Fierce Debate About Earth’s First Animal Heats Up

For years, an intense debate has raged in regards to the origins of animal life on Earth, dividing scientists into two camps: Those who assume sponges gave rise to animals and people who assume comb jellies (ctenophores) are accountable.

At first look, it might sound apparent which animal group got here first: the straightforward sponge. Bereft of muscle groups or neurons, sponges spend their lives anchored, filter-feeding detritus from ocean water. Ctenophores, then again, have muscle groups, neurons, and cilia for swimming—they’re cellular predators that hunt for his or her meals.

But evolution doesn’t all the time take the straightest path, and a 2008 analysis evaluating a whole lot of genes from a wide range of animal species and their relations surprisingly positioned the extra complicated ctenophores on the root of the animal household tree. A 2023 study inspecting linked genes (these situated shut to 1 one other on chromosomes) bolstered the so-called “ctenophore sister hypothesis.”

It was a stunning discovering. If true, it could recommend that early sponges had neurons and their descendants misplaced them over time. While it’s not essentially the most linear evolutionary path for an animal group to take, it’s a well-worn one. Whales, for instance, developed from terrestrial quadrupeds and, over many generations, misplaced their hind limbs after taking to the ocean.

Read extra: “In Search of the First Animals”

University of California, Berkeley, biologist Nicole King in contrast the concept that ctenophores developed first to “finding out that the guy you thought was your dad was not your dad.” And so, together with a postdoctoral researcher in her lab, phylogeneticist and computational biologist Jacob Steenwyk, she got down to examine.

“Jacob came in hypothesizing that the ctenophore sister idea was correct, while I hypothesized that the sponge sister idea was correct, and so we thought, why not? Let’s go for it,” King mentioned in a statement.

Together they created a dataset of conserved genes—sequences which can be current in all kinds of species and have confirmed to be remarkably resilient to alter over time. They analyzed the genes and winnowed their dataset right down to solely these genes that produced a constant end result for both the ctenophore or sponge speculation whatever the analytical methodology they used. Then they utilized statistical checks. The scientists concluded 62 p.c of their checks supported the sponge speculation, and none supported the ctenophore speculation (38 p.c have been inconclusive).

They revealed their findings in Science this week.

“I think the way we’ve done this analysis lends very strong support for the hypothesis that sponges evolved first, which is consistent with studies based on morphology. But I still think there’s room for investigating this question further,” King mentioned. “I hope that everyone interested will jump in, and together we’ll keep hammering on this.”

So the controversy rages on, however for now it looks like Team Sponge might have the higher hand.

Enjoying  Nautilus? Subscribe to our free e-newsletter.

Lead picture: Philip Hamilton / Ocean Image Bank

• Jake Currie

  Posted on November 14, 2025

  Jake Currie is a author primarily based in Brooklyn, NY.