Strange sea creature turns the tide on a debate
By Carl Zimmer
One of the greatest transformations 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 evolved into a staggering range of forms, like whales that weigh 200 tons, birds that soar 6 miles into the sky and sidewinders that slither across desert dunes.
Scientists have long wondered what the first animals were like, including questions about their anatomy and how they found food. In a study published this month, scientists found tantalizing answers in a little-known group of gelatinous creatures called comb jellies. While the first animals remain a mystery, scientists found that comb jellies belong to the deepest branch on the animal family tree.
The debate over the origin of animals has endured for decades. At first, researchers relied largely on the fossil record for clues. The oldest definitive animal fossils date back about 580 million years, although some researchers have claimed to find even older ones. In 2021, for example, Elizabeth Turner, a Canadian paleontologist, reported finding 890-million-year-old fossils of possible sponges.
Sponges would make sense as the oldest animal. They are simple creatures, with no muscles or nervous system. They anchor themselves to the ocean floor, where they filter 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 makeup reveals their kinship. They make certain proteins, such as collagen, that are produced only by animals. What’s more, their DNA shows they are more closely related to animals than to other forms of life.
Starting in the 1990s, as scientists gathered DNA from more animal species, they tried to draw the animal family tree. In some studies, the sponges ended up on the deepest branch of the tree. In this scenario, animals evolved a nervous system only after the sponges branched off.
But in the early 2000s, other scientists came to a surprisingly different conclusion. They found that the deepest branch of animals were comb jellies — slim, oval creatures that often grow a distinctive set of iridescent bands that flicker in the darkness of the deep ocean.
Many experts were reluctant to accept that conclusion, because it meant animal evolution was weirder than they had realized. For one thing, comb jellies were not as simple as sponges. They have a nervous system: A web of neurons circling their bodies controls their muscles.
To resolve the comb-jelly-versus-sponge debate, researchers around the world collected DNA from more species of ocean animals. And instead of looking at single genes, researchers figured out how to sequence entire genomes.
But the avalanche of new data failed to settle the debate. Some scientists ended up assembling a tree in which sponges were the deepest branch, 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 arise in different animal branches. A mutation may cause a single genetic letter, known as a base, to switch to a different letter. That mutation will then be inherited by an animal’s descendants.
But these mutations can be unreliable markers of history. A base may switch from one letter to another, and then millions of years later, it may switch back to the original one. Alternatively, the same base may switch to the same letter in two unrelated lineages. 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 kind of genetic change. On rare occasion, a huge chunk of DNA will get accidentally moved from one chromosome to another.
This massive mutation is less likely to deceive scientists. The odds that precisely the same chunk of DNA moves to precisely the same location a second time is astronomically low. It’s also next to impossible for that chunk to move back to exactly the spot from which it came.
“It’s direct evidence of something that happened,” Schultz said.
His team tracked the movements of genetic material in the chromosomes of nine animals, along with three single-celled relatives of animals. They found a number of chunks of DNA in precisely the same spot in the genomes of sponges and other animals. But these chunks were in a different position in comb jellies and single-celled relatives of animals. That finding led Schultz and his colleagues to conclude that comb jellies split off from other animals first.
The study raises intriguing new possibilities for what the common ancestor of living animals looked like. If comb jellies, with a nervous system and muscles, are the deepest branch on the animal tree, then early animals may have not been simple and spongelike. They had nervous systems and muscles too. Only later did sponges abandon their nervous system.
Schultz cautioned against thinking of comb jellies as living fossils, unchanged since the dawn of animals. “Something that’s alive today can’t be the ancestor of something alive today,” he said.
Instead, researchers are looking now to comb jellies to see how similar and different their nervous systems are from those of other animals. Recently, Maike Kittelmann, a cell biologist at Oxford Brookes University, and her colleagues froze comb jelly larvae so that they could get a microscopic look at their nervous system. What they saw left them baffled.
Throughout the animal kingdom, neurons are typically separated from one another by tiny gaps called synapses. They can communicate across the gap by releasing chemicals.
But when Kittelmann and her colleagues started to inspect the comb jelly neurons, they struggled to find a synapse 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 comb jelly nervous system forms one continuous web.
When Kittelmann and her colleagues reported their findings last month, they speculated yet another possibility for the origin of animals. Comb jellies may have evolved their own weird nervous system independently of other animals, using some of the same building blocks.
Kittelmann and her colleagues are now inspecting other species of comb jellies to see if that idea holds up. But they won’t be surprised to be surprised again. “You have to assume nothing,” she said.