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  The smoking gun: Support from a new technology
For the next ten years or so, the controversy raged. While new data and new arguments were brought in on both sides, none of the emerging research settled the matter to everyone's satisfaction. Though we often think of scientists as neutral, logical people, there was a lot of passion on both sides. Margulis continued to champion her hypothesis, and was called tenacious and audacious by her supporters. Her detractors called her the same things, but some said it with a whole lot less admiration!

Throughout the 1970s, while some scientists were debating Margulis' hypothesis, others were at work on a new technology that would eventually settle the matter: DNA sequencing — techniques that would allow us to read the chemical code that makes up our genes. DNA sequencing is one of the most powerful tools in biology. Because closely related species have similar genes, DNA sequencing can help us figure out how species are related to one another — and this was just what scientists needed to know to evaluate an important line of evidence on Margulis' checklist: whether mitochondria, plastids, and tubule structures have close bacterial relatives.

hypothesis comparison

Michael Gray and W. Ford Doolittle were interested in applying the new sequencing technologies to the debate about endosymbiosis. They wanted to know if the DNA inside plastids was more closely related to bacterial DNA or to the DNA inside the nucleus of the cell. If plastids evolved via endosymbiosis, we'd expect their DNA to have a similar sequence to that of free-living bacteria. On the other hand, if plastids evolved step-by-step inside the eukaryotic cell, we'd expect their DNA to be more like DNA in the nucleus.

Michael Gray W. Ford Doolittle

Michael Gray (left) and W. Ford Doolittle (right), both of Dalhousie University, Halifax, Nova Scotia.

By 1982, the results were in. In a paper that year, Doolittle and Gray summed up their results, as well as those of others: plastid DNA was much more similar to the DNA of free-living, photosynthesizing bacteria than it was to the DNA of the host cell. There was little doubt now: these organelles almost certainly evolved from endosymbionts.

Scientists still weren't certain about mitochondria, but just one year later they had genetic sequences from mitochondrial DNA too — and that DNA turned out to be remarkably similar to the DNA of free-living oxygen-using bacteria. This convinced most scientists that mitochondria had also evolved endosymbiotically from bacteria. Sixteen long years after Margulis had first published her ideas, the evidence was too powerful to ignore. Most scientists accepted her ideas about the importance of endosymbiosis. Evolutionary theory would have to make room for a new mechanism: lineages don't just split via speciation; they can also merge together via endosymbiosis to form a brand new lineage.

take a sidetrip
Learn more about theory change in science.

Gray photo courtesy of Michael Gray; Doolittle photo courtesy of W. Ford Doolittle

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