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Where did these organelles in eukaryotes come from? It was long suspected that maybe some of them had begun their origins as independent cells. For example in the mitochondria, the physically circular nature of their DNA and the content of the genetic code suggested that they were once bacteria that were acquired by eukaryotes, eventually becoming dependent on the host cell (Figure 5.17). The ancestral bacterium is thought to have been an alphaproteobacterium, a subphylum of bacteria. This process of endosymbiosis, championed by biologist Lynn Margulis (1938–2011), explains the origin of chloroplasts in eukaryotic photosynthetic organisms such as plants and algae. The chloroplasts were once independent photosynthetic cyanobacteria.

Figure 5.17 A schematic illustration of the concept of endosymbiosis. Chloroplasts and mitochondria were acquired initially as independent prokaryotic cells.

Endosymbiosis has even been invoked to explain the emergence of the eukaryotic nucleus. Another explanation is that the nucleus was a part of the original eukaryotic membrane that split off to form a separate structure (the exomembrane hypothesis). The origins of the eukaryotic nucleus remain an intriguing problem in cell biology.