May 17. (EUROPE PRESS) –
A novel approach based on chromosome structure has revealed that comb jellies, or ctenophores, were the first lineage to branch off the animal tree. Sponges were next, followed by the diversification of all other animals, including the lineage that led to humans.
Although the researchers determined that the ctenophore lineage branched off before the sponges, both groups of animals continued to evolve from their common ancestor. However, evolutionary biologists believe that these groups still share characteristics with the earliest animals, and that studying these early branches of the animal tree of life may shed light on how animals arose and evolved. to the diversity of species that we see around us today.
“The most recent common ancestor of all animals probably lived 600 to 700 million years ago. It’s hard to know what they looked like because they were soft-bodied animals and left no direct fossil record. But we can use comparisons between living animals to learn about our common ancestors,” said it’s a statement Daniel Rokhsar, professor of cell and molecular biology at the University of California, Berkeley and co-author of the new study published in Nature along with Darrin Schultz and Oleg Simakov of the University of Vienna.
Understanding the relationships between animal lineages will help scientists understand how key features of animal biology, such as the nervous system, muscles and digestive tract, evolved over time, the researchers say.
“We developed a new way to get one of the deepest possible looks into the origins of animal life,” Schultz, lead author and a former UC Santa Cruz graduate student and researcher at the Aquarium Research Institute, said in a statement. from Monterey Bay (MBARI) who is now a postdoctoral researcher at the University of Vienna. “This finding will lay the groundwork for the scientific community to begin to develop a better understanding of how animals have evolved.”
Most familiar animals, including worms, flies, mollusks, starfish, and vertebrates, including humans, have a head with a centralized brain, a gut that runs from the mouth to the anus, muscles, and other shared features that already exist. they had evolved at the time of the famous “Cambrian Explosion” some 500 million years ago. Together these animals are called bilaterals.
However, other animals, such as jellyfish, sea anemones, sponges, and ctenophores, have simpler body designs. These creatures lack many bilateral features—for example, they lack a definite brain and may not even have a nervous system or muscles—but they still share the features of animal life, notably the development of multicellular bodies from a fertilized egg. The evolutionary relationships between these various creatures—specifically, the order in which each of the lineages branched off the main trunk of the animal tree of life—has been a matter of controversy.
With the rise of DNA sequencing, biologists were able to compare gene sequences shared by animals to build a family tree that illustrates how animals and their genes evolved over time since the first animals arose in the Precambrian Period. But these phylogenetic methods based on gene sequences failed to resolve the controversy over whether sponges or comb jellies were the oldest branch of the animal tree, in part because of the deep antiquity of their divergence.Rokhsar said.
Just by looking at them, sponges appear quite primitive. After their free-swimming larval stage, they settle and generally stay in one spot, gently sweeping water through their pores to capture tiny food particles dissolved in the seawater. They have no nerves or muscles although its hard parts are good cleaners in the bathroom.
“Traditionally, sponges have been widely considered to be the oldest surviving branch of the animal tree, because sponges don’t have a nervous system, they don’t have muscles, and they look a bit like colonial versions of some single-celled protozoa,” Rokhsar said. . The sponge lineage retains many characteristics of the animal ancestor in the branch that leads to all other animals, including us. Specializations evolved that gave rise to neurons, nerves, muscles, guts, and all those other things we know and love as defining characteristics of the rest of animal life. Sponges appear to be primitive, as they lack those features.”.
The other candidate for the oldest animal lineage is the group of comb jellies, popular animals in many aquariums. While superficially resembling jellyfish, they are often bell-shaped, though with two lobes, unlike jellyfish, and usually tentacles, they are only distantly related. And while the jellyfish make their way through the water, the ctenophores propel themselves with eight rows of cilia arranged on the sides like combs.
To find out if sponges or ctenophores were the first branch of animals, the new study relied on an unlikely feature: the organization of genes into chromosomes. Each species has a characteristic number of chromosomes (humans have 23 pairs) and a characteristic distribution of genes along the chromosomes.
Rokhsar, Simakov, and their collaborators had previously shown that the chromosomes of sponges, jellyfish, and many other invertebrates carry similar sets of genes, despite more than half a billion years of independent evolution. This discovery suggested that the chromosomes of many animals evolve slowly and allowed the team to computationally reconstruct the chromosomes of the common ancestor of these various animals.
But the chromosome structure of ctenophores was unknown until 2021, when Schultz, then a graduate student at UC Santa Cruz, and his co-advisors, Richard Green of UCSC and Steven Haddock of MBARI and UCSC, determined the chromosome structure of the ctenophore Hormiphora californensis. It looked very different from those of other animals, which posed a puzzle, Rokhsar said.
The researchers joined forces to sequence the genomes of another ctenophore and sponge, as well as three single-celled creatures that are outside the animal lineage: a choanoflagellate, a philasterean amoeba, and a fish parasite called an ichthyospore. Approximate genome sequences for these non-animals already existed, but they did not contain the critical information needed for gene linkage at the chromosomal scale: where they are located on the chromosome.
Surprisingly, when the team compared the chromosomes of these various animals and non-animals, they found that ctenophores and non-animals shared particular combinations of genes and chromosomes, while the chromosomes of sponges and other animals rearranged in a distinctly different way.
“That was the irrefutable proof: we found a handful of rearrangements shared by sponges and non-ctenophores. In contrast, the ctenophores resembled non-animals. The simplest explanation is that the ctenophores branched before the rearrangements occurred.” said.
