Protistan Diversity

Protists are bewilderingly different. Oxygen-using prokaryotes established residence within other, larger cells. These endosymbionts evolved into mitochondria, giving rise to heterotrophic eukaryotes. Autotrophic eukaryotes also arose through endosymbiosis of a prokaryote by a eukaryote after a heterotrophic eukaryote engulfed an autotrophic cynobacterium. If the cynobacterium continued to function within its host cell, its photosynthesis would have provided a steady source of food for the heterotrophic host and thus given it a significant selective advantage. And because the cynobacterium had its own DNA, it could reproduce to make multiple copies of itself within the host cell. In addition, cynobacterium could be passed on when the host reproduced. Over time, the descendants of the origin cynobacterium evolved into chloroplasts. The chloroplast-bearing lineage of eukaryotes later diversified into the autotrophs green algae and red algae. On subsequent occasions during eukaryotic evolution green algae and red algae themselves became endosymbionts following ingestion by different heterotrophic eukaryotes. The heterotrophic host cells enclosed the algal cells in food vacuoles but the algae survived and became cellular organelles. The presence of the endosymbionts, which also had the ability to replicate themselves, gave their hosts a selective advantage. -Biology book page 331

 

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