Nature tries to solve a nearly intractable chicken-and-egg problem for evolutionary biologists. Which is the oldest kind of animal, a sponge or a comb jelly? They’re both simple sea creatures… but right now, sponges have taken the lead – for now:
Peering back through 600 million years of transformation is hard. It seems that every animal descends from ancestors on one of five branches near the base of the tree. But these five groups look very different from one another today. There are sponges, comb jellies, cnidarians (including sea anenomes, corals and jellyfish), bilaterally symmetrical animals (such as humans and clams) and obscure, microscopic worms called placozoans.
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For the better part of the past century, zoologists arranged these branches according to their judgements of what was simple and what was complex. Sponges fell to the bottom branch, and bilaterally symmetrical animals resided higher up. But in 2008, a genetic analysis published in Nature put comb jellies, rather than sponges, near the root of the evolutionary tree.
This arrangement rattled evolutionary biologists because it upended the idea that animal complexity increased over time. It implied that nerves and other characteristics evolved independently in different lineages, and were subsequently lost in sponges.
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[Michaël Manuel, an evolutionary biologist at the Institute of Biology Paris-Seine,] and his colleagues analysed 1,719 genes from an unparalleled range of species. It took computing power from Canada, Germany, Belgium and France to crunch the numbers. The team also tested several mathematical models that accounted for biological phenomena, including the fact that certain genetic changes are more likely than others. They chose a model called CAT, partly because of how well it reproduced sections of the animal tree that have already been confirmed.
The results from the CAT model placed sponges on the earliest branch of the animal family tree.