2023-05-17 18:08:08
Comb jellyfish or sponges? Until now, scientists had two hypotheses to understand the crucial moment in which, some 700 million years ago, a primitive organism gave rise to the variety of animals that exist today.
Approximately 700 million years ago one of the first key moments in animal evolution took place. For more than a century, scientists have been working to understand the crucial moment in which a primitive organism gave rise to the diverse variety of animals that exist in the world today. What was that animal and what it was like is the question that biologists are trying to solve. In a study published today in the journal Nature, a team today offers an answer, based on a new genetic analysis.
Searching among the most primitive looking animals, over the years and as technology and science have advanced, scientists have gradually narrowed the possibilities down to two groups: the sponges, that they spend their entire adult lives in one place, filtering their food from seawater; and comb jellyfish, voracious predators that travel the world’s oceans in search of food, producing a sticky substance with which they capture their prey. Currently living in the oceans are more than 150 species of comb jellyfish, such as the California gooseberry (Hormiphora californensis) that illustrates the article. Although they are little known, they are very abundant.
In this new study, they propose that the first to appear were those comb jellies (scientifically known as ctenphoros); sponges would have been the next to appear, followed by all other animals, including the lineage that led to humans.
To reach this conclusion, a team of researchers from the Monterey Bay Aquarium Research Institute (MBARI), the University of California, Berkeley, the University of California, Santa Cruz, and the University of Vienna mapped sets of genes that are always found together on a single chromosome, both in humans and in species as diverse as hamsters, crabs, or corals, for clear evidence that comb jellies are the brother group of all other animals.
In the words of Darrin Schultz, co-author and researcher at the University of Vienna, have used “genetics to travel back in time around a billion years to obtain the strongest evidence yet to answer a fundamental question about early events in animal evolution.”
With this type of genetic study, the authors explain, they can obtain information about how the nervous and digestive systems have evolved over time.
Chromosome structure
All of an animal’s genes are arranged in sequences on chromosomes. The sequence location of an individual gene can change over time, but changes in the links between genes on a particular chromosome are rare and largely irreversible.
The scientists examined links between genes on specific chromosomes that are conserved over time. They identified patterns that exist in a variety of animals and mapped those links back to the earliest point in animal evolution. In this way, they found strong evidence that comb jellies diverged of the tree of life before sponges, with which they had a common ancestor.
The team compares this evolutionary event to a genetic fork in the evolutionary path that occurred hundreds of millions of years ago. A solitary single-celled organism, the ancestor of all animals, traveled this road with its two offspring. A boy, who would become comb jellyfish as we know them today, took a path. As it evolved, the genes on its chromosomes stayed in a specific order and didn’t change much.
The other child of that unicellular organism took the other path and was the one that would end up becoming sponges and all the other animals as we know them today. Many of the genes on their chromosomes have been rearranged and fused together, and because these rearrangements are irreversible and are passed down from generation to generation, they are detectable even today.
By tracking these chromosome rearrangements, the scientific team found clear evidence that comb jellies, not sponges, are the sister group to all other animals. “Fingerprints of this ancient evolutionary event are still present in the genomes of animals hundreds of millions of years later,” compares Schultz, who believes that this research will help understand what makes animals animals, how they perceive their environment, how they eat and move, and the basic functions we all share.
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