At first glance, Mono Lake in California might look like a desolate place. Perched at the edge of the Sierra Nevada mountains, the lake shimmers in shades of turquoise and green, framed by eerie limestone towers known as tufa formations. Its waters are too salty and alkaline for fish, and its air can be heavy with a pungent smell when the wind shifts. Yet this unusual lake is far from lifeless. Beneath the surface, it hosts a carnival of strange organisms that have figured out how to thrive in conditions that would kill most other creatures.
Among them is a microscopic being that could hold answers to one of life’s greatest mysteries: how did the leap from simple, single-celled organisms to complex animals begin?
A Microscopic Clue From the Past
The star of this story is Barroeca monosierra, a single-celled organism belonging to a group called choanoflagellates. If the name sounds complicated, the idea is simple: choanoflagellates are the closest living relatives of animals. They aren’t animals themselves, but they carry enough similarities to help scientists trace the roots of our shared history.
Choanoflagellates live all over the world—in oceans, lakes, and rivers—but the ones in Mono Lake are unique. Instead of just drifting through the water eating bacteria (as their relatives usually do), this species seems to have developed a special relationship with bacteria. It doesn’t just swallow them—it houses them, allowing them to live within its colonies.
For scientists, this is groundbreaking. It means Barroeca monosierra is the simplest known organism with its own microbiome—a miniature world of bacteria living inside it. And that, in turn, could give us a glimpse into how early life forms began forming partnerships that eventually shaped animals, humans included.
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Why Mono Lake Is the Perfect Laboratory
Mono Lake is not your average lake. It’s one of the oldest lakes in North America, at least 760,000 years old, and possibly much older. Over time, it has become a closed basin—water flows in, but it has no outlet. This traps minerals and salts, making the water nearly three times as salty as the ocean and highly alkaline, with a pH similar to ammonia-based cleaners.
Add in toxic compounds like arsenic and cyanide, and you have a recipe for conditions that push life to its limits. Only highly specialized organisms can survive here. Among them are brine shrimp, alkali flies that walk underwater in silvery air bubbles, and the choanoflagellates.
This extreme environment forces evolution to get creative. It’s precisely the kind of place where scientists expect to find unusual survival strategies—and Mono Lake doesn’t disappoint.
From a Climbing Trip to a Discovery
The story of Barroeca monosierra’s discovery begins not with a planned expedition, but with a bit of scientific curiosity. Nearly a decade ago, UC Berkeley graduate student Daniel Richter scooped up a vial of water during a climbing trip near Mono Lake. Back in the lab, he peered at the sample under a microscope and was astonished.
Inside the water were large, hollow, spinning colonies of choanoflagellates. Each colony was made of around 100 identical cells arranged in a sphere, like a microscopic soccer ball. The cells beat their whip-like tails (called flagella) in unison, creating currents that made the entire colony twirl gracefully.
To Richter and his colleagues, these colonies looked familiar. They resembled a blastula, the hollow ball of cells that forms early in the development of animal embryos. Was this coincidence—or a living echo of ancient evolutionary steps?
The colonies were so unusual that the team decided to preserve them for future study, placing them in deep freeze. Years passed before the samples were revived by graduate student Kayley Hake, who would uncover their true secret.
The Surprise Inside
When Hake thawed the frozen colonies and examined them, something puzzling appeared. The hollow interior of the spheres contained DNA—but it didn’t belong to choanoflagellates. It was bacterial.
This was shocking. No one had ever documented bacteria living inside choanoflagellate colonies. To make matters more intriguing, the choanoflagellates had created an extracellular matrix—a sticky web-like structure—that appeared to house the bacteria, almost like an apartment complex.
This wasn’t a simple predator-prey interaction. It was a partnership.
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A Microscopic Time Machine
To understand what this means, it helps to step back. Life on Earth began about 3.5 billion years ago, but for most of that time, it consisted of single cells. Then, about 650 million years ago, something dramatic happened: groups of cells began working together, evolving into the first multicellular animals.
But how? That’s the mystery scientists are trying to solve. One possibility is that early cells learned to cooperate not just with each other, but with bacteria. If bacteria influenced how single cells grouped, communicated, or shared resources, they may have nudged evolution toward complexity.
Barroeca monosierra could be a living example of this ancient cooperation. By studying it, researchers may be able to see echoes of the partnerships that set the stage for animals—and eventually for us.
The DNA Detectives Step In
To identify exactly which bacteria were living inside the colonies, Nicole King’s team turned to UC Berkeley professor Jill Banfield, a pioneer in the field of metagenomics. This technique allows scientists to sequence all the DNA in an environmental sample, like a scoop of lake water, and piece together which species are present.
Banfield’s lab found that the bacteria inside the colonies were not the same as those floating freely in the lake. Instead, they formed a unique community, adapted to the oxygen-poor conditions inside the spheres. Some bacterial species thrived in this micro-habitat, while others were excluded.
Were the choanoflagellates choosing their partners? Or were the bacteria exploiting the colony as a safe haven? No one knows for sure. But the fact that such relationships exist at all is enough to shift how scientists think about early evolution.
A Bigger Lesson: Life Is About Partnerships
This discovery adds weight to a growing realization: life on Earth has never been a solo act. From the beginning, survival has depended on collaboration.
Consider mitochondria, the powerhouses of animal and plant cells. These tiny structures were once free-living bacteria that got absorbed by larger cells. Instead of being digested, they formed a permanent alliance, providing energy in exchange for protection. That partnership was so successful it became the foundation of all complex life.
Now, with Barroeca monosierra, scientists see another possible example of cooperation shaping evolution. It suggests that bacteria weren’t just bystanders in the rise of animals—they may have been co-architects.
Echoes in Our Own Bodies
If you think this is just about lake microbes, think again. The human body is home to trillions of bacteria—our microbiome. These microbial companions help us digest food, train our immune system, and even influence our mood. Without them, we couldn’t survive.
In a way, Barroeca monosierra is showing us an ancient version of the same story: animals and bacteria living side by side, shaping each other’s destiny.
The Challenges Ahead
As exciting as this discovery is, studying Barroeca monosierra isn’t easy. The organisms are elusive. In a recent field study, researchers found them in only six out of 100 water samples. They also don’t thrive easily in lab settings, making experiments slow and painstaking.
Still, Nicole King remains optimistic. “There’s a great deal more that needs to be done on the microbial life of Mono Lake,” she explained, noting that these interactions are at the heart of the lake’s entire ecosystem.
Future research may reveal whether the choanoflagellates actively cultivate their bacteria, how the bacteria influence colony formation, and whether similar relationships existed in the ancient oceans where animals first evolved.
The Team Behind the Discovery
This work is the result of collaboration. Alongside King and Banfield, the project has included graduate student Kayley Hake, former doctoral student Patrick West, electron microscopist Kent McDonald, and postdoctoral fellows Josean Reyes-Rivera and Alain Garcia De Las Bayonas. Funding came from the Howard Hughes Medical Institute and the National Science Foundation.
Together, this team has given us not just a scientific paper, but a new way of looking at the story of life.
Why It Captures the Imagination
What makes this discovery so captivating is not just the biology—it’s the perspective. In a world that often celebrates competition as the driving force of evolution, Barroeca monosierra suggests that cooperation may have been just as important.
It’s a reminder that the grand story of life on Earth might not be about survival of the fittest alone. It might also be about survival of the friendliest, survival of the partners, survival of those who learned to share.
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Final Thought
Barroeca monosierra is almost invisible to the naked eye. Yet, hidden inside its delicate, spinning colonies may be the blueprint for one of the most important transitions in Earth’s history: the birth of animals.
From a salty, toxic lake in California, this tiny creature whispers secrets from 650 million years ago. And in doing so, it shows us something profound: that sometimes, the path to greatness begins not with going it alone, but with reaching out to a neighbor and saying, “Let’s do this together.”
Featured image: GPT-5o Recreation.
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