Veronique Greenwood writes:

Once, long ago, the only players in the grand drama of life, predation and death were invisibly small and simple cells. Archaea and bacteria jigged and whirled through seas and ponds, assembled themselves into fortresses a few microns wide, and devoured films of organic matter. Then some of them began to change, and eventually the first eukaryote — the first organism to keep its genes locked away in a nucleus, to line its interior with ramifying compartments, and, crucially, to use mitochondria to make energy — appeared on the scene. We and all the rest of life visible to the naked eye are the descendants of that cell, the last common ancestor of all eukaryotes.

Scientists still understand relatively little about what happened during that transformation. One of the central conundrums is how and when our eukaryotic ancestor acquired its mitochondria, the powerhouse organelles that generate the cell’s energy. The mitochondrion was clearly once an independent bacterium, until some host cell (an archaeon or descendant of one, from all the evidence) engulfed it and turned it into a permanent symbiotic partner.

But the way eukaryotic cells engulf bacteria is energetically costly; it involves extensive and rapid remodeling of the cytoskeleton, the protein scaffolding beneath the cell membrane. A cell almost needs to possess mitochondria to do it, since mitochondria can wring about 18 times as much energy from a molecule of glucose as glycolysis and fermentation, the alternative metabolic processes. So scientists debate which came first: the mitochondrion or the engulfment process, known as phagocytosis.

The two options suggest vastly different origin stories for eukaryotes: Was the mitochondrion an afterthought, a late arrival in the evolution of the first eukaryote? Or did it come early, with its spectacular energy-generating powers, and drive the changes in our ancestor? [Continue reading…]