Researchers reexamined conserved genes to test competing hypotheses about Earth’s first animals. Using only genes that produced consistent results across methods, they found 62% of tests support a sponge-first origin, 0% supported ctenophores first, and 38% were inconclusive. Published in Science, the work strengthens the case for sponges but the authors stress further research is needed.
New Study Strengthens Case for Sponges as Earth’s First Animals
Researchers reexamined conserved genes to test competing hypotheses about Earth’s first animals. Using only genes that produced consistent results across methods, they found 62% of tests support a sponge-first origin, 0% supported ctenophores first, and 38% were inconclusive. Published in Science, the work strengthens the case for sponges but the authors stress further research is needed.
01:26 AM, 11/15/2025Science
Battle Over Earth’s First Animal Intensifies — New Analysis Favors Sponges
Scientists have long debated whether the planet’s earliest animals were simple, sessile sponges or the more complex, mobile comb jellies (ctenophores). At first glance, sponges seem the obvious ancestral form: they lack muscles and neurons, remain attached to substrates, and feed by filtering seawater. Ctenophores, by contrast, possess muscles, neurons and beating cilia, and are active predators.
That tidy picture was upended by a 2008 genomic analysis that surprisingly placed ctenophores at the base of the animal family tree. A 2023 study of linked genes—genes located near each other on chromosomes—further bolstered that “ctenophore-sister” hypothesis. If true, the result implied that sponges once had nervous systems that were later lost — an unusual but not unprecedented evolutionary pattern.
UC Berkeley biologist Nicole King and postdoctoral researcher Jacob Steenwyk directly revisited the question. Their approach focused on conserved genes—sequences that are widely shared across many species and change little over evolutionary time. They filtered their dataset to include only genes that gave consistent results for one hypothesis or the other across multiple analytical methods, then applied rigorous statistical tests to those loci.
Key findings: 62% of their tests supported the sponge-first hypothesis, 0% supported the ctenophore-first hypothesis, and 38% were inconclusive. The team published their results in the journal Science.
“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 said. “I hope that everyone interested will jump in, and together we’ll keep hammering on this.”
The study does not end the debate, but it shifts the balance of genetic evidence back toward sponges as the most likely earliest animal lineage. Future work—adding more genomes, refining phylogenetic methods, and integrating morphological and developmental data—will help clarify the deep branches of the animal tree.
Why it matters: Resolving which group branched off first helps scientists understand how key features such as neurons and muscles evolved or were lost, shaping our picture of early animal evolution.