The power plants in your cells were once free-living bacteria.
Mitochondria, which power our cells, and chloroplasts, which let plants photosynthesise, have their own DNA and divide on their own schedule, like bacteria. That is because they once were bacteria, swallowed by other cells long ago. The idea languished for decades before Lynn Margulis made the case.
Complex cells, the eukaryotic cells that make up animals, plants and fungi, contain compartments called organelles. Symbiogenesis, also known as endosymbiotic theory, holds that two of the most important, mitochondria and plastids such as chloroplasts, descend from bacteria that were engulfed by another cell and, instead of being digested, stayed on as partners. Mitochondria appear related to a group called the alphaproteobacteria; chloroplasts to cyanobacteria.
The idea has deep roots. In 1883 Andreas Schimper noticed that chloroplasts divide much as free-living cyanobacteria do, and floated the idea of a symbiotic union in a footnote. The Russian botanist Konstantin Mereschkowski set out a theory of symbiogenesis in 1905 and 1910. It gained little traction until electron microscopy and the discovery in the 1960s that mitochondria and plastids carry their own DNA. In 1967 Lynn Margulis marshalled the microbiological evidence, and the core theory is now widely accepted.
The clues are striking. Mitochondria and plastids reproduce by splitting in two, separately from the cell around them. Some have single circular chromosomes like bacteria. Their membranes contain proteins and lipids otherwise found in bacterial membranes. Over time, most of the guest's genes moved to the host's nucleus: human mitochondrial DNA is only about 16,000 base pairs long and carries 37 genes, whereas its bacterial relatives have thousands.
Not all of Margulis's ideas survived. Her proposal that cell tails such as flagella and cilia came from swallowed spirochaete bacteria has won little support, because these structures lack DNA and do not resemble bacteria closely. Still, the central insight changed evolutionary thinking: some of life's biggest leaps may have come from mergers and cooperation rather than slow competition alone.
Source: Wikipedia — Symbiogenesis · Text summarised from Wikipedia (CC BY-SA 4.0)
