The first multicellular animals to exist on Earth

Genetic research reveals that comb jellies may have been the first group of multicellular animals to appear, although they have more complex nervous systems than sponges.

Hormiphora californensis - a species of comb jelly. Photo: Darrin Schultz/2021 MBARI — *Hormiphora californensis* - a species of comb jelly. Photo: *Darrin Schultz/2021 MBARI*

Sponges ( Porifera ) have long been the leading contenders for the title of first multicellular animals due to their anatomical simplicity, for example, their lack of a nervous system. However, new data suggests that comb jellies ( Ctenophora ) take the top spot, despite having more complex nervous systems. The new study by an international team of scientists published in the journal Nature on May 17 shows that evolution is not a straightforward journey from simple to complex.

"The most recent common ancestor of all animals probably lived 600 or 700 million years ago. It's hard to know what they were like because they were soft-bodied animals and didn't leave a direct fossil record. But we can use comparisons with living animals to learn about their common ancestor," explains Daniel Rokhsar, a molecular biologist at the University of California, Berkeley, who was part of the research team.

Previous analyses of genetic sequences have yielded conflicting results, with some suggesting sponges came first, while others pointed to comb jellies. In the new study, the team compared the genomes of comb jellies, two types of sponges, two groups of single-celled animals (choanoflagellates and amoebas), a fish-like parasite, and fungi (ichthyospores) with those of other modern animals.

As it turns out, sponges and more modern animals share traits from a rare type of chromosome fusion and rearrangement event. But this doesn’t happen in comb jellies, whose genomes are arranged more like those of single-celled animals. So it’s likely that comb jellies evolved first, followed by sponges. Sponges then passed on their new chromosome arrangement to their descendants.

"Traces of this ancient evolutionary event are still present in the genomes of animals hundreds of millions of years later. The new study gives us a context for understanding what makes animals tick. It will also help us understand the basic functions we all have, such as how to sense our environment, eat, and move," said Darrin Schultz, a bioinformatician at the University of Vienna.

Thu Thao (According to Science Alert )

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