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The theory on the origin of the eukaryotic cells was believed to have started 3.5 - 2.7 bya (billion years ago) where there are only prokaryotic cells thriving the surface of the earth. As the temperature of the earth has increased, some of the prokaryotic cells have died and some have survived under the rocks, below the water levels and in humid places. From there on the existing prokaryotic cells needed to survive which gave way to the process of endosymbiosis.
The endosymbiotic evolution of the eukaryotic cells is composed of three characters: Prokaryotic host cell, alpha-proteobacterium (aerobic bacterium) and the cyanobacteria (photosynthetic organism). There exist two endosymbiotic events.
1. Prokaryotic cell phagocytized (or engulfed) the alpha-proteobacterium. The prokaryotic cell tries to eat the alpha-proteobacterium but it does not happen. What happened is that the alpha-proteobacterium survived inside the prokaryotic cell. After certain millions of years, they became one organism and we call it cell X. Inside the cell X is the alpha-proteobacterium which has evolved into mitochondria.
2. Cell X now an aerobic organism engulfed the cyanobacterium. Again, endosymbiosis occurred. The cyanobacterium didn't die inside the cell X thus finding its way to adapt to its new environment. Millions of years has passed and until they evolved into a new organism. We call it Cell Y. The cyanobacterium further evolved into the present chloroplast.
Most of the Cell X still survived and continued to evolve. Cell X is now the ancestor of the present eukaryotic animal cells. Cell Y is the ancestor of the photosynthetic organisms such as plant cells.
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