Researchers have long debated which branch of the animal family tree is the oldest—and as technology has advanced, some surprising contenders have emerged. At first, scientists thought that sponges came first, but about a decade ago, comb jellies became a possibility, too. Now, reports Laura Geggel for LiveScience, a new study makes the case that comb jellies actually came first.
It all comes down to a difference in approach. Researchers who were part of the study in Current Biology analyzed a huge genetic dataset and found that sponges were at the base of the evolutionary tree. But the researchers that were part of the Nature Ecology & Evolution study used a different method. Instead of looking at a gigantic quantity of data, they focused in on a smaller number of what they call “contentious relationships”—branches of the tree on which different types of data analysis produce contradictory results.
When they focused in on the individual genes of animals in contentious categories and compared them to those of their closest relatives, the researchers discovered that often, a difference of just one gene out of hundreds of thousands can deliver a completely different result. They then looked at each gene to determine the creatures' closest relatives, using that information to place them on the tree of life. This analysis consistently put comb jellies, not sponges, at the bottom of the tree.
That might come as a surprise to sponge-first supporters. Those who think that sponges came first often use the sponge’s much simpler genetic structure as support for the idea that it predates other, more complicated lifeforms. But this latest study suggests that comb jellies have specific genes that suggest they came first.
The comb jelly controversy has been alive and well since scientists first started to use genetic analyses to link species together. As Geggel reports, a 2008 study that supported comb jellies as the oldest animals threatened to topple the simple sponge from its place—and opinion has vacillated back and forth ever since.
“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,” says Antonis Rokas, who co-authored the paper, in a press release. It’s proof that scientists are constantly devising better—and different—ways to go in-depth with genetic data. Rokas tells George Dvorsky at Gizmodo: “Some of the controversies that we’ve examined, including the jellies/sponges one are devilishly difficult to decipher.” So don’t expect the debate to end anytime soon.