Sponges are so simple, in fact, that it can come as a surprise that they are animals at all, but their molecular makeup reveals their kinship. They make certain proteins, such as collagen, that are produced only by animals. What’s more, their DNA shows they are more closely related to animals than to other forms of life.

Starting in the 1990s, as scientists gathered DNA from more animal species, they tried to draw the animal family tree. In some studies, the sponges ended up on the deepest branch of the tree. In this scenario, animals evolved a nervous system only after the sponges branched off.

But in the early 2000s, other scientists came to a surprisingly different conclusion. They found that the deepest branch of animals were comb jellies — slim, oval creatures that often grow a distinctive set of iridescent bands that flicker in the darkness of the deep ocean.

Many experts were reluctant to accept that conclusion, because it meant animal evolution was weirder than they had realized. For one thing, comb jellies were not as simple as sponges. They have a nervous system: A web of neurons circling their bodies controls their muscles.

To resolve the comb-jelly-versus-sponge debate, researchers from around the world collected DNA from more species of ocean animals. And instead of looking at single genes, researchers figured out how to sequence entire genomes.