The Evolutionary Origin of Mitochondria and Mitochondrion-Related Organelles

Mitochondria and related organelles—hydrogenosomes, mitosomes and other mitochondrion-related organelles (MROs)—are essential compartments in almost all eukaryotic cells. Although mitochondria are renowned for their role in high-efficiency ATP synthesis via oxidative phosphorylation, they have a wide spectrum of other functions within eukaryotic cells including processes such as cofactor biosynthesis, lipid and amino acid metabolism to name a few. In addition, their functions markedly differ between major eukaryote lineages and even between cell types or life stages of the same organism. From an evolutionary standpoint, the origin of mitochondria was clearly an important step in one of the most significant evolutionary transitions in the history of life—the origin of modern eukaryotic cells. Yet, both the timing of mitochondrial symbiosis in eukaryogenesis and the role of mitochondria in this evolutionary transition remain shrouded in mystery and continue to inspire vigorous debate. In this chapter, we will summarise and discuss the various forms of evidence relating to mitochondrial origins, the integration of this symbiotic bacterium into the eukaryotic cellular lineage and the diversification of its properties in the eukaryote tree of life.