Margulis knew that other scientists had proposed similar ​​hypotheses about endosymbiosis in the past and were ridiculed for it. Why hadn’t the idea ever gained acceptance? In science, ideas can be rejected for many different reasons — and most of them applied in the case of this hypothesis:

1. Lack of evidence
Scientists strive to scrutinize the ​​evidence for everything, even things that seem obvious. This means that to be ​​accepted, a scientific idea must be more than just plausible; it must be ​​tested and supported repeatedly with multiple ​​lines of evidence. Earlier scientists had tried to test the endosymbiotic hypothesis, but they didn’t have the ​​technology that they needed to design a truly fair test of the idea — so there was simply no strong evidence for the idea. Sure, mitochondria look a lot like bacteria, but that wasn’t enough to convince scientists that they had once actually been bacteria.

2. Inconsistency with an accepted theory
Many scientists were skeptical of the endosymbiotic hypothesis because it didn’t seem to fit into the ​​theory of evolution as it was understood then. Between 1900 and 1950, biologists made many key discoveries in the field of genetics by focusing on small, random changes in DNA — mutations — that occur when a cell reproduces. These genetic “mistakes” were clearly an important mechanism of evolution, and many biologists thought that all evolution occurred as a result of the accumulation of many small mutations over time. However, the new hypothesis proposed big evolutionary advances through symbiosis — not slow and steady change through tiny mutations. The endosymbiotic hypothesis seemed, at first, to be a poor fit with what scientists of the day understood about how evolution works.

3. Parsimony
Scientists are more likely to accept simpler, or more ​​parsimonious, ideas over more complex ones, all other things being equal. And accepting the new idea would have made evolutionary theory more complex. Instead of proposing one main mechanism (the accumulation of small mutations over time), the theory would have had to incorporate symbiosis as an additional mechanism of evolutionary change. Scientists didn’t see why they should look for a new way to explain evolutionary change when the old way had so much supporting evidence and seemed to explain most of what they had ​​observed. Extra evidence was needed to convince them that evolutionary theory had to make room for an additional mechanism of change.

4. Biases
Scientists strive to work ​​objectively, but they are still human and vulnerable to biases just like everyone else. In this case, scientists had two big biases that tainted their reaction to the endosymbiotic hypothesis. First, ever since Darwin, evolution had been about competition between organisms fighting it out for territory, mates, and food. But the endosymbiotic hypothesis focused on cooperation. Evolutionary theory didn’t say that cooperation couldn’t happen, but scientists just weren’t used to the idea that evolution could occur as the result of two organisms working together.

Second, most of the scientists doing research in evolution at the time worked with relatively large animals — fruit flies, birds, and mice — not microorganisms like Margulis’ amoebae and bacteria. Scientists who worked with microorganisms knew that one organism living inside another was commonplace, but those working on large animals had seen few examples of endosymbiosis. Today, we know that endosymbiosis is common even in complex, multicellular animals (like the algae that live in giant clams and perform photosynthesis), but at the time, scientists working on large animals assumed it was exceedingly rare. These scientists had trouble accepting the hypothesis because they weren’t familiar with endosymbiosis from the animals they studied.

Obviously, scientists are not always won over by new ideas right away. This kind of resistance can make science progress slowly, but it also works to ensure that every new idea is thoroughly tested before gaining acceptance. In her first publication on the hypothesis, Margulis did her best to explain all the tests of the hypothesis that had already been done and which were still waiting to be performed …