One of the greatest changes in the history of life occurred more than 600 million years ago, when a single-celled organism gave rise to the first animals. In their multicellular bodies, animals evolved into a surprising array of forms, such as whales that weighed 200 tons, birds that soared six miles into the sky and sidewinders that glided across the sands of desert.
Scientists have long wondered what the first animals looked like, including questions about their anatomy and how they found food. In a study published Wednesday, scientists have found tantalizing answers in a little-known group of gelatinous creatures called comb jellies. While the first animals remain a mystery, scientists have discovered that comb jellies are among the deepest branches in the animal family tree.
The debate about the origin of animals has lasted for decades. At first, researchers relied more on the fossil record for clues. The oldest definitive animal fossils come from then 580 million years, although some researchers claim to have found even older ones. In 2021, for example, Elizabeth Turner, a Canadian paleontologist, reported a find 890-million-year-old fossils of possible sponges.
Sponges would make sense as the oldest animals. They are simple creatures, without muscles or nervous systems. They anchor themselves to the ocean floor, where they filter the water through a maze of holes, picking up bits of food.
Sponges are so simple, in fact, that it might come as a surprise that they’re animals at all, but their molecular makeup reveals their kinship. They produce certain proteins, such as collagen, that are only produced by animals. Furthermore, their DNA shows that they are more closely related to animals than to other life forms.
Beginning in the 1990s, as scientists gathered DNA from more animal species, they attempted to draw the animal family tree. In some studies, the sponges ended up in the deepest branches of the tree. In this scenario, animals evolved a nervous system only after sponges branched off.
But in the early 2000s, other scientists came to a surprisingly strange conclusion. They found that the deepest branch of animals are comb jellies — slim, oval-shaped creatures that often grow a distinctive array of iridescent bands that flash in the darkness of the deep ocean.
Many experts are reluctant to accept that conclusion, because it means that animal evolution is more unusual than they realize. For one thing, comb jellies are not as simple as sponges. They have a nervous system: A web of neurons running around their bodies controls their muscles.
To resolve the comb-jelly-versus-sponge debate, researchers from around the world collected DNA from more species of ocean animals. And instead of looking at single genes, researchers figured out how to sequence the entire genome.
But the avalanche of new data has failed to settle the debate. Some scientists ended up assembling a tree where sponges were the deepest branches, while others ended up with comb jellies.
The new study, published in the journal Nature, relied on a new method for using DNA to track animal evolution.
In previous studies, scientists looked at how certain mutations appeared in different branches of the animal. A mutation can cause one genetic letter, known as a base, to move to another letter. That mutation is inherited in an animal’s offspring.
But these mutations may not be reliable markers of history. A base can move from one letter to another, and then millions of years later, it can return to the original. Alternatively, the same base can move to the same letter on two unrelated lines. That parallel evolution creates the illusion that the two lineages are closely related.
In the new study, Darrin Schultz, an evolutionary biologist at the University of Vienna, and his colleagues looked instead at a different type of genetic change. On rare occasions, a large chunk of DNA is accidentally transferred from one chromosome to another.
This massive mutation is more likely to fool scientists. The probability of the same chunk of DNA moving to the exact same location a second time is astronomically low. It’s also nearly impossible for that chunk to return to the exact place it came from.
“This is direct evidence of something that happened,” said Dr. Schultz.
His team tracked the movements of genetic material in the chromosomes of nine animals, including three single-celled relatives of the animals. They found a number of DNA fragments in exactly the same place in the genomes of sponges and other animals. But these chunks are in a different position in comb jellies and single-celled animal relatives. That search led Dr. Schultz and his colleagues concluded that comb jellies first diverged from other animals.
“It’s a fresh look with a fresh approach to the question,” said Antonis Rokas, an evolutionary biologist at Vanderbilt University, who was not involved in the study.
In a 2021 study, Dr. Rokas and his colleagues also dropped in favor of comb jellies. He said the new analysis provided a strong confirmation.
“I’ve learned not to say the debate is over,” said Dr. Rokas. “But this is what moves the needle.”
The study raises intriguing new possibilities for what the common ancestor of living animals looked like. If comb jellies, with their nervous systems and muscles, are the deepest branch on the animal tree, then the first animals may not have been simple and sponge-like. They also have nervous systems and muscles. Sponges later abandoned their nervous system.
Dr. warned. Schultz opposes thinking of comb jellies as living fossils, unchanged since the dawn of animals. “Something alive today cannot be the ancestor of something alive today,” he said.
Instead, researchers are now looking to comb jellies to see how similar and different their nervous systems are from other animals. Recently, Maike Kittelmann, a cell biologist at Oxford Brookes University, and his colleagues froze comb jelly larvae so they could see their nervous system. They were shocked by what they saw.
Throughout the animal kingdom, neurons are usually separated from each other by small gaps called synapses. They can communicate across space by emitting chemicals.
But when Dr. Kittelmann and his colleagues examined comb jelly neurons, they had difficulty finding a synapse between neurons. “At that point, we were like, ‘This is weird,'” he said.
In the end, they failed to find any synapses between them. Instead, the comb jelly nervous system forms a continuous web.
When Dr. Kittelmann and his colleagues reported their findings last month, they considered another possibility for the origin of animals. Comb jellies may have evolved their own unique nervous systems separately from other animals, using some of the same building blocks.
Dr. is now investigating. Kittelmann and his colleagues looked at other species of comb jellies to see if that idea held up. But they will not be surprised to be surprised again. “You don’t have to think about it,” he said.