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Your Oldest Ancestor Was Probably Sponge Like

A new study may settle a long-running debate about which creature was the first to evolve from a universal common animal ancestor

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There’s been a long-simmering rivalry in the phylogenetics world. The divisive topic: What creature is last common ancestor of all animals?

Researchers have split into two camps. Some say it was a simple sponge-like creature that first formed, while others believe it was the more complex creature like the comb jelly, which has a nervous system, a gut and the ability to move. Now, as Nicola Davis at The Guardian reports, researchers from the University of Bristol have weighed in with new research, concluding the most likely last common ancestor was sedentary and sponge like.

Researchers work out the relationship between organisms by creating the family trees, similar to the family trees most people had to make in grade school. In the past, scientists had to rely on physical features to decide which animals are related to which, according to  a video press release, and which ones may have shared a common ancestor. But in recent decades, researchers have turned to DNA for a more detailed look into animal relationships.

At the very root of that tree lies one animal: the “last common ancestor” of all animal life (not to be confused with LUCA, the last universal common ancestor of all life on earth, including plants, animals and all those weird new kingdoms). DNA doesn’t tell us just what type of animal that first creature was, so researchers have created evolutionary models to work backward to figure it out. If we know the characteristics of the first animal to split off from the universal common ancestor, it will tell us a lot about that original creature.

The traditional view is that sponges were the first animals. But since 2008 several models have suggested comb jellies were the first branch to split off the animal tree. In fact, in October, one study used the “molecular clock” technique of DNA analysis to show comb jellies split from the trunk of the tree 88 to 350 million years ago. Another study earlier this year also pointed to comb jellies, though an additional one published around the same time pointed to sponges.

To assess these competing claims, Davis reports that researchers from the University of Bristol used statistical analysis to assess the datasets of the various evolutionary models. What they found is that models that find sponges at the root of the evolutionary tree explain the data much better than the comb jelly models. The research appears in the journal Current Biology.

It’s not totally surprising. The comb jelly hypothesis has its flaws. For instance, if the first animals had guts and nerves, it would mean that for more simple creatures like sponges to evolve, they would have to lose these sophisticated elements to become sedentary filter feeders. The ancestors of animals are also known to be choanoflagellates, a type of filter-feeding organism lending some weight to the sponge camp.

The debate may seem pedantic, but as co-author Davide Pisani explains in a press release, identifying the first animals has big implications for biology. “The fact is, hypotheses about whether sponges or comb jellies came first suggest entirely different evolutionary histories for key animal organ systems like the nervous and the digestive systems,” he says. “Therefore, knowing the correct branching order at the root of the animal tree is fundamental to understanding our own evolution, and the origin of key features of the animal anatomy.”

Antonis Rokas, researcher from Vanderbilt University who has published studies suggesting comb jellies were at the root of the tree, tells Davis that the debate is not over—but it’s getting close. “With this study, the authors have significantly tipped the balance toward the sponges-sister hypothesis,” he says. “But I will eagerly await to see what are the effects of adding additional genomes from both sponge and ctenophore lineages, as well as models that do not reduce the information provided from the data, before considering the debate solved.”

Whatever the case, both lineages are super, super old. So next time you see a sponge or comb jelly, make sure to thank your grandparents.

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

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