One of the largest transitions in the history of life occurred more than 600 million years ago, when a single-celled organism gave rise to the first animals. With their multicellular bodies, animals have evolved into a dizzying array of shapes, like 200-ton whales, birds that soar six miles into the sky and sidekicks that glide across desert dunes.
Scientists have long wondered what the first animals were like, including questions about their anatomy and how they found food. in Stady Published Wednesday, scientists have found tantalizing answers in a little-known group of gelatinous creatures called comb jellies. While the first animals are still a mystery, scientists have found that comb jellies belong to the deepest branch of the animal family tree.
The debate about the origin of animals has been going on for decades. At first, researchers relied largely on the fossil record for clues. The oldest specific animal fossils date to about 580 million years, although some researchers claimed to have found an even older one. In 2021, for example, Elizabeth Turner, a Canadian paleontologist, reported a discovery 890-million-year-old fossils of possible sponges.
It makes sense that sponges are the oldest animal. 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 pores, trapping bits of food.
Sponges are so simple, in fact, that it can come as a surprise that they are animals at all, but their molecular structure reveals their kinship. They make certain proteins, such as collagen, that only animals produce. Moreover, their DNA shows that they are more closely related to animals than to other forms of life.
Starting in the 1990s, when scientists collected DNA from more animal species, they attempted to map out the animal’s family tree. In some studies, sponges ended up on the deepest branch of the tree. In this scenario, the animals developed a nervous system only after the sponges bifurcated.
But in the early 2000s, other scientists came to a surprisingly different conclusion. They found that the deepest branch of the fauna was the jelly candies—thin, oval creatures that often grow a distinctive array of iridescent bands that flicker in the darkness of the ocean depths.
Many experts were reluctant to accept this conclusion, as it meant that animal evolution was stranger than they had realised. For one thing, comb gels aren’t as simple as a sponge. They have a nervous system: a network of neurons running around their bodies that control their muscles.
To resolve the jelly-comb-sponge debate, researchers from around the world collected DNA from more species of ocean animals. Instead of looking at single genes, the researchers figured out how to sequence the entire genome.
But the torrent of new data failed to settle the controversy. Some scientists ended up assembling a tree in which the sponge was the deepest branch, while others ended up cutting out the comb jelly.
The new study, published in the journal Nature, builds on a new method of using DNA to track the evolution of animals.
In previous studies, scientists looked at how specific mutations appear in different branches of animals. A mutation may cause one genetic letter, known as the base, to be transformed into a different letter. This mutation will then be inherited by the animal’s descendants.
But these mutations can be unreliable historical markers. The base might shift from one letter to the next and, millions of years later, back to the original letter. Alternatively, the same base may denote the same letter in two unrelated lineages. This parallel development creates the illusion that the two lineages are closely related.
In the new study, Darren Schultz, an evolutionary biologist at the University of Vienna, and his colleagues instead looked at a different kind of genetic change. In rare cases, a large portion of DNA is crossed accidentally from one chromosome to another.
Scientists are unlikely to be fooled by such a huge mutation. The odds of the exact same piece of DNA moving to the exact same location a second time are astronomically low. It is also impossible for that piece to return to the exact place it came from.
“It’s direct evidence that something happened,” said Dr. Schultz.
His team tracked the movements of genetic material in the chromosomes of nine animals, along with three single-celled relatives of the animals. They found a number of pieces of DNA in exactly the same place in the genomes of sponges and other animals. But these pieces were in a different position in comb jellies and their single-celled animal relatives. This discovery led Dr. Schultz and his colleagues to conclude that comb jellies separated from other animals first.
“It’s a new look with a new approach to the issue,” said Antonis Rocas, an evolutionary biologist at Vanderbilt University who was not involved in the study.
In a 2021 study, comb jelly was endorsed by Dr. Rocas and colleagues. He said the new analysis provided a strong confirmation.
“I’ve learned not to say the controversy is over,” said Dr. Rocas. “But this moves the needle.”
The study raises new and intriguing possibilities for what the common ancestor of living animals might have looked like. If comb jellies, with their nervous system and muscles, were the deepest branch of the animal tree, perhaps early animals weren’t simple and sponge-like. They also had nervous systems and muscles. Only later did the sponge give up its nervous system.
Dr. Schultz cautioned against considering comb jellies as living fossils that have not changed since the dawn of animals. “Something alive today cannot be the ancestor of something alive today,” he said.
Instead, the researchers are now looking to comb the jellies to see how similar and different their nervous systems are to those of other animals. Recently, Mike Keitelman, a cell biologist at Oxford Brookes University, and her colleagues froze comb jelly larvae so they could get a microscopic look at their nervous system. What they saw left them confused.
Throughout the animal kingdom, neurons are usually separated from each other by tiny gaps called synapses. They can communicate across the gap by releasing chemicals.
But when Dr. Kettleman and her colleagues began examining the metatarsal gel neurons, they struggled to find the synapses between the neurons. “At that point, we were like, ‘This is curious,'” she said.
In the end, they failed to find any synapses between them. Instead, the metacarpophalangeal nervous system forms one continuous network.
According to Dr. Kettleman and her colleagues I reported their findings last month, and speculated another possibility for the origin of animals. Comb jellies may have evolved their peculiar nervous system independently of other animals, using some of the same building blocks.
Dr. Kittelmann and her colleagues are now examining other types of comb jelly to see if this idea holds up. But they will never be surprised again. “You shouldn’t assume anything,” she said.