Ancient symbiotic gene hints at mitochondrial origin

Long ago, in the drama of life, predation, and death, the only participants were simple cells too small to be seen. Archaea and bacteria float and sink in oceans and ponds, forming fortresses a few micrometers wide, devouring thin films of organic matter. Then some of them started to change and eventually the first eukaryote came along, it was the first organism to lock its genes in the nucleus, which was lined with staggered compartments, and most importantly, it Use mitochondria to generate energy. We and all visible life are descendants of this cell, the last common ancestor of all eukaryotes.

Scientists still know very little about what happened during this transition. One of the central puzzles is how and when eukaryotic ancestors acquired mitochondria, the organelle “powerhouses” that generate cellular energy. Mitochondria apparently used to be an independent bacterium until some host cell (by all evidence, an archaea or a descendant of an archaea) engulfed it and made it permanent Symbiotic partner. But the way eukaryotic cells engulf bacteria is expensive in energy terms; it requires extensive and rapid remodeling of the cytoskeleton, the protein scaffolding beneath the cell membrane. A cell almost needs mitochondria to do this, because mitochondria extract about 18 times more energy from a single glucose molecule than glycolysis and fermentation (alternative metabolic processes). So scientists debate which came first: mitochondria or the process of phagocytosis known as phagocytosis.

The two alternatives represent very different eukaryotic origin stories : Did mitochondria emerge after the fact, after the evolution of the first eukaryotes? Or did it appear earlier, with astonishing energy-producing capabilities, that drove changes in our ancestors? A recent paper in Molecular Biology and Evolution takes an interesting look at this chicken-and-egg question more than 1.5 billion years ago. The researchers sequenced the DNA of more than 30 parasitic and symbiotic bacteria that survive on the host’s resources after being engulfed by eukaryotic cells without being digested. Scientists realize that the ability to live inside eukaryotic cells appears to be much older than expected. This suggests that some form of phagocytosis predates mitochondria, setting the stage for the coming revolution, and it brings us some important caveats.

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