We still don’t know what the world’s first animal looked like, but scientists say it arose roughly 700 million years ago from a soup of single-celled organisms floating in the ocean. The multi-celled creature thrived, multiplied and evolved, at some point splitting into two distinct species.
One species kept evolving, eventually producing virtually all the animals on Earth — dinosaurs, humans, cats, mosquitoes. The other species, the “sister to all other animals,” took its own, narrower evolutionary path.
Now, after years of fierce debate, scientists have the clearest evidence to date which animal alive today is the sister’s true descendant: It’s the mysterious comb jelly, several species of which flourish in Monterey Bay.
The scientific consensus siding with the gelatinous deep-sea creature — over the other leading contender, the simple sponge — gelled over the summer after a team of Northern California researchers led by Darrin Schultz, a 30-year-old biologist at the Monterey Bay Aquarium Research Institute, provided the evidence in the scientific journal Nature in May. In the months since the report was published, the scientific community worldwide has embraced both the team’s findings and its novel approach, with many scientists now predicting that the team’s work will change the way evolution is studied.
“It is an extraordinary result,” said Max Telford, a zoologist at University College London who has spent years researching the subject and had always believed the sponge was the sister. “The new analyses are plain for anyone to see.”
Previously, scientists on both sides of the debate had largely relied on the traditional technique of comparing animals’ individual genes, but the NorCal team found a way to compare their entire chromosomes.
Composed of scientists from Moss Landing-based MBARI, UC Santa Cruz, UC Berkeley and the San Francisco-based Chan Zuckerberg Initiative, the team discovered that patterns in sponge chromosomes matched those found in virtually all other animals’ chromosomes. But the patterns in comb jelly chromosomes were distinctly different.
“I think it really is a rewrite-the-textbook kind of moment,” said Steven Haddock, a marine biologist at MBARI and adjunct professor of ecology and evolutionary biology at UC Santa Cruz. Haddock worked on the study with Schultz, having met the ambitious young scientist at UCSC when Schultz was working on his Ph.D. in biomolecular engineering and bioinformatics.
Haddock said a lot of researchers who thought the sponge was the sister species have said, “OK, finally I’m convinced. They grudgingly admit that this is a totally new way to look at it.”
Many scientists had championed the sponge theory because the animals are so simple. Their bodies are basically just a tube.
Comb jellies, on the other hand, are complex, sporting eight rows of hair-like cilia that help them move through the ocean. The cilia reflect light, so the animals appear to pulse with electricity like drifting deep-sea marquees. And unlike sponges, comb jellies have nerve and muscle cells.
Schultz’s team first made international waves in 2021 when it released the full genetic code of the 13 chromosomes of the California sea gooseberry, an oval-shaped comb jelly less than two inches long that lurks in the depths of the Monterey Bay.
That research paved the way for the new study, but the work was often grueling.
Comb jellies are transparent and mostly made of water, and they live up to six miles below the ocean’s surface. It’s extremely difficult to find and capture them with scuba gear.
Luckily, MBARI’s remotely operated vehicle (ROV), named Doc Ricketts in honor of marine biologist Ed Ricketts, who inspired the character “Doc” in John Steinbeck’s “Cannery Row,” was available to Schultz’s team. The ROV can dive down 2.5 miles and has cameras and robotic arms for finding and collecting samples.
Schultz said the work was only possible because the ROV and DNA sequencing technologies advanced at the right time. “The ability to go down and collect a rare organism from the deep sea, take that one small animal and make a perfect genome from it … is just a really cool synergy of technology that helped us achieve this,” he said.
After unraveling the genetic code of the California sea gooseberry, Schultz’s team kept collecting and sequencing other unique sea creatures from Monterey Bay, compared their chromosomes and made their ground-breaking discovery.
When Schultz first presented the team’s initial findings in June 2022 at an evolutionary biology conference in Roscoff, France, the audience was visibly startled.
“People were freaking out,” Haddock said. “They were like, ‘Finally!’’’
Telford, the London-based zoologist, and other researchers believe that a new suite of software programs — which Schultz developed himself and shared with the world — will open new doors for scientists who’ve devoted their lives to studying evolution.
Anthony Redmond, an evolutionary geneticist at Trinity College Dublin whose own work had pointed to the sponge, said the tools will let scientists track species’ evolutionary trajectories in ways “we haven’t been able to do up to now.”
Schultz, meanwhile, has moved on to a postdoctoral research position at the University of Vienna, where he says he intends to use the new techniques to “re-create the history of animal genome evolution.”
Eventually, Schultz said, he hopes to catch a glimpse of what life was like hundreds of thousands of years ago — and perhaps even discover what Earth’s first animal looked like and how it came to be.
