Sponge or jelly? Though it sounds like a choice between desserts, it turns out that either sponges or comb jellies are the key to understanding the origin of animals, what the first animals were like and when the first brains evolved.
That is because one of them was the first animal group to split from other animals and begin evolving separately – but it has long been unclear which.
A new analysis points the finger at jellies. The study has been praised by scientists on both sides of the debate because it brings clarity to a confused area, but it isn’t being interpreted as the definitive answer.
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The first complex animals evolved around 600 million years ago. Unlike their simpler ancestors, they had large bodies made of many cells, which developed into specialised organs.
That ancestral population gave rise to all the major animal groups, from starfish and insects to birds and mammals, but we don’t know what the first animals looked like or how they behaved.
It would help to know the order in which the different animal groups split from each other, but this is contentious. It comes down to the two groups that are known to be older than the rest: comb jellies, also known as ctenophores, and sponges. Sponges live stationary lives in the sea and have no nervous systems, while comb jellies superficially look like jellyfish and do have nervous systems.
“The ancestor of animals I can definitely say was neither a ctenophore nor a sponge as we see them today,” says Antonis Rokas at Vanderbilt University in Tennessee. But their DNA can tell us which genes were present in the first animals, revealing what those first animals were like.
For many decades, it was assumed that sponges were the first to split and were therefore the oldest and most primitive animals.
Yet recent studies have thrown that into doubt. In 2008, researchers led by Casey Dunn, now at Yale University, obtained DNA from 29 distantly related species. They found that the . Five years later, this claim was bolstered when of a comb jelly, the warty comb jelly (Mnemiopsis leidyi).
A row has raged ever since, and on which group split first. support ctenophores as the first group to split, but back .
To resolve this, researchers led by Rokas and Dunn have reanalysed all the genetic data from comb jellies, sponges and related groups.
“One thing we’ve noticed over the years is that, very often, sometimes, we talk past each other,” says Rokas. The genetic data can be analysed in different ways, giving different answers, and this means the arguments over which study is right are often based on apples-to-oranges comparisons. “You start getting confusion because not everybody has the same benchmark dataset,” he says.
Rokas, Dunn and their team have now found ways to compare all the data and analyses within one framework. “We’re able to interpret the behaviours of the different models in a way that’s standardised and systematic,” says Rokas.
The team found that most analytical methods supported ctenophores as the first group to split. “They all point to the same answer,” says Rokas. Only one software package pointed to sponges, and only if it was set up in a particular way.
That means it is less likely that sponges were the first group to split, says Rokas. “I would say this moves the needle towards [ctenophores] pretty robustly.”
“This is an important advance, in terms of the clarity of the analyses, and the fact that everything is so well documented that it can be reproduced,” says Nicole King at the University of California, Berkeley, who co-authored a .
However, King and Rokas both say that the question will rumble on. A key problem is that only a handful of sponges and comb jellies have had their DNA sequenced. Many more genomes are needed.
“We have no idea how the models will start behaving as we start using more and more genomes,” says Rokas.
“I’ve been a little bit disappointed in some of the rapid responses that I’ve seen on social media, which assert that this changes everything, because it doesn’t,” says King.
In particular, it is difficult to figure out which parts of comb jellies’ genomes come from the common ancestor of animals. “Ctenophore genomes are very fast-evolving,” says King. “So living ctenophores don’t represent the ancestral condition.”
Resolving the question isn’t just a minor detail. – it could change our understanding of the evolution of the nervous system and brain.
If the brainless sponges were first to split, this implies that the first animals didn’t have brains and that brains only evolved once, on the line that led to comb jellies and other animals. Yet if , the story changes, says Rokas. “You have to assume either the nervous system was present in ancestors and was lost in sponges […] or alternatively, the nervous system evolved twice independently, once in ctenophores and once in the rest of animals.”
For Leonid Moroz at the University of Florida, this shouldn’t be controversial. “The nervous system evolved multiple times, definitely at least twice, maybe three times or more,” he says. Moroz compares the nervous system to complex eyes, which evolved independently in many animal groups.
King is unconvinced by this, arguing that the nervous systems of comb jellies have many similarities to those of other animals. They diverged a long time ago, so it’s not surprising there are also differences, but the similarities point to a common origin, she says.
Genetics alone won’t settle either question, says Moroz. “You have to have people from many different backgrounds working on these projects,” says his colleague Andrea Kohn, also at the University of Florida.
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