Part 2/9:
This shared cellular architecture implies that eukaryotes arose just once in Earth's history, approximately two billion years ago, forming a singular evolutionary event. Although bacteria and archaea possess a greater diversity of genes, they have not acquired the internal complexity seen in eukaryotic cells. The question arises: what set eukaryotes apart? The answer may lie in their acquisition of mitochondria—double-membraned organelles descended from endosymbiotic bacteria—which transformed early eukaryotic evolution by providing a new capacity for energy generation and cellular complexity.