Carl Zimmer has a new brief article up discussing an interesting point which was raised during his research for On The Origin Of Eukaryotes.
No living eukaryote, whether animal, plant, fungus, or protozoan, has completely lost its mitochondria since that symbiotic milestone some 2 billion years ago. It wasn’t the only time that two species merged, however. Plants, for example, descend from algae that engulfed a species of photosynthesizing bacteria. Many protozoans have swallowed up photosynthetic partners as well.
Absorbing an external organism through a cellular membrane and safely enclosing it in a bubble takes complex molecular machinery. One theory is that such a system evolved in eukaryotes, not prokaryotes, which gave them an advantage in integrating mitochondria.
But today, there’s a provocative new alternative to consider. Maybe a lot of today’s prokaryotes are also the result of an ancient merger. The idea comes from James Lake of the University of California, Los Angeles, a veteran researcher on the early history of life. In my essay, I describe how Lake first proposed in the early 1980s that the host cell that gave rise to eukaryotes belonged to a lineage of prokaryotes he dubbed eocytes. Now, a quarter of a century later, new studies on genomes are strongly supporting his eocyte hypothesis.
It is still not clear how prokaryotes handled fusion, but there is evidence that it happened. The double membrane of gram negative bacteria seem to be one indication of a bacteria swallowing another at some point in the ancient past.
It will be interesting to see if Lake’s new hypothesis fares as well as his eocyte hypothesis is doing. If he’s right, this symbiosis had an impact on the history of life on par with the origin of eukaryotes. Gram-negative bacteria were the first photosynthesizers, for example, and were then swallowed up by the ancestors of plants. And the same lineage also gave rise to the bacteria that became our own mitochondria. Our cells, in other words, are not just microbes within microbes; they are microbes within microbes within microbes: a true Russian doll of evolution.
Carl Zimmer is the author of Evolution: The Triumph of an Idea and Parasite Rex : Inside the Bizarre World of Nature’s Most Dangerous Creatures among others. We highly recommend these books.