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The Day the Eukaryotes Changed

June 24, 2013

About three billion years ago, simple, single-celled organisms without much in the way of skill sets ruled the world.  Then something simultaneously creepy and liberating happened: Bacteria parasitized these proto-plants and creatures.  Some of these invaders became mitochondria, the minute, organic engines that provide energy for cellular activity.  Others developed into chloroplasts, the organelles that produce chlorophyll, the pigment that allows plants to conduct photosynthesis.

Or maybe that was two billion years ago.  Or a billion years.  That’s the problem:  No one is in agreement on the date.  But two UC graduate students have developed a method that may pinpoint the dates – or at least the general eras – when these literally life-changing events occurred.  And in one case it was pretty darn recent, at least from the perspective of geologic time:  about 900 million years ago.

The two researchers, Patrick Shih and Nicholas Matzke, who both earned doctorates at Berkeley in 2013,  focused on the gene for ATP synthase, which is essential to the functioning of ATP, a molecule that aids in intercellular energy transfer.

Unlike previous researchers, who worked with microbial fossils, Shih and Matzke studied contemporary chloroplasts and mitochondria, reasoning they are evolutionarily similar to the ones utilized by Old School microorganisms hundreds of millions of years ago.

“Our single-celled ancestors had these genes, and we have them now,” Shih said in a phone interview, “and they’ve been conserved very well over all that time.”

Employing Bayesian statistics – a discipline that characterizes reality as degrees of probability – the two researchers determined that mitochondria first appeared about 1.2 billion years ago, and chloroplasts showed up about 900 million years ago.

Their findings will be published in the journal Proceedings of the National Academy of Sciences; an online version of the paper is now available.

Shih, who will soon begin a postdoctoral fellowship at the Joint BioEnergy Institute in Emeryville, acknowledged that people sussing out the time line for the evolution of eukaryotes (nucleated microorganisms) tend to have strong opinions.

“But we don’t feel like we have to aggressively defend our data,” says Shih.  “We’re starting to get some emails (from the science community), and they’ve been generally positive.  But we’re not so much concerned with being right as we are about contributing to the discussion.  I mean, it’s going to be very hard to argue any data strongly unless you have a time machine to back you up.”

–Glen Martin

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