Sponge v comb jellies: which was evolution’s first trailblazer?

While life on Earth has flourished for billions of years, much of it has been single-celled and microscopic. None of the first organisms had brains, or even neurons (nerve cells). None of them could “think”. The first animals to evolve were also brainless: harnessing hormones or other chemicals, rather than neurons, to coordinate their bodies. But some soon evolved central nervous systems – and the first “thoughts” were pulsed.

For decades, biologists have assumed that this only happened once and was a one-way process. Once animals had evolved brains, why would they lose them? But in the past 15 years, evidence has accumulated that this may be wrong; that sponges and other brainless animals that exist today may be descended from brainy ancestors that lost their minds.

These huge mysteries about the origins of neurons and brains all turn on a seemingly simple question: which of two ancient animal groups, the sponges or the comb jellies, was the first to diverge from other animals and begin evolving independently? The oldest confirmed animals are from between 635m and 485m years ago. It is thought the first group of simple animals soon began separating into distinct populations which then evolved in different ways – giving rise to different animals.

Biologists had thought sponges were the first group to break away from the common animal ancestor, as today sponges are almost the only animals lacking neurons. From this viewpoint, sponges are a relic of a bygone era. “It was never really robustly tested,” says Casey Dunn, professor of ecology and evolutionary biology at Yale University. “It was just what was in the textbooks.”

Then in 2008, Dunn’s team published a landmark study that used genetic material from 21 animal groups to build a family tree. The more similar two animals’ DNA, the more closely they are related. To everyone’s surprise, it did not show sponges branching first, but second. The first breakaway group was unexpected: the comb jellies. “And then this thing [heated debate] kicked off for 15 years,” says Dunn.

It was a surprise because comb jellies, also called ctenophores, don’t look simple. Found in the oceans, the blob-like jellies propel themselves through water using slender threads called cilia, and are mostly active predators, unlike the sedentary, filter-feeding sponges. Crucially, comb jellies have neurons connected in a “neural net”: a simple brain. Compared with sponges, comb jellies look much more complex – or rather, as Dunn points out, they are complex in ways that resemble humans. It seems counterintuitive that they were the first to split from the other animals.

There are two ways to explain comb jellies splitting first, says Aoife McLysaght, professor of genetics at Trinity College Dublin. One interpretation is that neurons were already present in the last common ancestor of all today’s animals but that sponges subsequently lost theirs. Alternatively, neurons may have evolved independently in comb jellies and in other groups. The evidence was bolstered in 2013, when Dunn’s group published the first complete genome of a comb jelly and analyses pointed to comb jellies breaking away before sponges.

However, the rebuttals began with the original 2008 study. Many geneticists suspected Dunn’s analyses were flawed. Claims and counter-claims have shot back and forth “like a ping-pong match”, says Darrin Schultz, a postdoctoral scientist at the University of Vienna.

“The subsequent debate is all about the technicalities of how you analyse the DNA sequence,” says McLysaght. It’s not as simple as plugging in lots of DNA sequences and seeing what the computer spits out; choices must be made. “There’s going to be bits of each genome that just aren’t comparable to others.” Researchers must also decide which species to include, and factor in complications such as parts of the genome changing faster than others. If you don’t make the right choices, you can get the wrong answer even if you have good data, she says. “We think it is probably an artefact.”

One prominent study, published in 2017 by Davide Pisani, professor of phylogenomics at the University of Bristol, and his colleagues, compared previous analyses. They found that studies that captured a particular type of mutation were more likely to show sponges splitting first; in contrast, models that showed comb jellies splitting first had tended to ignore these mutations. While at the time the Guardian reported this as “Evolution row ends”, it is far from over.

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In 2021, McLysaght and her colleague Anthony Redmond entered the debate. They suspected that analyses finding that comb jellies split first had been thrown off by a well-known but difficult- to-control problem. If some groups of animals evolve much faster than others, by sheer chance they will develop some of the same genomic features, making them look closely related. The pair controlled for this in their new analyses that found sponges were the first to split. However, Dunn’s group published their own reanalysis later that year that examined 15 existing studies and performed additional analyses. Their study backed the comb jelly.

Then in May this year, Schultz and colleagues came at the problem in a new way. Instead of using the exact sequence of the DNA to compare the animals, they looked at overall structure: which genes were on which chromosomes, and in what order. These large-scale patterns change more slowly than individual DNA letters so are better for studying the most ancient evolutionary shifts. The team found genomic patterns that were shared by the comb jellies and the non-animals sampled, and a different set of patterns shared by sponges and all other animals. This suggested that comb jellies were indeed the first to start evolving separately. “I think this is strong evidence towards resolving it,” says Schultz.

“I find that type of analysis very compelling and I find the logic of it very compelling,” says McLysaght. Nevertheless, she isn’t convinced. “There’s been a lot of choices made along the way there as well,” just like in the earlier studies. However, she says she’s open to changing her mind.

Dunn also highlights recent studies of the biology of comb jellies, which he says support his case. For example, a 2021 study identified some of the chemicals produced by comb jelly neurons, and found they are unlike those in the neurons of other animals. In April this year, the same group showed that comb jelly neurons also connect with one another differently. The neurons of most animals are separate cells which communicate by firing chemicals across a gap called a synapse. However, comb jelly neurons fuse with one another, becoming a single interconnected web. This is all compatible with comb jellies having evolved neurons independently, says Dunn.

We also need to be more open to the idea that sponges really did give up on neurons, says Schultz. Their lifestyle is slow and passive, but it works for them. “Sometimes it’s better to just not do anything really and just sit there.” Dunn agrees that “having a central nervous system is very expensive”. If a group of animals doesn’t need to “think”, evolution will remove their brains. “We burn so many calories in our heads. Why would you maintain this expensive information-processing organ if you don’t need it?”