Endosymbiosis: the dinner that changed the world

What is endosymbiosis, and how did it lead to life as we know it? Georgia Mills spoke to Maureen O'Malley...

Maureen - There’s been quite a debate about this and in the late 1960s, Lynn Margulis, very famous for the gaia hypothesis, she put forward this idea that it had been around for 100 years or maybe even longer, that it’s due to endosymbiosis. That one cell swallowed up and retained another cell inside it, and that this cell eventually became the mitochondrion. Once you understand how the mitochondrion gets there, you might understand how you get this kind of intracellular complexity that makes eukaryote cells.

Georgia - So one cell eats another one, and it doesn’t get digested, it just stays there. It’s now thought that before this unusual meal took place the cells were already starting to become more complex on their inside – somehow creating these compartmental barriers.

莫林——你想象的是他们得到一个突变of some kind that generates all kinds of membranes inside the cell, and gives the cell the capacity to move things around in a more sophisticated way. Ultimately, having those membranes inside the cell helps make compartments and so when, eventually, one day this cell gets a bacterium inside it, it holds onto that bacteria with the membrane and eventually converts it to the mitochondrion.

The same things happens, a bit later on (another half billion years later) with what’s called the chloroplast. So this was a free-living, photosynthesising bacterium which gets swallowed up by one of these complexifying cells. And because of these membranes and other structures inside, it’s held there and made to do work.

Georgia - In this acquisition - what are these cells doing? Are they trying to eat them, or is it a mistake and, either way, why doesn’t it get destroyed when they’re inside this cell?

Maureen - First, I’m sure it happens many, many times and by far the majority get destroyed or digested. I think there are different points of view but you can imagine, if you’ve got a membrane that allows you to swallow up other cells, this is a great source of food.

So you’ve got all these other bacteria and archaea out there, you’re a cell that’s getting bigger yourself because you’ve got different membranes and compartments inside, you want food. And if you’re able to swallow up cells, you’re basically taking them into your body, which is just one cell surface, and then if you happen not to digest one or if one of them is something like a parasite even and manages to stay there despite you trying to digest it, but eventually it might happen that the cell that’s ingested is retained.

Once that happens and if it can be passed on to the next generation and, if either it’s not causing harm or it starts to cause a little bit of advantage, that thing will be retained. The exact circumstances are still, I think, quite a mystery.

Georgia - Do we have any evidence? How do we know that cells didn’t just build their own mitochondria? Why do we think there was this external event?
Maureen - This is a great question and I think it’s a very nice historical one too, and this is why Lynn Margulis is so important to the story. People had thought, way back in the 1890s and even before, that perhaps the mitochondrion and especially the chloroplast were bacteria. They looked like bacteria, they had these membranes around them. They in some ways seemed to do metabolic activities that looked like bacteria but that isn’t conclusive evidence.

So what Lynn Margulis did is she gathers together some of the structural evidence. Look at their membranes; what kind of membranes have they got? She also looks at what they’re doing and how they’re doing it. She looks at the structure of various parts of these endosymbionts. What she’s then able to do is draw on the molecular evidence.

Now she doesn’t do any of this molecular stuff, but what happens is in the 70s and 80s, what begins to flourish is the idea of using molecules to understand evolution. So you take molecules from an organism and then, using various evolutionary models, you’re able to extrapolate back to the past. As people look at the DNA from the mitochondria and then DNA from the chloroplast, by the beginning of the 1980s it’s conclusive the DNA of the chloroplast, and the DNA of the mitochondrion is the DNA that’s the most closely related to contemporary bacteria. And because of the other structural things of having membranes, having things inside the mitochondria and chloroplast, everybody is convinced by this. It takes a few years, but what was once a sort of strange, marginal idea even as late as the 60s, becomes a completely standard thing to think by the 80s and 90s.