Sarah Kaplan writes: 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.” [Continue reading…]