5.2 Atypical Cells and Endosymbiosis

The endosymbiotic theory proposes that mitochondria and chloroplasts descend from free-living bacteria engulfed by an ancestral host cell. Instead of being digested, the internal cells supplied useful metabolism: aerobic respiration in the mitochondrial lineage and photosynthesis in the chloroplast lineage. Over evolutionary time the relationship became obligatory.

Both organelles contain circular DNA, bacterial-like 70S ribosomes and divide by a fission-like process. Their size overlaps bacterial dimensions. Each is bounded by two membranes, consistent with an inner bacterial membrane plus an outer membrane associated with engulfment. Molecular phylogenies place mitochondrial genes with alphaproteobacteria and chloroplast genes with cyanobacteria.

The evidence is powerful because its components converge. Circular DNA alone would not be decisive; plasmids and other DNA circles also exist. The combined distribution of membranes, ribosomes, division, genes and phylogenetic relationships is unlikely under a model in which organelles arose independently inside eukaryotes.

Many genes from the ancestral symbionts moved to the host nucleus or were lost. Modern mitochondria and chloroplasts import numerous proteins encoded by nuclear genes and cannot normally survive outside the cell. Endosymbiosis therefore describes ancestry, not a claim that an organelle today is simply a captured bacterium.

The sequence of events matters. Mitochondria, or highly reduced derivatives, occur across essentially all eukaryotic lineages, implying an early acquisition in eukaryote history. Chloroplasts occur only in photosynthetic branches and were acquired later by an already mitochondrion-containing ancestor. Some algae subsequently acquired plastids by engulfing another eukaryotic alga, producing additional membrane layers.

A scientific theory is not a guess waiting to become a law. It is an explanatory framework supported by evidence and open to refinement. Endosymbiosis cannot be replayed as the original event, but it predicts observable similarities and phylogenetic relationships that competing accounts must also explain.