What were the first animals? The fierce sponge-jelly battle that just won’t end

Which animals arrived first? For more than a century, most evidence suggested that sponges, immobile filter feeders lacking muscles, neurons, and other specialized tissues, were the first animal lineages to emerge. Then, in 2008, a genomic study revealed a fearsome rival: dazzling, translucent predators called comb jellies, or ctenophores, with nerves, muscles and other sophisticated features.

This single study sparked a debate which has raged for almost 20 years, sparking heated discussions about how complexity evolved in animals. But after dozens of studies — some of which analyzed and reanalyzed the same data and came to different conclusions — the debate has become entrenched, some researchers say.

“While it would have been healthy for people to engage with curiosity and interest in discovering the truth together, it became a battle,” says Nicole King, an evolutionary biologist at the University of California, Berkeley, who co-authored a paper last November that landed cautiously on “team sponge.”

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She has since requested to withdraw the document due to flaws identified after its publication, and is reconsidering whether she wishes to participate in the debate in the future. Scientists, including King, say a different approach is needed: one in which researchers from both sides work together to answer the question.

New ideas – and attitudes – would catalyze progress, they say. “We need to think outside the box,” says Leonid Moroz, a neuroscientist at the University of Florida in Gainesville, whose work has supported comb jellies as a lineage at the root of the animal family tree.

The first animals emerge

About 600 to 800 million years ago, radically different organisms appeared. Instead of being made up of isolated cells, like all previous lives, these creatures were made up of multiple interacting cells. Multicellularity has been so successful that it has sparked an explosion of innovative body shapes and new ways of feeling and responding to environments.

In the blink of an evolutionary eye – perhaps within a few tens of millions of years – five major groups of animals appeared. Besides the ancestors of today’s sponges and comb jellies, there were placozoans (now represented by blob-like marine invertebrates); cnidarians (modern members of which include jellyfish and sea anemones); and bilaterians which exhibit mirror-image body symmetry early in their development, which would give rise to invertebrates including starfish, snails and spiders, and vertebrates including humans (see “Tree of life – now with two options”).

Fossil evidence of early animals is sparse and difficult to decipher – a porous cavity here or a branching tube there. Identifying the first animal lineage, as well as knowing its modern descendants, is another way to better understand these early creatures. “Knowing this will tell us something, but not everything, about what these early animals might have looked like,” says Max Telford, an evolutionary biologist at University College London. Evolutionary biologists sometimes call this first animal the “sister” of other animal groups, because it shares a common parent with all of them.

For more than a century, most scientists placed the sponge lineage at the base of the animal tree, mainly because modern sponges lack many of the features that define other animals, including specialized tissues such as muscles, nerves and intestines, which were thought to have evolved later. “If the sponge tree was right, everything would fall into place,” says Telford. But when scientists turned to rapid genome sequencing to confirm this seemingly established picture, it collapsed.

The epic battle of evolutionary biology

Casey Dunn, an evolutionary biologist now at Yale University in New Haven, Conn., never anticipated starting a debate that would last several decades. But the advent of faster, cheaper DNA sequencing technologies in the early 2000s prompted Dunn and his colleagues to build the tree of animal life using genomic data — one of the first efforts of its kind.

They analyzed thousands of genetic sequences from 77 organisms – from sponges and sea spiders to chickens and corals. The study was the first to include data on the comb jelly genome, Dunn says, but “at first we didn’t know we would get a result other than sponges being sisters to the rest of the animals.” Their 2008 conclusion that comb jellies, not sponges, were the first group of animals was a bombshell.

The discovery prompted two types of responses, Dunn says. One of them was open-minded curiosity. “Maybe the preconceived ideas contained in the first chapters of zoology textbooks are not accurate,” he says. “There was another response where various people said, ‘Hey, the sponges have always been the sisters and they always will be.'”

Dozens of other papers followed, some using new data sets, different analysis methods, or both. Some have given further support to comb jellies as a lineage at the root of the animal family tree, others have re-established sponges as sisters to all other animals (see “Ping-Pong Articles”). Journals published essays, perspectives, and other expert analyses, while institutional press releases and media coverage sometimes presented each advance as the final word. “It’s kind of a back and forth of trying to refute each other and then with each subsequent statement saying that this is the answer,” King says.

Unlike the sponge sister hypothesis, which fits perfectly with the apparent simplicity of modern sponges, placing comb jellies at the root of the animal tree raises new questions. One is how complex tissues such as nerves, muscles, and intestines might be present in early animals but absent in members of some later lineages. One possibility is that these tissues evolved not just once, but independently within multiple lineages. Another option is that these features were present in early animals but were lost in later lineages, including sponges. Some biologists believe that a similar loss occurred in the placozoan lineage, whose modern members also lack a nervous system and muscles.

The two camps tend to separate themselves by discipline, observes Antonis Rokas, an evolutionary geneticist at Vanderbilt University in Nashville, Tennessee. Supporters of sister sponges mostly have a background in zoology and evolutionary and developmental biology. To these scientists, the gradual accumulation and elaboration of complex traits was compelling.

Those who argue for a sister in comb jelly, meanwhile, are often trained in genomics and are more open to the idea that complexity could evolve independently and was systematically won and lost. Rokas counts himself in this group. “I’m not going to say that’s the answer because I’ve been here long enough,” he said.

The struggle to look back

The challenge of reconstructing the origin of animals that existed hundreds of millions of years ago using modern genome sequences is similar to that faced by astrophysicists discerning the early history of the Universe from the night sky, Rokas says. “The signal is weak, it has traveled a long distance, and there are several factors that can erode that signal.”

To determine which lineage – jelly comb or sponge – originated the animal family tree, scientists look for signs of the small number of genetic variations that arose within a very specific window of time: after the first animal lineage diverged and before the next one bifurcated. These are all that distinguish this first lineage from successive branches before each begins to follow its own evolutionary path.

But it is difficult to find this genetic signal. According to the researchers, this window could have lasted less than five million years, less than the time since the appearance of humans and chimpanzees (Pan troglodytes) shared a common ancestor, meaning it didn’t take long for a lot of changes to accumulate. And the trace of the signal quickly faded: After animal lineages began to evolve independently, around 600 million years ago, these early genetic variations would have quickly been lost or obscured by further changes within specific lineages.

When looking for such a weak signal, seemingly minor decisions can exert a major influence on the conclusion. In a 2021 analysis, for example, Rokas and colleagues found that whether a study reaches a sponge sister or comb jelly sister conclusion may depend on which non-animals — called outgroups — are included in the analysis, as well as various hypotheses about how different gene sequences evolve.

“These are good people who are sincerely trying to get a good answer, but the puzzle is really difficult,” says evolutionary biologist Sean Carroll of the University of Maryland in College Park.

Change of direction

To understand once and for all how animals evolved, many scientists say new approaches are needed. One of the most promising solutions is to infer evolutionary relationships using the physical position of genes on different chromosomes, instead of using their sequences, as all previous efforts have done. These alterations occur less frequently than sequence changes and are much less likely to be reversed, leaving potentially indelible marks in the genome of living organisms.

In a 2023 paper, evolutionary geneticist Darrin Schultz, now working at Stanford University in Palo Alto, California, and his colleagues examined the arrangement of genes in the chromosomes of comb jellies, sponges, and other organisms, and found patterns that strongly support the comb jellies’ lineage as sister to other animals. For example, comb jellies shared many chromosome-wide arrangements with related unicellular animals. Additionally, these arrangements were absent in sponges, jellyfish, and bilaterians.

King says that the Science The article she co-wrote in November, which marked her entry into the debate, was an attempt to bring another approach to the problem – but that the need to retract undermined her.

Aware that seemingly minor decisions about how to analyze data can influence conclusions, she and her co-author, evolutionary biologist Jacob Steenwyk, also at the University of California, Berkeley, took a “kitchen sink” approach. They analyzed the new and existing data sets in every way possible to identify the sets of genes most likely to give consistent answers about whether the sponge or comb-jelly lineage is sister to the other groups.

Their study supported a sister sponge but said the debate was far from settled. Scientists on both sides praised his measured tone. “It was awesome,” Dunn says. But for him, the analysis didn’t hold water, so Dunn began corresponding with King and Steenwyk to better understand and replicate their analysis.

Dunn and several of his colleagues discovered technical errors, detailed in a letter submitted to Science end of December. He says that, when corrected, the data strongly supports comb jellies as the first animal lineage. “We definitely made a mistake,” says King, who with Steenwyk wrote in a Jan. 9 letter in Science that they intend to withdraw the study. Science says it will soon issue a formal notice of retraction.

King had hoped to help forge a truce between the opposing camps. Although she is now likely to abandon the search for the first animal lineage – once her work with Steenwyk, a postdoc in her lab, is complete – she still hopes that researchers from the “comb jelly” team and “sponge team” will come together to agree on the best approaches and start writing papers together. King says she would have liked to release the study as a preprint so that errors could be caught sooner. But she feared that a pre-publication would invite unfair criticism because of the entrenchment of the debate.

Dunn, however, believes the debate is overblown: “This issue has been injected into more drama than there actually is. » Responses to Schultz’s 2023 study suggest both sides are far from détente. A paper published last December by evolutionary biologist Richard Copley of the French National Center for Scientific Research in Villefranche-sur-Mer questioned the statistical significance of the shared chromosome arrangements identified by the 2023 study and the strength of its conclusion that the comb jelly lineage diverged first.

Schultz supports the study – and its approach. But settling the debate could take decades, he says. “For these really difficult questions, the science moves slowly.”

More data could be useful in the short term. Rokas says studies have relied too much on fanciful models and theories rather than new data. Very few species of comb jellies and sponges have had their genomes sequenced. Earlier in January, Dunn traveled to St. Helena in the South Atlantic Ocean, one of the world’s most isolated islands, to collect invertebrates, including comb jellies.

Meanwhile, the quest for the first animal lineage led to discoveries about comb jellies and sponges. For example, Dunn says, studies sparked by the debate have found that the comb-shaped nervous system, lacking neuronal junctions called synapses, is very different from the nervous systems of other animals. “We are learning so much about these animals,” he says. “People present this as a ping-pong match that stops, but that couldn’t be further from the case.”

This article is reproduced with permission and has been published for the first time January 27, 2026.