What is Endosymbiosis?

I have covered of eukaryotic cell development and concentration gradients by providing the assumption that cells generate lots of energy. Thus, I believe it is high time that the evolution of energy generation be discussed. In this post, I will not be going into the details of how organisms chemically produce energy but I will talk about the importance of the mechanism and how deeply rooted the necessity for energy is ingrained in complex organisms. The chemical details will be covered under mechanisms of respiration.

We have established the importance of energy in creating and maintaining complex systems in the past. However, by the time maintaining complexity became a problem life faced, there were already countless organisms living on the planet. This poses a huge question: how did each individual organism in need of energy develop such an intricate energy harvesting system? Since natural selection works by chance, it would be a statistically impossible feat for each organism to develop the same mechanism. This question led researchers to look into the issue and after years of research, the theory of endosymbiosis emerged.

Fueling Life with Sugar

To explain this theory, I will run through the proposed progression of events and then I will talk about the evidence proposed that is consistent with the story.

Imagine we are in the ancient oceans of Earth. The land is barren rock and the air is very dense in carbon dioxide. In the oceans, life is brimming to the edges, single celled bacteria dominating the seas. At a certain point, an organism arose that was able to use carbon dioxide to generate glucose. These organisms were able to use this glucose the same way as life had been for billions of years by breaking it down into acids and alcohols (glycolysis) to generate ATP. However, because these cells were able to make their own energy molecule, they were able to expand and out-compete the other cells. Eventually, the majority of the oceans were filled with these glucose generating bacteria.

Poison in the Air

However, as these bacteria made their energy, they released oxygen into the atmosphere. Over time, so many bacteria were present using this process that the air began to saturate with oxygen. Unfortunately for the bacteria in the oceans, this was bad news. Oxygen was poison to them; this is why it was being expelled. Oxygen has a tendency to carry electrons wherever it goes. This is bad news for life because both proteins and DNA can be damaged by these molecules. If you have ever been told antioxidants are good for you, the reason is because they absorb the charged oxygen molecules, protecting your cells.

As oxygen levels grew, the glucose producing cells began to die, choking on their own fumes. Life in the ocean reached a low point as all the bacteria struggled in this hostile environment. As usual, when there is no organism capable of thriving, nature is under a huge pressure to select for traits that can exploit the environment. Over time, this is exactly what happened. A species of bacteria formed which was able to use oxygen to gain an advantage. In fact, this bacteria was able to use oxygen in such a way that it could extract 15 times as much ATP from glucose with it. This process was called aerobic respiration.

Aerobic Respiration

This moment is one of the most important moments in the history of life. In a world run by the currency of energy, being able to generate 15 times the energy as everyone else opens so many doors for opportunity. Imagine if your current income was multiplied by 15 for the duration of your life; the possibilities that opens up are enormous. All restrictions that were placed on these organisms in terms of energy management were lifted instantly. Everything that it took to survive became so easy for these organisms, allowing them to specialize into eukaryotic cells. As time went on, these organisms dominated more and more of the population, pushing all other life to the brinks of extinction.

As we all know, defeated organisms do not give up without a fight. In fact, in the end, they actually won the battle. Here is how:

Endosymbiosis At Work

Over the millions of years that the tyranny of aerobic respiration threatened domination, other life was hard at work trying to keep up. While it was impossible for nature to randomly generate the unique system of aerobic respiration again, organisms found a shortcut around their massive handicap: symbiosis. If the other life forms could provide an indispensable service to these energy producers, then it would be easy for an energy exchange to evolve.

The Fragility Factor

As it turned out, these energy factories, although numerous, were incredibly small and fragile. What they all needed desperately was a way to protect themselves from the elements. In fact, they were often eaten by the larger cells in their vicinity. Eventually, one of these larger cells mutated in such a way that it took in these small organisms without digesting them. By doing this, the large cell solved the problem of protection for the little guy. In return, the energy that was generated by the little organism could be shared with the larger one. You can think of this relationship like a relationship between roommates. In return for protecting your roommate from bullies, he gives you a ton of cash on demand. Who wouldn’t take that deal?

Energy Explosion

At this point, life had unleashed a huge window of opportunity. The combination of having both a massive energy source and a large space to make use of it means that evolutionary potential is off the charts. With so much extra energy, evolution was free to run wild. Nature had the resources to create limitless proteins and chemicals. As time passed, this pair of organisms began to surpass all others in the race for resources.

Initially, evolution triggered a massive restructuring of the internal environment of the cell, generating machines known as organelles that could specialize in jobs that were needed to improve the life of the cell. This internal reorganization is what gave rise to the eukaryotic cell. Diversity exploded, and organisms diverged like never before, expanding into environments too hostile for their simpler counterparts. By feeding the flames of life with oxygen, organisms became a godlike entity, seemingly unstoppable in conquering the oceans of Earth. However, this is only the first half of the story.

Double Endosymbiosis and Energy Balance

Like a tipping scale, the atmosphere of our planet was constantly changing based on the life forms’ use of gaseous molecules. Early on, the carbon dioxide rich air had transformed into an oxygen dense environment. However, now that life was consuming oxygen faster than ever and releasing carbon dioxide, the composition of the air was slowly changing back. As a result, the original oxygen producing, carbon dioxide consuming cells were in demand again. And what better way to harness their power than to capture them just like their close siblings?

Photosynthetic Life and Moving In

This is exactly what happened; oxygen producing cells were captured and harnessed for their glucose (and oxygen) producing capabilities, giving rise to the photosynthetic branches of life we see today. The oxygen production and consumption balance that followed was determined by the population of these two classes of organisms.

Over time, these energy producing guests became residents. They slowly lost their own DNA as it was incorporated into the host cell’s genome. More and more independent function was given up in order to create the energy generation process more efficient. This converted the cells into the organelles known as mitochondria and chloroplasts.

Explaining the Evidence

Now that I have told the story, let us examine the evidence that has been discovered to support the theory:

All mitochondria have a little bit of DNA that is unique to them. This is genetic information is stored within each mitochondria, not the nucleus. This supports the theory that, at one point, the ancestors of mitochondria were their own organism with DNA. In fact, there is even more DNA inside the chloroplasts themselves, indicating they may have become incorporated into cells later than the mitochondria, as the story above narrates.

Even more convincing is the fact that mitochondria are able to control their own division. When more energy is needed in a cell (like muscle) mitochondria are able to divide in order to meet energy production needs.

In addition to this evidence, mitochondria and chloroplasts have 2 membranes. This indicates they were once individual cells. The reason is because of how cells take in large things. To take in something big, cells pinch their cell membrane around the object, engulfing it in a membrane vesicle. The two membranes of a mitochondria indicate it was taken in; one membrane was that of the small cell and the other is the vesicle formed around it. Interestingly enough, chloroplasts often have been seen with 4 membranes, indicating that they took in organisms that had taken in a chloroplast (double endosymbiosis).

Conclusion

This story of endosymbiosis answers the question of how energy production developed in all organisms. However, at the same time it raises fundamental questions about what it takes to be living and dead. While we often define life as having certain characteristics (generate energy, reproduce, make cells, etc.), the theory of endosymbiosis makes us question the definition of death. How is it that something that was once alive allowed itself to become “dead” at all?

In short, nature doesn’t care how we define living and dead. At the end of the day, nature will select for whatever works best. Just like the debate of whether viruses are living or dead, the discussion of life in endosymbiosis is irrelevant and purely technical.

Rather than debate about such trivial things, it is a lot more satisfying to simply take a step back and appreciate the beauty so obviously present in the complexities of biology.