Scientists: Jellies Are the Oldest Animals on Earth

One of the hottest debates in science today concerns two very, very chill animals. Some scientists argue that sea sponges were the first-ever animals to evolve; others say jellies came first. There’s evidence to support both sides, but the latest research, published in the new journal Nature Ecology and Evolution, lands hard in favor of Team Jelly.

Sponges and comb jellies like the one shown above are certainly uncomplicated animals. Both have bodies that are essentially modified tubes, with no eyes, no brains, and no hearts. It might not be an exciting setup, but it’s been working out for these animals for millions and millions of years.

Determining the exact age of a boneless animal is much harder than dating, say, a dinosaur. After death, soft bodies essentially melt away, leaving almost no clues behind—except for the genes they’ve passed down to the next generation. Each animal’s genes is like a map of its evolutionary history. The trick is figuring out how to read it accurately.

Paper co-author Antonis Rokas is an evolutionary biologist at Vanderbilt University. He and his team had grown dissatisfied with existing phylogenomic (genetic map-reading) techniques, which can yield incomplete or ambiguous results.

“The current method that scientists use in phylogenomic studies is to collect large amounts of genetic data, analyze the data, build a set of relationships and then argue that their conclusions are correct because of various improvements they have made in their analysis,” Rokas said in a statement. “This has worked extremely well in 95 percent of the cases, but it has led to apparently irreconcilable differences in the remaining 5 percent."

What was needed, the researchers decided, was a more precise instrument, one that used only the most pertinent information instead of huge piles of data. In this case, that meant looking only at the relationships between different animals’ genetic maps.

“When you look at a particular gene in an organism, let's call it A,” Rokas said, “we ask if it is most closely related to its counterpart in organism B? Or to its counterpart in organism C? And by how much?"

The researchers say the results of this new tactic are clearer and more decisive than any research on the subject to date. And those results say comb jellies, not sponges, were the first animals to evolve.

"We believe that our approach can help resolve many of these long-standing controversies and raise the game of phylogenetic reconstruction to a new level," Rokas said.