinous like jellyfish, but the similarity ends there. In body plan, jellyfish resemble the largely sessile, almost plantlike sea anemones, corals and other cnidarians: a group that dates back at least 550 million years. While jellyfish and other cnidarians have nerve cells that form a loose network in their bodies, comb jellies have a more sophisticated nervous system with a rudimentary brain and cellular connections called synapses that are also found in flies, humans and most other animals.
Yet, detailed looks at the genomes of two species of comb jellies suggest, surprisingly, that they are the more primitive animals, and not the jellyfish, sea anemones or corals, as has long been thought. It’s even possible that the sophisticated comb jelly lineage may have evolved before the brainless, gutless, muscle-less sea sponges. …
The notion that comb jellies may elbow out sponges from the base of the animal tree of life is a radical one. If true, it means that comb jellies evolved nerves, muscles and other complex features, which in some ways resemble our own, independent of the ancestor that led to most animals.
Alternatively, and even more difficult for biologists to accept, is the possibility that the last common ancestor of all animals might have possessed complex features that remained in the comb jellies but were lost without a trace in the sponges, jellyfish and their kin. Both options muddle traditional assumptions that multifaceted features do not pop up and vanish willy-nilly over evolutionary time. …
- Leonid Moroz, a neurobiologist at the University of Florida’s Whitney Laboratory for Marine Bioscience in St. Augustine, recently sequenced the genome of the sea gooseberry comb jelly, Pleurobrachia bachei. The new genetic findings imply that “there may be many ways to make a complex animal,” he says.
- Others disagree. “It would be remarkably fascinating if comb jellies evolved neurons and muscles independently, and astonishing if they were at the base of the tree,” says Graham Budd, a paleontologist at Uppsala University in Sweden. “It is effectively saying animals evolved twice. Frankly, I’m not ready to believe it.”