Inheritance, Metabolic Variation, and Evolution of Eukaryotes

The striking differences between eukaryotic and prokaryotic cells have led to many speculations about the evolutionary relationship of these two great classes of living organisms. A popular theory is that mitochondria, which are characteristic of most eukary-
otes, arose from aerobic bacteria. After cyanobacteria had developed and oxygen had become abundant, a symbiotic relationship could have arisen in which small aerobic bacteria lived within cells of larger bacteria that had previously been obligate anaerobes. Sequence similarities of proteins suggest that these symbionts may have been related to present-day methanogens60 and thermophilic sulfur bacteria. The aerobes presumably used up any oxy-
gen present, protecting the surrounding anaerobic organisms from its toxicity. The elationship became permanent and led eventually to the mitochondriacontaining eukaryotic cell. Further symbiosis with cyanobacteria or prochlorophytes could have led to the chloroplasts of the eukaryotic plants. A fact that supports such ideas is the existence among present-day organisms of many endosymbiotic relationships. For example, the green paramecium (Paramecium bursaria) contains, within its cytoplasm, an alga (Chlorella), a common green plant that is quite capable of living on its own. Perhaps by accident it took up residence within the paramecium. Some dinoflagellates contain endosymbiotic cyano-
bacteria66 and recently a ciliate that contains endosymbiotic purple photosynthetic bacteria has been discovered. These bacteria do not produce O2 but utilize products of the host ciliates’ metabolism such as acetate, lactate, and H2 as electron donors for photosyntheses. They also utilize O2 for respiration and may protect their hosts from the toxicity of O2, just as may have happened in the distant past. According to this theory the symbionts would eventually have lost their photosynthetic ability and have become mitochondria. The elationship of mitochondria to bacteria is also supported by many biochemical similarities. Fossils of bacteria and blue-green algae have been obtained from rocks whose age, as determined by geochemical dating, is more than 3 × 109 years. However the first eukaryotic cells may have appeared about 1 × 109 years ago70 and started to evolve into the more than one million species that now exist.