we are all microbes

Astrobiology | AM: In Microcosmos, you detailed four specific microorganisms that you thought were involved, through symbiogenesis, in the creation of various eukaryotic cells, the type of cells that animals and plants are made of. At the time, those ideas were not well accepted. Has that changed?

Lynn Margulis: Well, we've won three out of four.

Nobody today doubts that chloroplasts began as cyanobacteria. Chloroplasts are the little green dots in the cells of plants and algae, in which all photosynthesis occurs. Photosynthesis, the conversion of sunlight as energy to food and cell material, is fundamentally a bacterial virtuosity. It began in a specific group of oxygen-producing photosynthetic bacteria that, by definition, are cyanobacteria. If they're green, they're photosynthetic. They make food only in the sunlight, because they require sunlight for their source of energy. They take carbon dioxide out of the atmosphere, and fix it, that is, chemically change it to food and body, and they produce oxygen as waste. That series of changes is done by cyanobacteria exclusively. They're the only organisms that can make the oxygen and make the food that everything else needs.

Well, you say, can't plants do that? And the answer is yes, but plants are something that hold up cyanobacteria. That's all plants are. It's the cyanobacteria in the plants that do that transformation. You say, well, can't algae in the water, green water scum, can't they do it? And the answer is, yes, but the algae are something that brings the little green things inside the scum to the light. So the answer is: nothing but cyanobacteria can make our food and produce our oxygen.

We like to call them the greater bacteria, or the greatest bacteria, because they are. And they're in only three forms: they're in cyanobacteria (what used to be called blue-green algae) all by themselves; or they're in algae; or they're in plants. But fundamentally, if you cut them out of the plant cell, and throw away the rest of the plant cell, the little green dot is the only thing that can do that oxygen production. That is the greatest achievement of life on Earth, and it occurred extremely early in the history of life. Who knows whether it's 3 billion years ago, or 2.7 billion, or 3.5 billion, but it's that kind of time. And the idea that those little organelles, those little bodies inside of cells, started as free-living cyanobacteria is completely accepted by everybody who even thinks about these problems.

So that's one out of four.