A Monocercomonoides cell is seen under an electron microscope. (Naoji Yubuki)

Life on Earth used to be simple.

Once upon a time, every organism on the planet was a single, simple cell. Scientists call them prokaryotes. They are about as basic as a living thing can be — just little balloons of DNA and protein, with no grander goals in life than to swim around, eat and occasionally duplicate themselves to produce more swimmers and eaters.

Then, about 1.5 billion years ago, something strange and spectacular happened. One prokaryote engulfed another, and instead of digesting it, he put the little guy to work. They established an endosymbiotic relationship: The smaller internal cell performed lots of helpful tasks — such as making energy and building proteins — and in exchange, the bigger cell kept it safe and well-fed. This lucky bit of teamwork gave rise to the complex (a.k.a. eukaryotic) cell that exists today, from curious single-celled protists to the cells that make up all plants, fungi and animals — including us.

The eukaryotes are a weird and diverse lot, but at the cellular level we've all got the same basic components: a nucleus to store our DNA, and mitochondria — the descendants of that ancient swallowed organism — to make energy and perform other essential functions. Other internal structures, called organelles, may vary, but those two are so universal that biologists assumed we couldn't exist without them.

"They're part of the definition of eukaryotic cell," said Anna Karnkowska, an evolutionary biologist at the University of British Columbia. "If you open a biology textbook to a picture of a eukaryotic cell, that's what you'll see."

Which is why she was so shocked to find a eukaryote that didn't have any mitochondria at all: a single-celled relative of the giardia parasite called Monocercomonoides.

The discovery, which Karnkowska made with other biologists when she was a post-doctoral fellow at Charles University in Prague, seems to rip up that textbook illustration. One of her co-authors compared it to finding a city with no utilities or public works department.

"This is the first example of a eukaryote lacking any form of a mitochondrion," the researchers write in their study, which was published Thursday in the journal Current Biology, "demonstrating that this organelle is not absolutely essential for the viability of a eukaryotic cell."

And now that one mitochondria-less organism has been found, they expect others will start popping out of the woodwork.

"It's a whole new world of weird stuff," Karnkowska said. "Eukaryotic cells are sometimes very weird and very diverse and for a biologist it's very interesting to study ... but I think even I maybe underestimated how great they are."

Of course, in biology, as in all science, it's difficult to prove a negative. The classic response to "We didn't find anything" is always, "Are you sure you looked hard enough?"

But Karnkowska and her colleagues searched pretty hard. In their initial examination of Monocercomonoides (its pronunciation, which you can attempt at your own peril, is something like "mon-oh-sir-ko-mon-oi-dees") they found no trace of the maze-like structures or their signature proteins. When they sequenced the organism's genome, there was no trace of the genes associated with mitochondria or any of their essential functions.

"They’ve done as good a job as they can possibly do," said University of British Columbia researcher Ryan Gawryluk, who was not involved in the study. "I think the find will hold up."

Gawryluk, who specializes in mitochondrial evolution, added that he'd almost been expecting a discovery like this. Monocercomonoides, which lives in the gut of chinchillas, is related to a range of single-celled protists that dwell in oxygen-less environments. Because the process by which mitchondria produce energy requires oxygen, they've all had to develop alternative mechanisms for powering themselves.

"If there were a group where I would have predicted you would have found this it was this group," Gawryluk said.

But mitochondria still serve other necessary functions, most important the construction of special iron-sulfur proteins that cells need to catalyze reactions and translate their genes. So even the life forms that survive without oxygen have stripped-own versions of the organelle.

"We started to think that mitochondria were essential even if they don't produce energy but because of this essential function," said Vladimir Hampl, a professor of protistology at Charles University who oversaw the study.

Monocercomonoides forces him to rethink that assumption. Not only did it shift energy production to other organelles, but it came up with a new way of making iron-sulfur proteins that allowed it to get rid of the mitochondria altogether. It swallowed a bacteria that had its own mechanism for iron-sulfur protein synthesis, then borrowed the genes for the pathway.

This process, known as horizontal gene transfer, happens in prokaryotes all the time. But scientists are beginning to realize it must also be part of the evolution of complex life. Exactly how common it is in eukaryotes is the subject of much debate — and this find is likely to add more fuel to the flames.

But, as Hampl pointed out, there's a certain kind of poetry to the process. More than a billion years ago, a cell swallowed a bacteria to gain mitochondria. Sometime since, it swallowed a bacteria to get rid of them.

"It is kind of analogous," he said.

But the direction of development is opposite, and that's an important lesson for us, he added. We may like to think of evolution as a progressive march toward more and more complexity — life started with tiny single cell organisms and ended up with us.

But, as Hampl points out, “evolution is not linear.” Life doesn't relentlessly pursue complexity so much as it pursues efficiency. We become more complex when we need to be, to take advantage of a food source or fill an ecological niche. But we never add on an organ just for the heck of it. If a cell can streamline its system and avoid an extra energy expense, it will do so. And that’s exactly what Monocercomonoides seems to have done.

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