Life › Life & humans today › Common descent & the tree of life
Lynn Margulis
1938–2011; endosymbiosis theory 1967· Scientist· Microbes & cells
Well establishedStrong fossil, genetic or geological evidence.
American biologist who championed the idea that complex cells arose when cells swallowed bacteria that became permanent partners.
See it on the Life map →What happened
In 1967, as Lynn Sagan, she published 'On the Origin of Mitosing Cells', reportedly after about 15 journals rejected it. She argued that mitochondria, which make a cell's energy, and chloroplasts, which do photosynthesis, began as free-living bacteria engulfed by a host cell. Others had floated versions earlier, but she gathered the evidence and pushed it into the mainstream. Her claim that cell tails and hairs (flagella and cilia) also came from bacteria was not accepted. She also co-developed the Gaia hypothesis with James Lovelock.
Why it matters
Endosymbiosis, once dismissed, is now textbook biology, and it showed that partnerships, not only competition, built major branches of life.
Discoveries and legacy
DNA studies from the late 1970s showed mitochondria descend from alphaproteobacteria and chloroplasts from cyanobacteria, as she argued. She was elected to the U.S. National Academy of Sciences in 1983.
Still debated
Some of her later contrarian views were widely rejected. What kind of host cell swallowed the first bacterium, and how, is still investigated.
Connections (2)
▸Lynn Margulis championed the endosymbiotic origin of Mitochondria: endosymbiosisWell established
The connection
Margulis argued in 1967 that mitochondria, the cell's power plants, descend from oxygen-using bacteria swallowed by a host cell that kept them instead of digesting them. She pointed to their own DNA, their bacteria-like ribosomes and their habit of dividing in two like bacteria. Earlier versions of the idea had been dismissed; she made it testable.
When and how
Her 1967 paper in the Journal of Theoretical Biology, published as Lynn Sagan while at Boston University, and her 1970 book Origin of Eukaryotic Cells.
The evidence
Gene sequencing from the late 1970s placed mitochondria among the alphaproteobacteria; their own circular DNA and double membranes fit a swallowed bacterium.
Debates and open questions
Whether the host was already a complex cell or a simple archaeon, and whether the merger came before or after the nucleus evolved.
▸Lynn Margulis championed the bacterial origin of Chloroplasts: algae capture cyanobacteriaWell established
The connection
Russian botanist Konstantin Mereschkowski suggested in 1905 that chloroplasts, the green bodies that do photosynthesis in plant cells, were captured cyanobacteria. Margulis revived and extended the idea in 1967, arguing that algae and plants gained photosynthesis by engulfing such bacteria. Like mitochondria, chloroplasts have their own DNA and bacteria-like ribosomes.
When and how
Mereschkowski, 1905; Margulis, 1967 and her 1970 book; supported in the 1970s by RNA comparisons, including work by Ford Doolittle's group in Canada.
The evidence
Chloroplast genes sit inside the cyanobacteria branch of the gene tree, and the amoeba Paulinella shows a separate, younger capture within roughly the last 100 million years.
Debates and open questions
Whether all ordinary chloroplasts trace to a single capture, and which living group of cyanobacteria is closest to the original partner.