First Animals: The comb jellyfish was the first

What is the earliest evolution of animals? Which group made up the first branch in the animal family tree? Using chromosome analysis, a study has now shown that comb jellyfish were the first to split about 700 million years ago — even before sponges, which were once thought to be the most primitive representatives of the animal kingdom. Therefore, the comb jellyfish is among all modern animals at the genetic level still most closely related to the protozoa from which the first animals arose.

Several hundred million years ago, the first multicellular animals of the oceans arose from single-celled microorganisms. But which of the animal groups that can still be found today are the most primitive? Until now, science has generally assumed it’s the sponge. They have neither organs nor nervous system and they act as a group of single-celled organisms. Another candidate was the comb jellyfish (Ctenophora), which looks like jellyfish but is distantly related to them. Although genetic analyzes showed that both sponges and comb jellyfish are very primitive, they were unable to elucidate which of the two groups made up the first branch of the animal’s family tree.

Sponges, comb jellyfish, and protozoa were examined

A team led by Darren Schultz of the University of Vienna in Austria has solved this mystery. Rather than analyzing how genes evolve over time, Schultz and his colleagues focused on the order in which genes are arranged on chromosomes. This trait changes very little over the course of evolution. In an earlier study, the research team showed that the chromosomes of sponges, jellyfish, and many other invertebrates carry very similar chromosomal genotypes, even though they have evolved independently for more than half a billion years.

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For the current study, the team compared the chromosomal makeup of two species of sponge, two species of comb jellyfish and three single-celled unicellular creatures. Result: The arrangement of genes in comb jellyfish was more similar to that of protozoa at key points. “That was the gist of it,” says co-author Daniel Rokhsar of the University of California, Berkeley. “We found a handful of rearrangements that are shared by sponges and all other animals except the comb jellyfish. In contrast, the comb jellyfish is similar to non-animals. The most plausible explanation is that the comb jellyfish branched before the reorganization.”

A window into the past

“Fingerprints of this ancient evolutionary event can still be found in the genomes of animals hundreds of millions of years later,” says Schultz. Chromosome analyzes thus open a window into a largely unknown era of early animal development. “The most recent common ancestor of all animals probably lived 600 or 700 million years ago,” Rokhsar explains. “It’s hard to know what they looked like because they are soft-bodied animals that have left no direct fossil traces. But we can do comparisons between living animals to get a sense of our common ancestors. With our analysis, we dig deep into the past where we have no hope of finding fossils — but by comparing genomes, We learn about these very early ancestors.”

The study thus provides an important building block for understanding how animal phyla relate to one another – from primitive organisms such as comb jellyfish and sponges to worms and arthropods to vertebrates such as humans. It can also help understand how important features such as the nervous system, muscles, and digestive system develop. “This research gives us context for understanding what makes animals animals,” Schultz says. “It will help us understand the basic functions that we all share, such as environmental awareness, nutrition, and movement.”

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Source: Darren Schultz (University of Vienna, Austria) et al., Nature, Available Here. doi: 10.1038/s41586-023-05936-6

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