Defining the Endosymbiotic Theory
The Endosymbiotic Theory proposes that eukaryotic cells evolved from prokaryotic cells living inside other prokaryotic cells. Specifically, it suggests that mitochondria and chloroplasts, key organelles within eukaryotic cells, originated as free-living bacteria that were engulfed by a larger host cell. Instead of being digested, these smaller cells established a symbiotic relationship, benefiting both the host and the endosymbiont.
Key Principles of Endosymbiosis
This theory posits a sequential process. First, an ancestral anaerobic prokaryote engulfed an aerobic (oxygen-breathing) bacterium. Over time, this engulfed bacterium evolved into the mitochondrion, providing the host cell with efficient energy production. Later, in the lineage leading to plants and algae, a eukaryotic cell (which already contained mitochondria) engulfed a photosynthetic cyanobacterium. This cyanobacterium then evolved into the chloroplast, enabling the host cell to perform photosynthesis.
Mitochondria and Chloroplasts as Examples
The strongest evidence for the Endosymbiotic Theory lies in the characteristics of mitochondria and chloroplasts. Both organelles possess their own circular DNA, similar to bacterial DNA, and divide independently by binary fission, much like bacteria. They also have ribosomes that resemble bacterial ribosomes and are sensitive to antibiotics that affect bacterial protein synthesis, further supporting their prokaryotic ancestry.
Evolutionary Significance of the Theory
The Endosymbiotic Theory is a cornerstone of modern evolutionary biology, explaining a major transition in the history of life: the origin of eukaryotic cells. This development paved the way for the evolution of multicellular organisms and the vast diversity of complex life we see today. By integrating specialized energy-producing and photosynthetic capabilities, early eukaryotic cells gained a significant evolutionary advantage, driving the diversification of life on Earth.