21.2: A history of the idea

In the early 1880s, German botanist Andreas Schimper used the most cutting-edge microscopes available to study plant cells. He noted that chloroplasts grew and divided independently of the rest of the cell. This was a novel insight, and confounded expectations. Copying the experiments of his contemporary Louis Pasteur, Schimper demonstrated that a plant cell which has lost its chloroplasts cannot regenerate them. Like life itself, in the modern day, all chloroplasts descend from other chloroplasts. Schimper’s 1883 conclusion was that chloroplasts were endosymbiotic cyanobacteria. Another botanist, this one from Russia, Konstantin Merezhkovsky (also spelled Mereschkowski) built on this idea, proposing in 1905 that algae and plant cells were chimeras between a protozoan (a protist) and a cyanobacterium nestled inside it. However, critical empirical tests were missing at the turn of the century: there was no DNA sequencing available then, nor sufficiently advanced microscopy to investigate Merezhovsky’s ideas to the extent they deserved. Thus, the hypothesis of chloroplast endosymbiosis withered shortly after it was first suggested. The technology that would show it to be true had not yet been invented. An idea this radical needed testing to remain viable; Without it, the idea went dormant for several decades.

Shortly thereafter, American biologist Ivan Wallin developed an interest in mitochondria. In 1922, he noted that the little rod-shaped organelles were so similar to bacteria that the two were frequently mistaken for one another. Intriguingly, the same staining techniques that worked to highlight mitochondria also worked on bacteria. Wallin concluded that, “mitochondria are symbiotic bacteria in the cytoplasm of the cells of all higher organisms.” Again, the idea was plainly stated but did not gain widespread acceptance due to the lack of confirmation from other more precise lines of evidence.

At the same time as Wallin’s work in America, another botanist in Russia independently came up with the same idea expressed decades earlier by Merezhovsky. In 1924, Boris Kozo-Polyansky published a paper again documenting the similarities between chloroplasts and cyanobacteria, extrapolating the larger insight that, “new organisms occur not only by divergence of the lineages but also by their convergence and fusion.” Again, the ground was not fertile for the reception of the provocative notion, and it once more faded into obscurity.

Figure \(\PageIndex{1}\): Lynn Margulis in 2005. (Photo by J. Pedreira via Wikimedia Commons.)

The idea of endosymbiosis would see a revival and ultimate validation in 1967 when Lynn Margulis (then Lynn Sagan) [yes, that Sagan] proposed that not only was the endosymbiosis notion true for chloroplasts, it also applied to mitochondria, and maybe flagella too. A rhetorical force of nature, when Margulis championed endosymbiosis, it stuck. Widespread scientific consensus wasn’t only the result of Margulis’s tireless advocacy, but also because of the advance in scientific technology since Merezhovsky’s day. For one thing, electron microscopy now existed, and was capable of showing how similar the internal structure of chloroplasts was to the architecture of cyanobacteria. Moreover, it showed the double-membraned envelope of chloroplasts, a signature of having been enfolded from the exterior to the interior of the host eukaryote. Seeing these details simply wasn’t possible prior to sufficiently detailed imaging technology.

More recently, the ability to extract and sequence DNA has shown the similarity between the genes of organelles like the mitochondrion and the chloroplast and free-living bacteria. The two structures shared many of the same genes, in the same way that familial cousins do. This was the detail that cinched the case for most scientists. With multiple lines of evidence converging on the same answer, it was time to accept that answer: Eukaryotes are chimeras resulting from endosymbiosis.

Because eukaryotes are capable of feats that could never be accomplished by a lone prokaryote, a philosophical corollary is that evolutionary innovation can be achieved by cooperation rather than solely through competition. The idea of endosymbiosis offers a different lens through which to view the evolution of life on Earth: it turns “survival of the fittest” on its head, and suggests instead “survival of the most well-integrated team.”

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

Which scientist finally saw success in establishing the idea of an endosymbiotic origin for mitochondria and chloroplasts among the wider scientific community?

a. Ivan Wallin

b. Boris Kozo-Polyansky

c. Konstantin Merezhkovsky

d. Lynn Margulis

e. Andreas Schimper

Answer

d. Lynn Margulis

Exercise \(\PageIndex{2}\)

What was the reason that the wider scientific community accepted mitochondria and chloroplasts as obligate endosymbionts in the late 1960s and 1970s?

a. The discovery of cyanobacteria, which were previously unknown

b. Advances in the technologies of microscopy and genetic sequencing

c. Documentation that mitochondria respond to the same laboratory staining techniques as bacteria

d. Records that chloroplasts reproduce independently of the rest of the cell

Answer

b. Advances in the technologies of microscopy and genetic sequencing