Mitochondria, the power plants of the eukaryotic cell, were free-living bacteria billions of years ago.
Senior Fellow John Archibald (Dalhousie University) of the CIFAR program in Integrated Microbial Biodiversity has a new book out that explores how single-celled organisms came together billions of years ago and laid the building blocks for the development of complex life.
News & Ideas talked with him about his book.
Q. The title of your book is One Plus One Equals One: Symbiosis and the evolution of complex life. Explain what that means.
Essentially it’s a description of the process of endosymbiosis giving rise to an organelle. That is, you take one type of cell, you add another type of cell, and given enough time what results is essentially a more complex cell that is a merger of the two previously distinct organisms.
Symbiosis describes two different organisms living in close association with one another. There are various forms of symbiosis, and many of them are mutually beneficial relationships: two organisms living together, each providing the other with something useful. It could be shelter, it could be nutrients, or something like that.
A more specific and intimate form of symbiosis is what we refer to as endosymbiosis, where one type of organism comes to live inside another organism. We usually think of endosymbiosis in terms of cells, so one cell living inside another.
If endosymbiosis goes on for long enough, what ends up happening is the two partners, the outer cell and the inner cell, become so intertwined with one another in terms of their biology that they can no longer be teased apart again. This process has resulted in what are called organelles – permanent subcellular entities inside complex cells that used to be self-sufficient organisms.
Q. What are some examples?
From a human perspective, the most interesting example is our mitochondria. These are the energy-converting, subcellular entities inside each and every one of our cells.
As strange as it may seem, mitochondria are derived from once free-living bacteria and they have the DNA to prove it. Our mitochondrial genome is bacterial in nature and distinct from the DNA that is present in the nucleus, which is the main DNA-containing compartment of our cells. This is something that was not obvious when mitochondria were first discovered.
Q. How long ago did mitochondria first arise?
What we can say for sure is that it was at the very least a billion years ago, and perhaps two or more billion years ago.
Biologists categorize cells into two very distinct forms. There are prokaryotes, things like simple bacteria. The others are what we call eukaryotes, defined, by and large, by the presence of a nucleus and in many cases a mitochondrion. Human beings and all other animals and things like plants and fungi are eukaryotes, along with a whole myriad of single-celled entities.
So the burning question in the field of early cell evolution is when did this fundamental transition actually happen, when and how did a relatively simple prokaryotic type of cell structure evolve into a complex eukaryotic cell?
We don’t yet have a firm answer for that, but it is increasingly clear that the endosymbiotic event that gave rise to mitochondria seems to have occurred very close to, and perhaps was concomitant with, the origin of the eukaryotic cell itself.
Q. Are chloroplasts the same?
The chloroplast story is similar in many ways, different in others. It’s similar in that chloroplasts, which are the light-gathering organelles of plants and algae, have a genome, as mitochondria do. That chloroplast DNA is very clearly related to a group of bacteria called cyanobacteria, known for their photosynthesis.
So it turns out that at some point during the evolutionary history of eukaryotes, a cyanobacterium came to reside within a eukaryotic cell, and similar to the process I described for mitochondria, there was a period where the two cells became increasingly intimate in terms of the biochemical associations between them. And that endosymbiotic cyanobacterium was eventually converted into what is now the chloroplast.
The evolution of chloroplasts was a landmark event in the history of life, given that all of the macroscopic organisms like trees and plants and shrubs and so forth, all of these organisms owe their photosynthetic abilities to this primordial endosymbiotic event.
Q. Without the bacterial progenitors of mitochondria colonizing those cells originally, would we have complex life, multi-cellular life?
Some biologists argue that the eukaryotic cell simply could not have arisen, would not have been possible were it not for the endosymbiotic event that gave rise to mitochondria. The basic idea is that the complexity of the eukaryotic cell, all the bells and whistles that we tend to think of as being quintessentially eukaryotic, must have been energetically ‘expensive’ to evolve – it’s hard for some researchers to think how all of that could have arisen without the energy that the mitochondrion could provide.
Others are less enthusiastic about that idea. The more traditional view of how eukaryotic cells and mitochondria came to be is that it was a much more gradual process whereby a prokaryote, a very simple bacteria-like cell, gradually increased its complexity to the point that it was essentially a eukaryote minus a mitochondrion. And it was this mitochondrion-free eukaryote that was the engulfer, if you will, of the bacteria that became the mitochondrion. Even more complex cells evolved from that point on.
Q. Writing a book obviously is an awful lot of work. What compelled you to do it?
The idea that I might take on a project like this came to me at a retirement party for a senior colleague of mine, Michael Gray. He’s basically Mr. Mitochondrion. He has been a leading figure in mitochondrial DNA research for many, many years. I was listening to people tell stories about their days in Mike’s lab, going back to the 1970s.
And I thought that the discovery of the evolutionary origins of mitochondria and chloroplasts was a story that needed to be told to a general audience, but also that I needed to learn how to tell it. So this book wasn’t simply a case where I was an expert on a topic and I wanted to write about that topic, it was as much a learning exercise as anything else.
Q. What’s compelling to you about that story?
I’m interested in how the process of science ultimately led to a fuller understanding of the true complexities of the microbial world. The basic concept of symbiosis dates back to the late 1800s, but it’s only relatively recently that we’ve been able to use molecular tools to gain insight into how single-celled organisms and organelles are related to one another.
Testing the so-called endosymbiont hypothesis for the origins of chloroplasts and mitochondria was one of the first and most fundamental triumphs of modern molecular biology. It was a remarkable piece of scientific detective work, one in which brand-new laboratory techniques were used to test old ideas.
The book has historical themes running through it. I felt it was important to convey the history of early ideas about symbiosis and the nature of mitochondria and chloroplasts, and the reasons why such ideas were for the most part dismissed by mainstream biologists in the pre-molecular era. Our views on the microbial world have been transformed by DNA-based technologies, and I wanted to capture the excitement of this important phase of cell evolution research from the perspective of the scientists themselves.