Endosymbiosis--the symbiotic relationship where one symbiotic organism living inside another! Where did this theory come from? Biologist Lynn Margulis proposed the idea of endosymbiosis about 50 years ago. She proposed it as the explanation for how complex cells evolved. Read why she was called a “biology rebel” in this Scientific American Blog post. Her idea was not readily accepted, but in 1966 she finally convinced the recently founded Journal of Theoretical Biology to publish one of her papers. In 1970, she published a book called Origin of Eukaryotic Cells. The discovery of better technology in the 1980s, greatly improved our ability to study DNA. As a result, more genetic evidence became readily available. Margulis’ theory gained support and is now the dominant explanation for the origin of chloroplasts and mitochondria inside eukaryotic cells. Detailed information can be found here.

A vast variety of assimilation events are thought to have occurred throughout the evolution of different branches of the tree of life. Especially in the groups of microorganisms known as protozoans, evidence exists for tertiary and quaternary endosymbiosis, i.e. an organism that had already absorbed one cell being absorbed itself by another cell. Microbiologists are constantly updating their understanding of the origin and diversification of eukaryotic organisms as new evidence becomes available. Annual reviews like this one communicate the current understanding to the biology community.

When assimilation events occur, there is often an exchange of genetic material between the host cells and the cells entering the host. As a result, the genome present in the nucleus of a eukaryotic cell does contain genes that encode proteins targeted back to the endosymbionts, which are now the mitochondria or chloroplasts. This animation illustrates both the assimilation and the gene transfer.

While there are nuclear genes that encode chloroplast and mitochondria proteins, the cell still depends on the binary fission of these structures for the synthesis of new ones. Chloroplasts have been removed in experiments by exposure to chemicals or deprivation from light. After removing those chemicals or re-exposing to light, no new ones form. For mitochondria, a similar thing happens when sperm and egg unite. Both have mitochondria that become present in the fertilized egg, or zygote. However, the paternal mitochondria are chemically tagged and destroyed by the cell. The maternal ones are not. As the embryo grows, only maternal mitochondria are present. Even though paternal DNA is present in the nucleus, no paternal mitochondria are ever produced. This is summarized in the idea of a human “Mitochondrial Eve “ who lived in East Africa between 100,000 and 200,000 years ago. Can you explain the evidence that supports the Endosymbiotic Theory?

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