Genome reveals comb jellies’ ancient origin


Animals evolved gradually, from the lowly sponge to the menagerie of tentacled, winged and brainy creatures that inhabit Earth today. This idea makes such intuitive sense that biologists are now stunned by genome-sequencing data suggesting that the sponges were preceded by complex marine predators called comb jellies. 

Although they are gelatinous like jellyfish, comb jellies form their own phylum, known as ctenophores. Trees of life typically root the comb jellies’ lineage between the group containing jellyfish and sea anemones and the one containing animals with heads and rears — which include slugs, flies and humans. Comb jellies paddle through the sea with iridescent cilia and snare prey with sticky tentacles. They are much more complex than sponges — they have nerves, muscles, tissue layers and light sensors, all of which the sponges lack. 

“It’s just wild to imagine” that comb jellies evolved before sponges, says Billie Swalla, a developmental biologist at the University of Washington in Seattle and a leading member of the team sequencing the genome of the comb jelly Pleurobrachia bachei. But the team is suggesting just that, in results they presented at the annual meeting of the Society for Integrative and Comparative Biology, held on 3–7 January in San Francisco, California.

Despite comb jellies’ complexity, DNA sequences in the Pleurobrachia genome place them at the base of the animal tree of life, announced Swalla’s colleague Leonid Moroz, a neurobiologist at the University of Florida in Gainesville. Another team presented results from genome sequencing for the comb jelly Mnemiopsis leidyi, and found that the phylum lands either below, or as close to the base as, sponges on the tree.

Written By: Amy Maxmen
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  1. This piece could not have turned up at a better time for me. I am doing the Introduction to Genetics and Evolution course with Duke Uni through Coursera and sadly have spent a greater amount of my time dealing with creationist threads on the course forum (I know I should ignore it but I love a good fight).

    Anyway, one of the accusations levelled at science by them is our own confirmation bias, and how we do not allow any science in that does not fit our neat little framework and presents problems.

    Here is a prime example for me to use to show that that is just not true. The idea that there is one big scientific conspiracy to maintain the Status quo is just rubbish.

    It is an interesting find too by the way, I will follow this one with much interest.

  2. I read something like that a couple of years ago. Then it claimed it was perhaps not correct, and now it is true again? Time will tell. Either way, another theory lately is that sponges are not the direct ancestors of more complex animals, but an early sidebranch. Perhaps did the bilaterian animals evolve from a common ancestor that did not include cnidaria or other so-called primitive animals. Then again, jellyfish and their relatives are said to have a bilaterian larval stage, so who knows.

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