Biologists have been looking for the earliest animals for over a century, narrowing the options to two groups: sponges and comb jellies.
According to a new study, ctenophores were the first lineage to branch from the animal tree. Sponges came next, followed by diversifying all other animals, including human origin.
Although the ctenophore lineage diverged before sponges, both groups of animals have continued to expand from their common ancestor.
However, evolutionary scientists think that these groupings still share traits with the earliest animals and that researching these early animal trees of life branches will help us understand how animals came to be and developed into the variety of species we see today.
“All animals’ most recent common ancestor 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.
The researcher said, “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 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.
We developed a new way to take one of the most profound 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 develop a better understanding of how animals have evolved.
Most familiar animals, including worms, flies, mollusks, sea stars, and vertebrates — including humans — have a centralized brain, a gut that runs from mouth to anus, muscles, and other shared features that had evolved by the time of the famed “Cambrian Explosion” 500 million years ago. These animals are known together as bilaterians.
Rokhsar said, “Traditionally, sponges have been widely considered to be the earliest surviving branch of the animal tree because sponges don’t have a nervous system, they don’t have muscles, and they look a little bit like colonial versions of some unicellular protozoans, And so, it was a nice story: First came the unicellular protozoans, and then sponge-like multicellular consortia of such cells evolved and became the ancestor of all of today’s animal diversity. In this scenario, the sponge lineage preserves many features of the animal ancestor on the branch leading to all other animals, including us. Specializations evolved that led to neurons, nerves, muscles, guts, and all those things we know and love as the defining features of the rest of animal life. Sponges appear to be primitive since they lack those features.”
DNA sequencing has been used to construct a family tree. However, it has failed to resolve the controversy over whether sponges or comb jellies were the earliest branches of the animal tree.
He said, “The results of sophisticated sequence-based studies were split. Some researchers did well-designed analyses and found that sponges branched first. Others did equally complex and justifiable studies and got ctenophores. There hasn’t been any convergence to a definitive answer.”
In 2019, a bioluminescent deep-sea sponge was discovered. Because sponges lack nerves and muscles, they have historically been regarded as the earliest surviving branch of the animal tree.
Jellyfish, sea anemones, sponges, and ctenophores lack many bilaterian characteristics but share the hallmarks of animal existence.
The evolutionary relationships among these diverse creatures have been controversial due to the deep antiquity of their divergence.
A new study has discovered that comb jellies and ctenophores are also possibilities for the oldest animal lineage. This is owing to the structuring of genes into chromosomes, each species having a unique chromosome number and gene distribution along chromosomes. This shows that the first animals originated billions of years ago from single-celled protists.
The other candidate for earliest animal lineage is the group of comb jellies, popular animals in many aquariums. At the same time, they look superficially like jellyfish — they often have a bell-like shape, although with two lobes, unlike jellyfish, and usually tentacles — they are only distantly related.
While jellyfish squirt through the water, ctenophores push themselves with eight rows of beating cilia arrayed like combs down their sides. The 1-inch-diameter sea gooseberry is a frequent ctenophore along the California coast.
The new study relied on an unusual attribute to determine whether sponges or ctenophores were the oldest branch of animals: the arrangement of genes into chromosomes. Each species has a different chromosome number — humans have 23 pairs — and another gene distribution throughout chromosomes.
Rokhsar, Simakov, and others had previously demonstrated that many invertebrates’ chromosomes contain comparable sets of genes. However, the chromosome structure of ctenophores was unknown until 2021, when Schultz and his co-advisers identified the chromosome structure of Hormiphora californensis.
This demonstrated that ctenophores and non-animals shared specific gene-chromosome combinations, whereas sponges and other animals’ chromosomes were rearranged differently.
Bolinopsis microptera is a ctenophore that has shared rearrangements with non-ctenophore animals. This shows that ctenophores diverged before the rearrangements, still present in animal genomes hundreds of millions of years later.
This study is funded by the David and Lucile Packard Foundation.