Mitochondria and the Complexity of Life

Our bodies are made up of trillions of cells that work in concert to perform all the basic processes required for life. Inside each cell are veritable factories of energy production called mitochondria, which help support the energy expenditure necessary for these small cells to maintain the much larger organism as a whole. In fact, all multicellular organisms carry mitochondria within their cells, falling under the domain of life known as Eukarya, meaning “true nucleus”. Life before the emergence of eukaryotes was dominated by single-celled organisms from the domains of Archaea and Bacteria, which remain major groups of life to this day. How do eukaryotes fit into the family tree of life on Earth, and where do mitochondria come in?

Popularized by Lyn Margulis’ 1967 article in the Journal of Theoretical Biology, it is now well-accepted that mitochondria were once free-living bacteria that were acquired by a pre-eukaryotic archaeal cell in an endosymbiotic event. The acquisition of the endosymbiont bacteria by the host archaea provided advantages that spurred the co-evolution of both species, leading to an inseparable integration to form the eukaryotic cell. Phylogenetic analyses currently place the original bacterial endosymbiont as a close relative to the alphaproteobacteria family, and the original host as potentially related to the Asgard archaea. However, the exact nature of the first two partners that gave rise to complex multicellular life remains a mystery lost to time.

MITOCHONDRIA-EARLY HYPOTHESIS:

The acquisition of mitochondria pre-dates modern eukaryotic features. An example is the “Hydrogen Hypothesis”, which proposes that the original proto-mitochondrion was an alphaproteobacteria that could ferment or respire depending on oxygen availability, and the archaeal host was an anaerobe that could not tolerate oxygen. Under low oxygen conditions, the alphaproteobacteria would produce nutrients (hydrogen) to the host as a byproduct of fermentation. Under high oxygen conditions, the alphaproteobacteria would remove oxygen harmful to the host by respiring.

MITOCHONDRIA-LATER HYPOTHESIS:

The original archaeon host already exhibited most eukaryotic features prior to acquiring the mitochondria. After being engulfed, the alphaproteobacteria escaped the digestion machinery to coexist with its host in the cytosol, eventually ceding all autonomy to its host and becoming an energy production center.

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- Karen Yeung