Sea creatures shed light on the origin of the animal kingdom – 05/23/2023 – Science

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 have evolved into an incredible variety of forms, such as whales that weigh 200 tons, birds that soar 10 kilometers across the sky, and snakes that slither across desert dunes.

Scientists have long wondered what the first animals looked like, including questions about their anatomy and how they found food. In a study published last Wednesday (17), scientists found tantalizing answers in a little-known group of gelatinous creatures called bioluminescent ctenophores, popularly known as comb jellyfish, or sea carambola.

Although the earliest animals remain a mystery, scientists have discovered that hawksbills belong to the deepest branch of the animal family tree.

The debate about the origin of animals has lasted for decades. Early on, researchers relied heavily on the fossil record for clues. The oldest definitive animal fossils date back to around 580 million years, although some researchers have claimed they have found even older ones. In 2021, for example, Canadian paleontologist Elizabeth Turner reported finding fossils 890 million years old, possibly from sponges.

Sponges would make sense as the oldest animal. They are simple creatures, without muscles or nervous systems. They anchor themselves to the ocean floor, where they filter water through a maze of pores, trapping plankton and small foods.

Sponges are so simple, in fact, that it might be surprising to learn they’re animals, but their molecular makeup reveals their parentage. 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 forms of life.

Starting in the 1990s, as scientists collected DNA from more species, they tried to draw the animals’ family tree. In some studies, the sponges ended up on the lowest branch of the tree. In this scenario, the animals developed a nervous system only after the sponges branched out.

But in the early 2000s other scientists came to a surprisingly different conclusion. They found that the deepest branch of the animals were the hawksbill jellyfish — slender, oval-shaped creatures that often develop a set of iridescent stripes that flash in the deep darkness of the ocean.

Many experts were reluctant to accept this conclusion, because it meant that animal evolution was stranger than they realized. For one thing, comb jellyfish weren’t as simple as sponges. They have a nervous system: a web of neurons around their bodies that controls their muscles.

To settle the comb jellyfish versus sponge debate, researchers around the world have collected DNA from more species of ocean animals. And instead of looking at individual genes, they figured out how to sequence entire genomes.

But the avalanche of new data has failed to resolve the debate. Some scientists ended up putting together a tree in which sponges were the deepest branch, while others stuck with comb jellyfish.

The new study, published in the journal Nature, relies on a new method that uses DNA to track animal evolution.

In previous studies, scientists looked at how certain mutations arose in different animal branches. A mutation can cause a single genetic letter, known as the base, to change to a different letter. This mutation will then be inherited by an animal’s offspring.

But these mutations may not be reliable as markers of history. A base can change from one letter to another and, millions of years later, go back to the original. Alternatively, the same base can change to the same letter in two unrelated lineages. This 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 (Austria), and his colleagues looked at a different kind of genetic change. On rare occasions, a large piece of DNA will be accidentally moved from one chromosome to another.

This massive mutation is less likely to mislead scientists. The chances of the exact same piece of DNA moving to exactly the same location a second time are astronomically small. It is also almost impossible for that piece to return exactly to where it came from.

“It’s direct evidence that something happened,” Schultz said.

His team tracked the movements of genetic material in the chromosomes of nine animals, along with three single-celled animal relatives. They found several 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 hawksbill jellyfish than in single-celled animal relatives. This discovery led Schultz and his colleagues to conclude that hawksbills first separated from other animals.

The study raises intriguing new possibilities about what the common ancestor of living animals looked like. If hawksbill jellyfish, with a nervous system and muscles, are the deepest branch of the animal tree, then the first animals may not have been simple and spongy. They also had nervous systems and muscles. Only later did the sponges leave his nervous system.

Schultz cautioned against thinking of hawksbill jellyfish as living fossils, unchanged since the appearance of animals. “Something that is alive today cannot be the ancestor of something alive today,” he said.

Instead, researchers are now examining hawksbill jellyfish to see how their nervous systems are similar to and different from those of other animals. Recently, Maike Kittelmann, a cell biologist at Oxford Brookes University, and her colleagues froze hawksbill jellyfish larvae so they could get a microscopic look at their nervous system. What they saw left them baffled.

Throughout the animal kingdom, neurons are normally separated from one another by tiny gaps called synapses. They can communicate across the gap by releasing chemicals.

When Kittelmann and his colleagues began to inspect the neurons in the hawksbill jellyfish, however, they had trouble finding a synapse between the neurons. “At that point, we thought, ‘This is curious,'” she said.

In the end, they couldn’t find any synapses. Instead, the hawksbill’s nervous system forms a continuous web.

When Kittelmann and his colleagues reported their findings last month, they speculated yet another possibility for the animals’ origin. Hawksbill jellyfish may have evolved their own strange nervous system independently of other animals, using some of the same building blocks.

Kittelmann and his colleagues are now inspecting other species of hawksbill jellyfish to see if this idea holds up. But they will not be surprised if they are surprised again. “You shouldn’t assume anything,” she said.

^{Translated by Luiz Roberto M. Gonçalves}