Mitochondria are believed to have originated from α-Proteobacteria through endosymbiosis. Recent genetic and genomic data challenge the classical endosymbiont hypothesis, suggesting mitochondria may have evolved alongside the eukaryotic cell. The mitochondrial genome has undergone significant variation across eukaryotic lineages, with many genes derived from sources outside α-Proteobacteria. The discovery of DNA within mitochondria and their distinct translation systems supported the endosymbiont hypothesis. Advances in DNA sequencing and cloning have confirmed the bacterial origin of mitochondrial and plastid genomes, identifying α-Proteobacteria as the closest relatives. Phylogenetic analyses of mitochondrial and bacterial rRNA sequences show mitochondrial rRNA sequences originate from within Bacteria, not Archaea or Eukarya. The mitochondrial genome of *Plasmodium falciparum* is the smallest known, while plant mitochondrial genomes are large. The mitochondrial genome of *Reclinomonas americana* resembles a bacterial genome, with operon-like gene clusters and bacterial-like rRNA structures. The Rickettsiales, a group of α-Proteobacteria, are closely related to mitochondria, but their genomes are independently reduced. Phylogenetic reconstructions suggest mitochondria and Rickettsiales share a common ancestor, though their relationship remains debated. The discovery of free-living α-Proteobacteria, such as SAR11, has led to discussions about their potential relation to mitochondria. However, the exact evolutionary path remains unclear due to challenges in phylogenetic analysis, including systematic errors and limited gene content in mitochondrial genomes. The endosymbiotic models for mitochondrial origin include the "archezoan scenario" and the "symbiogenesis scenario." The archezoan scenario posits that mitochondria originated from a primitive amitochondriate eukaryote, while the symbiogenesis scenario suggests a bacterial endosymbiont in an archaeal host. The hydrogen hypothesis proposes that eukaryotes arose from a symbiotic relationship between an archaeon and a eubacterium. Both scenarios have their merits, but neither provides a definitive answer. Mitochondria-related organelles (MROs), such as hydrogenosomes and mitosomes, show extreme genome reduction and lack mtDNA. These organelles retain some mitochondrial functions but have evolved independently. The discovery of transitional MROs in certain eukaryotes has blurred the distinction between mitochondria and MROs. The mitochondrial proteome has evolved significantly, with many genes transferred to the nucleus via endosymbiotic gene transfer. The mitochondrial proteome is a mosaic of α-Proteobacterial and eukaryotic origins, with many proteins derived from other sources. Comparative genomics and proteomics have revealed the complex evolutionary history of the mitochondrial proteome, with many proteins showing lineage-specific adaptations. In conclusion, mitochondrial evolution is a complex process involving endosymbiosisMitochondria are believed to have originated from α-Proteobacteria through endosymbiosis. Recent genetic and genomic data challenge the classical endosymbiont hypothesis, suggesting mitochondria may have evolved alongside the eukaryotic cell. The mitochondrial genome has undergone significant variation across eukaryotic lineages, with many genes derived from sources outside α-Proteobacteria. The discovery of DNA within mitochondria and their distinct translation systems supported the endosymbiont hypothesis. Advances in DNA sequencing and cloning have confirmed the bacterial origin of mitochondrial and plastid genomes, identifying α-Proteobacteria as the closest relatives. Phylogenetic analyses of mitochondrial and bacterial rRNA sequences show mitochondrial rRNA sequences originate from within Bacteria, not Archaea or Eukarya. The mitochondrial genome of *Plasmodium falciparum* is the smallest known, while plant mitochondrial genomes are large. The mitochondrial genome of *Reclinomonas americana* resembles a bacterial genome, with operon-like gene clusters and bacterial-like rRNA structures. The Rickettsiales, a group of α-Proteobacteria, are closely related to mitochondria, but their genomes are independently reduced. Phylogenetic reconstructions suggest mitochondria and Rickettsiales share a common ancestor, though their relationship remains debated. The discovery of free-living α-Proteobacteria, such as SAR11, has led to discussions about their potential relation to mitochondria. However, the exact evolutionary path remains unclear due to challenges in phylogenetic analysis, including systematic errors and limited gene content in mitochondrial genomes. The endosymbiotic models for mitochondrial origin include the "archezoan scenario" and the "symbiogenesis scenario." The archezoan scenario posits that mitochondria originated from a primitive amitochondriate eukaryote, while the symbiogenesis scenario suggests a bacterial endosymbiont in an archaeal host. The hydrogen hypothesis proposes that eukaryotes arose from a symbiotic relationship between an archaeon and a eubacterium. Both scenarios have their merits, but neither provides a definitive answer. Mitochondria-related organelles (MROs), such as hydrogenosomes and mitosomes, show extreme genome reduction and lack mtDNA. These organelles retain some mitochondrial functions but have evolved independently. The discovery of transitional MROs in certain eukaryotes has blurred the distinction between mitochondria and MROs. The mitochondrial proteome has evolved significantly, with many genes transferred to the nucleus via endosymbiotic gene transfer. The mitochondrial proteome is a mosaic of α-Proteobacterial and eukaryotic origins, with many proteins derived from other sources. Comparative genomics and proteomics have revealed the complex evolutionary history of the mitochondrial proteome, with many proteins showing lineage-specific adaptations. In conclusion, mitochondrial evolution is a complex process involving endosymbiosis