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Figure 1 The main competing theories of eukaryotic origin. Schematic diagrams describing the Archezoa (a) and anti-Archezoa (b) hypotheses, and their archaeal (a) and fusion (f) versions as envisioned from genomic and biochemical perspectives. AR, archaeon; BA, bacterium; CH, chimeric prokaryote; AZ, archezoon; EK, eukaryote; MAN, mitochondrial ancestor; FLA, free-living a-proteobacterium; RLE, rickettsia-like endosymbiont; N, nucleus with multiple chromosomes; E, endomembrane system; C, cytoskeleton; M, mitochondria. From Emelyanov VV (2003) Mitochondrial connection to the origin of the eukaryotic cell. European Journal of Biochemistry 270(8): 1599-1618.

chlorophyll a and b are chloroplasts). Mitochondria are often referred to as cellular 'power plants' and are responsible for oxidative phosphorylation; this enables aerobic respiration (which yields 15 times as much ATP from glucose than does anaerobic respiration alone), while chloroplasts allow for photosynthesis. The acquisition of these organelles would clearly have been an energetic boon to the eukaryotes possessing them.

Unlike theories on the origins of the eukaryotes, the 'endosymbiotic theory' has remained relatively unchanged for decades, has a host of strong evidence, and is almost universally accepted. The endosymbiotic theory dates back to 1883 when Andreas Schimper observed that chloroplasts divided similar to cyanobac-teria (blue-green algae which are bacteria despite their common name). In 1905 Konstantin Mereschkowsky suggested that plastids were originally endosymbionts

(beneficial organisms living within another organism), and in the 1920s, Ivan Wallin suggested the same for mitochondria. These ideas were based on visual similarity when viewed under light microscopy and were largely ridiculed until the later half of the 1960s, after which Lynn Margulis helped popularized the 'endosymbiotic theory' in her book Origin of Eukaryotic Cells (1970).

The 'endosymbiotic theory' states that mitochondria and plastids evolved from an engulfed bacteria and cyanobacteria, respectively. It is believed that a host cell engulfed an anaerobic bacterium (perhaps as prey or a parasite) and that over time a symbiosis arose, eventually becoming an obligate symbiosis (a mutually beneficial interaction required by both participants) before evolving into mitochondria. Later a mitochondria-harboring host engulfed a cyanobacterium, which similarly evolved into plastids. The mitochondrial event is believed to have occurred near the origin of eukaryotes themselves, and the plastid event about a half billion years later.

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