New research based on chromosome structure has revealed that: comb jellies, or ctenophores, were the first lineages to branch from the tree. animals. Sponges were next, followed by diversification of all other animals, including the lineage that went to humans.
Although the researchers determined that the ctenophore lineage diverged before 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 examining these early branches of the animal tree of life can shed light on how animals arose and evolved into the diversity of species we see today.
“The closest common ancestor of all animals probably lived between 600 and 700 million years ago.. It’s hard to know what they looked like because they were soft-bodied animals and did not leave a direct fossil record. But we can use comparisons between living animals to learn about our common ancestors,” he said. Nature With Darrin Schultz and Oleg Simakov of the University of Vienna.
relationships between lineages
Understanding the relationships between animal lineages will help scientists understand how key features of animal biology evolved, Like the nervous system, muscles, and digestive system, the researchers said.
“We’ve developed a new way to get one of the deepest possible insights into the origins of animal life,” Schultz, lead author of the study, said in a statement. Said. “This finding will lay the groundwork for the scientific community to begin developing a better understanding of how animals evolve.”
Most familiar animals, including worms, flies, mollusks, starfish, and vertebrates, including humans, had a head with a central brain, a gut extending from the mouth to the anus, muscles, and other common features that already existed. It developed during the famous ‘Cambrian Explosion’ about 500 million years ago. Together; these animals are called bilateral.
But, Other animals such as jellyfish, sea anemones, sponges and ctenophores have simpler body designs.. These creatures don’t have many dual features—for example, they don’t have a specific brain and may not even have a nervous system or muscles—but they still share features of animal life, particularly the development of multicellular bodies from a fertilized egg.
The evolutionary relationships between these various creatures, especially the order in which each of the lineages branched from the main trunk of the tree of life, has been a matter of debate.
first animals discovered
With the rise of DNA sequencing, biologists were able to compare gene sequences shared by animals to create a family tree that shows how animals and their genes have evolved over time since they emerged in the Pre-Cambrian Period. However These phylogenetic methods, based on gene sequences, have failed to settle the debate over whether sponges or comb jellies are the oldest branch of the animal tree.Partly because of the profound antiquity of their differences, Rokhsar said.
Sponges look rather primitive just by looking at them. After the free-floating larval stage, they settle by gently opening their pores to catch small food particles dissolved in the seawater, and often stay in one place. They have no nerves or muscles, but their hard parts make good cleaning in the bathroom.
“Traditionally, sponges have been considered the oldest surviving branch. “Because sponges have no nervous system, no muscles, and look like colonial versions of some single-celled protozoa,” adds Rokhsar.
The sponge lineage retains many of the features of the branch animal ancestor that led to all other animals, including ourselves. They developed expertise that uncovered neurons, nerves, muscles, guts, and everything else we know as defining features of the rest of animal life. Sponges look primitive because they lack these features.”
Another group that is a candidate for the oldest animal lineage is the honeycomb jellyfish group., popular animals in many aquariums. Although superficially similar to jellyfish (unlike jellyfish they are two-lobed and often bell-shaped, although they are often tentacled), they are only distantly related. And as jellyfish navigate through the water, ctenophores move with eight rows of cilia arranged sideways like combs.
Organization of genes in chromosomes
To learn whether sponges or ctenophores were the first branch of animals, andThe new study was based on a little-known feature: the organization of genes on 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 shows that the chromosomes of many animals evolve slowly and allowed the team to computationally reconstruct chromosomes from common ancestor from these various animals
However chromosomal structure of ctenophores was not known until 2021Schultz, 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. Rokhsar said it looks very different from other puzzle-forming animals.
sequencing genomes
Researchers join forces Sequence the genomes of another ctenophore and sponge, plus three single-celled creatures outside the animal lineage: a choanoflagellate, a philasterean amoeba, and a fish parasite called an ichthyospore. Approximate genome sequences were already available for these non-animals, but they did not contain the critical information needed to link genes at the chromosome scale: where they are located on the chromosome.
Surprisingly, when the team compared the chromosomes of these various animals and non-animals, ctenophores and non-animals shared certain combinations of genes and chromosomes, while there are chromosomes sponges and other animals rearranged themselves in a completely different way.
“This was the irrefutable evidence: we found a handful of rearrangements shared by sponges and non-ctenophores. In contrast, ctenophores were similar to non-animal ones. The simplest explanation is that ctenophores branched out before rearrangements occurred“, He claimed.
Reference work: https://www.nature.com/articles/s41586-023-05936-6
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