Comb jellies were the first animals on Earth.

More than a century it took several generations of researchers to discover which was the oldest branch of the animal family tree, that first differentiated organism from the plant world that could have given rise to the diverse variety of fauna existing in the world today, including humans.

With the advancement of science and technology, the hypotheses were clarified until reaching two possibilities in the original bifurcation, which pointed to sponges and comb jellyfish, also known as ctenophores.

These two groups of atypical specimens were the most distantly related to the rest of the animals. In turn, they could be considered sister groups to each other, although sponges spend their entire adult lives in one place, filtering food from seawater, and jellyfish are voracious predators that move through the world’s oceans in search of food. of food.

The lineage of ctenophores branched out before that of sponges, so both groups of animals continued to evolve from their common ancestor.

Now, a new study published in the journal Nature would show that ctenophores (comb jellyfish) were the first lineage to break off in the animal tree. Sponges followed, and later came the diversification of all other animals, including the human lineage. To find the answer, the researchers used a new approach based on the chromosomal structure passed down by these primitive creatures.

The team – made up of researchers from the Monterey Bay Aquarium Research Institute (MBARI), the University of California (Berkeley and Santa Cruz) and the University of Vienna—mapped sets of genes that are always found together on the same chromosome, from humans and hamsters to crabs and corals, to clearly demonstrate that ctenophores are group-by-group siblings. all other animals.

This work determined, in effect, that the lineage of ctenophores branched out before that of sponges, so that both groups of animals continued to evolve from their common ancestor. However, evolutionary biologists believe that these groups continue to share characteristics with the first specimens of their species.

The most recent common ancestor of all animals probably lived 600 to 700 million years ago. It’s hard to know what it was like because they were soft-bodied animals and didn’t leave a direct fossil record.

Daniel Rokhsar, co-author

As they explain in the note with which they present the discovery, understanding the relationships between the animals will help us understand how the first specimens of the fauna appeared and how they evolved over time to reach the diversity of species that we see today. development of key features of our anatomy, such as the nervous system or the digestive system.

define an animal

“The most recent common ancestor of all animals probably lived between 600 and 700 million years ago. It’s hard to know what it was like because they were soft-bodied animals and didn’t leave a direct fossil record. But we can use comparisons between living animals to learn about our common ancestors,” says Daniel Rokhsar, professor of Molecular and Cellular Biology at the University of California at Berkeley and co-author of the paper with Darrin Schultz and Oleg Simakov of the University of Vienna.

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Jellyfish, sea anemones, sponges, and ctenophores have simpler body constitutions than worms or flies.

“We’ve developed a new way of looking at the origins of animal life, using genetics to travel back in time a billion years to obtain the strongest evidence yet to answer a fundamental question about the earliest events of animal evolution,” says Darrin Schultz.

In the absence of fossils, the researchers compared the genomes of very ancient ancestors.

Jellyfish, sea anemones, sponges and ctenophores are known to have simpler body constitutions than worms, flies, molluscs and vertebrates in general. They lack a definite brain and may not even have a nervous system or muscles, but they share the characteristics of animal life, most notably the development of multicellular bodies from a fertilized egg.

The evolutionary relationships between these various creatures, and specifically the order in which each of the lineages branched off from the main trunk of the animal tree of life, has been controversial. With the rise of DNA analysis, it is now possible to compare shared gene sequences to build a family tree that illustrates how animals and their genes have evolved over time, since the first organisms arose in the Precambrian.

“This research gives us a context for understanding what makes animals animals, as it helps us understand the basic functions we all share,” says the lead author.

Detect original link

All of an animal’s genes are arranged in sequence on chromosomes. The location of an individual gene sequence can change over time, but changes in the links between genes on a given chromosome are rare and largely irreversible.

When Schultz and his collaborators examined the genome at the chromosomal level of comb jellies, they observed a cluster of genes very different from patterns in other animals. More importantly, they found shared genetic patterns between ctenophores and three single-celled non-animal organisms, while those patterns were mixed in every other animal, from sponges to sparrows.

They represent him as a solitary single-celled organism, the ancestor of all animals, with two young. One would evolve into comb jellies, with the genes on their chromosomes staying in a specific order over time and not changing much. On the other hand, the other descendants could have evolved into sponges and all other animals as we know them today. In this second case, many of the genes on their chromosomes have been rearranged and fused. And since these rearrangements are irreversible and transmitted from generation to generation, they are still detectable today.

By tracking these rearrangements, the team found clear evidence that comb jellies, not sponges, are the sister group to all other animals. “Traces of this ancient evolutionary event are still present in animal genomes hundreds of millions of years later,” concludes Schultz.

Reference:

D. Schultz et al. “Ancient genetic links support ctenophores as siblings to other animals.” Nature (2023)

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