All the experts agree nowadays that endosymbiosis played an essential role: somehow a bacterium got captured and domesticated inside another cell, a host, where it became a mitochondrion. Once present and abundant within early eukaryotic cells, mitochondria delivered vast quantities of energy, far beyond anything previously available, allowing increases in size and complexity among these new cells and the multicellular creatures that evolved from them. A salient feature of the increased complexity was containment—in particular, containment of genetic material. More specifically, that meant
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