The merger that created complex cells may have happened only once in Earth's history

Lane argues that eukaryotic cells, the type with a nucleus and internal organelles that make up all plants, animals, and fungi, arose from a rare merger between two simple prokaryotic organisms, in which one engulfed or absorbed the other rather than digesting it. This host-guest relationship eventually became permanent, with the guest evolving into the mitochondrion. What makes this striking is how singular the event appears in the fossil and genetic record: bacteria had already existed for roughly two billion years without ever independently stumbling onto this arrangement, and no comparable second origin of complex cellular life has been found. Lane treats this rarity as evidence that the leap to complexity was a lucky, contingent accident rather than a predictable evolutionary destination. Takeaway: the jump from simple to complex life may have depended on a single improbable biochemical accident.