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How Bacteria May Have Jumpstarted the First Animal Life

Evolutionary biologist Nicole King
John D. and Catherine T. MacArthur Foundation
Evolutionary biologist Nicole King

Stories about our origins exert a powerful draw. Who doesn’t want to know where he or she has come from? Who and what came before? For evolutionary biologist Nicole King, of UC Berkeley, those questions just go back a bit further than for most of us. She has dedicated her research career to figuring out how the first multicellular animal life came to be.

King discovered a love of fossils as a kid, then realized that the fossil record doesn’t hold answers to the questions she had. Instead, she found choanos, her nickname for the tiny organisms, called choanoflagellates, that she’s been studying for more than fifteen years.

Choanos are the closest living relative of animals. Roughly speaking, they look like a sperm cell fitted with one of the cone collars dogs have to wear after operations. That’s odd, but what’s really special about choanos is the fact that they can live either as independent cells or as rosettes – round colonies of cells that grow from an individual choano. In other words, they can go between being unicellular and something just shy of what King is comfortable calling truly multicellular.

Choano rosettes
Credit Mark J. Dayel / Creative Commons Attribution-Share Alike 3.0 Unported
Choano rosettes

To King, it seemed like a great model for investigating how something choano-like might have given rise to the first animals hundreds of millions of years ago. There was just one problem: King couldn’t reliably induce choanos to form rosettes in the lab. After years of trying, she finally gave up and decided to try a different approach.

And, of course, that’s when it happened.

King wanted to sequence the choanos’ DNA. In order to do that, she needed to separate the choanos from the bacteria that grow alongside them. The bacteria are food for the choanos, and come included in the water samples when choanos are brought into the lab from the environment.

One of King’s students started treating choanos with different combinations of antibiotics to knock down the bacteria, and saw something completely unexpected. Choanos treated with certain antibiotics formed rosettes - lots of them. Other antibiotics eliminated rosettes. And adding bacteria back to the choanos could reverse the effect of the antibiotics.

By sheer serendipity, King had found the trigger she’d been looking for and, instead of being something innate in the choanos, it was coming from bacteria in their environment. Her lab has since pinpointed the type of bacteria responsible and the exact molecule they use to induce rosette formation in choanos. Another student discovered that bacteria also control whether choanos reproduce by sexual fusion of two cells or by an asexual budding process, in which one cell splits into two daughters.    

In fact, King says she’s starting to think that bacteria may control every important part of a choano’s life cycle. Still, she says there are more questions than answers at this point: Why do choanos respond to certain bacterial chemicals by forming rosettes? Were bacteria involved in the origin of multicellular plants and fungi, or the dozen or so other times that multicellular life arose?

With all that science has uncovered in recent years about the influence bacteria have on our own lives – from digestive function to mental health – it’s worth contemplating the idea that we might not be here, at all, if it weren’t for bacteria.