More than 600 million years ago, the first multicellular organisms evolved in the ocean, giving rise to all animals alive today. Scientists have long wondered which was the first group to break off from this common ancestor and begin evolving separately. Over the years, competing studies have frustrated researchers by bouncing back and forth between two possibilities: comb jellies and sponges.
It’s “like a ping-pong match,” Darrin Schultz, a developmental biologist at the University of Vienna, tells New Scientist’s James Dinneen. “People feel like they’ve been banging their heads against the wall.”
Now, a new study published Wednesday in Nature points once more to ancestors of comb jellies as the first to split off.
“I’ve learned not to ever say the debate is over,” Antonis Rokas, an evolutionary biologist at Vanderbilt University who was not involved in the study, tells the New York Times’ Carl Zimmer. “But this moves the needle.”
When it comes to primitive creatures, sponges might seem like the more plausible choice—they lack muscles and a nervous system and have a type of cell called a choanocyte, which bears resemblance to single-celled non-animals called choanoflagellates, per Science’s Elise Overgaard. Sponges are such simple organisms that they’re often mistaken for plants. But comb jellies—oval-shaped animals that, despite their name, are not closely related to jellyfish—are more complex. It would complicate our understanding of evolution if they did indeed split off first.
Previous research into comb jelly and sponge evolution has looked at small-scale changes in DNA sequences, per New Scientist. But this method is not always reliable—mutations in DNA may arise for a while and then, over millions of years, change back, per the Times. Additionally, the same mutation can arise separately in lineages that are unrelated.
So, in the new study, scientists tried a different approach based on chromosome structure. Usually, despite pieces of DNA getting swapped and changed, genes stay on the same chromosome. But sometimes, in a rare and near-irreversible event, huge portions of DNA move to a new chromosome. These larger-scale patterns are less variable over long periods of evolution, per New Scientist.
The team examined the chromosomes of comb jellies, sponges and three different kinds of single-celled ancestors of animals. They found that groups of genes in comb jellies and in the single-celled organisms were arranged in the same manner—but those genes in the sponges and other animals were arranged in a different way, per a statement. The finding suggests the ancestors of comb jellies may have branched off from the common ancestor before these gene rearrangements happened, which is why they do not share the gene patterns found in sponges and other animals.
“That was the smoking gun,” co-author Daniel Rokhsar, a molecular and cell biologist at the University of California, Berkeley, says in the statement.
This study could change the way we think of animal evolution, including pushing back the evolution of neuron-like cells by 100 million years, Shultz tells Science. But not all researchers are convinced that the new paper can resolve years of debate.
“For 150 to 200 years, people have always assumed sponges are near the base of the tree,” Kenneth Halanych, a marine biologist at the University of North Carolina Wilmington, tells New Scientist. “You need multiple sources of the strongest data to really convince people.”