The Unique Merger That Made You (and Ewe, and Yew): All sophisticated life on the planet Earth may owe its existence to one freakish event.
The endosymbiotic event in which an archaea host took on an endosymbiotic prokaryote that would evolve into mitochondria is posited as a rare and critical event, never repeated in all of Earth's history, a step change rather than a gradual one, a step that enabled an explosion of complexity via vastly expanded cellular energy budgets. Perhaps the rarity of this event even explains the Fermi paradox.
The energetics of genome complexity by Nick Lane and William Martin (Nature, 2010) is only 5 pages long and has been an excellent follow-up read. I've found it fascinating and quite accessible, despite not being a biologist. (However, I admit I still don't understand the scaling that results in 0.003 fW/Mb (or ".0005 fW per gene, a 230,000-fold reduction") in the first paragraph of page 3.)
If I am not mistaken, the Lane and Martin's argument is that the critical innovation of mitochondria is their separation of DNA that must exist in numerous copies near the metabolic machinery it controls (mitochondrial DNA), from the DNA that needs only to exist in small copy numbers (nuclear DNA). Large prokaryotic cells need many copies of their entire genome spaced throughout the cell to control their respiratory metabolic activity (specifically, a genome per some area of bioenergetic membranes), a task eukaryotes accomplish using miniaturized mitochondrial genomes (that only contain the small number of necessary genes for this task).
Freed from the need to have so many copies of all their genes, eukaryotes could develop larger, more complex, and more specialized nuclear genomes (about 3000 megabases of DNA, or 20,000 genes, compared to an average prokaryote's 6Mb of DNA and 5,000 genes) while growing much larger (40,100 picograms vs an average prokaryote's 2.6 picograms). The overall result is a vastly enhanced energy budget per megabase of DNA (0.76 picowatts/Mb in eukaryotes vs. 0.08 pW/Mb), or a even higher enhancement of power per gene (115 femtowatts per gene in eukaryotes vs. 0.1fW/gene in prokaryotes) when considering the lower gene density of eukaryotes (about 12 genes per megabase in eukaryotes vs 500-1000 genes per megabase in prokaryotes).