Comb Jellies Outdated Sponges as the Oldest Animal in the World

Friday - May 19 2023, 05:54 UTC - 1 year ago

In a new study published this week in Nature, researchers used a novel chromosome-based approach to reveal that Comb Jellies were the first lineage to diverge from the animal tree of life, preceding sponges. This research, providing new insights into early animal evolution, refines our understanding of how key biological features evolved.

For more than a century, biologists have wondered what the earliest animals were like when they first arose in the ancient oceans over half a billion years ago.

Searching among today’s most primitive-looking animals for the earliest branch of the animal tree of life, scientists gradually narrowed the possibilities down to two groups: sponges, which spend their entire adult lives in one spot, filtering food from seawater; and comb jellies, voracious predators that oar their way through the world’s oceans in search of food.

In a new study published this week in the journal Nature, researchers use a novel approach based on chromosome structure to come up with a definitive answer: Comb jellies, or ctenophores (teen’-a-fores), were the first lineage to branch off from the animal tree. Sponges were next, followed by the diversification of all other animals, including the lineage leading to humans.

Although the researchers determined that the ctenophore lineage branched off before sponges, both groups of animals have continued to evolve from their common ancestor. Nevertheless, 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 can shed light on how animals arose and evolved to the diversity of species we see around us today.

"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 across living animals to learn about our common ancestors," said Daniel Rokhsar, University of California, Berkeley professor of molecular and cell biology and co-corresponding author of the paper along with Darrin Schultz and Oleg Simakov of the University of Vienna. "It’s exciting — we’re looking back deep in time where we have no hope of getting fossils, but by comparing genomes, we’re learning things about these very early ancestors." .

Understanding the relationships among 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 take one of the deepest glimpses possible into the origins of animal life," said Schultz, the lead author and a former UC Santa Cruz graduate student and researcher at the Monterey Bay Aquarium Research Institute (MBARI) who is now a postdoctoral researcher at the University of Vienna. "This finding will lay the foundation for the scientific community to begin to develop a better understanding of how animals have evolved." .

Most familiar animals, including worms, flies, mollusks, sea stars, and vertebrates — and including humans — have a head with a centralized brain, a gut running from mouth to anus, muscles and other shared featues.