Biologists at the University of California, Berkeley, are continuing to investigate a longstanding question in evolutionary biology: which organism is at the root of the animal family tree? This month, a new study from UC Berkeley researchers reported in Science that sponges—not comb jellies—are likely at the base of the animal lineage. The findings challenge a previous 2023 study from another UC Berkeley lab that used different genetic analysis methods and supported comb jellies as the earliest animals.
Until 2008, most scientists believed sponges were the earliest animals. That view shifted when genomic data suggested comb jellies could be first. The debate has persisted as various studies have split between supporting either sponges or comb jellies.
Nicole King, professor of molecular and cell biology at UC Berkeley and senior author of the latest paper, commented on the significance of this research. “I think we all want to know where we came from,” she said. She described how surprising it was when earlier evidence pointed to comb jellies rather than sponges: “The hypothesis that comb jellies evolved first — overturning years of evidence supporting the contrary — was ‘like finding out that the guy you thought was your dad was not your dad.’”
The question matters because it affects how biologists understand key features such as muscles and neurons. Sponges lack these traits, while ctenophores (comb jellies) possess them. If ctenophores were first, it would suggest either multiple independent evolutions or losses of these features among early animals.
Jacob Steenwyk, postdoctoral fellow at UC Berkeley and lead author of the new study, explained: “Muscles and neurons are key animal innovations, things that make us uniquely who we are. Understanding how those features evolved reveals an important part of our history.” He added that under one scenario—ctenophores evolving first—evolutionary explanations become more complicated. “But for the sponge-sister hypothesis, it’s a little bit simpler — just one innovation of muscles and neurons.”
King noted that reconstructing ancient evolutionary events is difficult due to conflicting findings across studies. “We are not arguing that our study settles the debate — only the community can decide that,” she said. “What we are saying is that we’ve found really strong evidence that favors only one hypothesis.”
Steenwyk pointed out both teams at Berkeley have been testing new analytical methods to address unresolved questions about early animal evolution: “That’s what’s exciting about these two papers: What are the new types of evidence that we can bring to this old debate? Then, given the totality of evidence, we can see where the chips lie.”
In 2023, Daniel Rokhsar—a professor at UC Berkeley—and collaborators used an approach examining gene linkages on chromosomes (synteny), which indicated support for comb jellies as being earliest.
For their recent work, Steenwyk and King developed a dataset with conserved genes from 100 organisms and applied an integrative method designed to reduce bias by including only genes yielding consistent results across different analyses. Their statistical tests showed 62% support for sponges being first; none supported ctenophores as earliest.
King concluded: “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. I hope that everyone interested will jump in, and together we’ll keep hammering on this.”
Funding for this research came from HHMI and other sources.
Related links include:
– The original Science article detailing these findings.
– A Science perspective discussing phylogenomic incongruence.
– A story by HHMI.
– The 2023 Nature paper (Ancient gene linkages support ctenophores as sister to other animals) presenting alternative findings.
– Coverage by UC Berkeley News in 2023.
