Symbiogenesis: an Evolutionary Accident (that Reshaped Life on Earth Forever)

By delving in the history of our Earth’s evolution, Max Walk explores the three domains of life, and how symbiogenesis was a remarkable accident that led to life as we know it today.

Arguably, some of the most impactful ‘accidents’ in history have been the events that led to our creation. From the Big Bang to the emergence of cellular life on earth, we would simply not exist were it not for these important episodes. However, not many people are aware of another remarkable incident in Earth’s history that gave rise to a completely new form of life. In an event that has baffled evolutionary biologists, the origin of all multicellular organisms on earth was made possible when one primitive prokaryotic cell found itself inside another. Allow me to explain.

The term ‘prokaryote’ is a fancy-sounding name for one of the two main types of cell that embody life. Prokaryotes are single-celled organisms that lack a nucleus. The archetypal prokaryotes are Bacteria: microscopic ‘bugs’ that are present everywhere and even colonise your gut. Yet there is another notable group of less-well-known prokaryotic organisms called Archaea. Derived from the Greek for ‘primitive’, these ancient entities played a crucial role in the creation of the first eukaryotic cell alongside bacteria. More on these later.

This brings us to the Eukaryotes, organisms whose cells contain organelles such as the nucleus and mitochondria. Organelles are essentially ‘miniature organs’ within the cell that carry out specific functions. Mitochondria, the ‘powerhouses of the cell’, for instance, carry out aerobic respiration. Eukaryotic organisms can be multicellular, but some, such as amoeba, are single-celled. Due to being more structurally sophisticated than their prokaryotic counterparts, eukaryotes are termed ‘complex life’ by biologists. Almost all of the visible wonders of life to the naked eye – including us humans – are eukaryotes.

Now that we have established what the 3 domains of life are (Bacteria, Archaea, and Eukaryotes), let me cast your mind back to the age of microbes, a time when prokaryotes ruled the planet. During this primitive age, long before dinosaurs roamed the planet, single-celled bacteria/archaea were the only organisms that inhabited Earth. This period of microbial domination lasted from the emergence of the first living cell until the chance event that unlocked the possibilities of complex life: an incident that involved a bacterium finding itself inside an archaeon, completely by accident.

For reasons that biologists still do not fully understand, the bacterium got swallowed by an archaeon and was kept prisoner within the archaeal cell thereafter. This sort of phenomenon is quite normal in nature; after all, a core weapon our immune system uses to combat bacterial infection involves white blood cells gobbling up and digesting pathogens - a process called phagocytosis. However, what is baffling about this event that happened millions of years ago is that the bacterium, which would become the precursor of our mitochondria, wasn’t destroyed, but coexisted inside the archaeon in a mutually beneficial association known as a symbiosis. The result of this accident was that the hybrid cell, part bacteria and part archaea, became the first eukaryote. This is what is known as Symbiogenesis, or the Endosymbiotic theory; it is the accepted explanation for the origin of complex life on earth.

In the classical theory of evolution proposed by Darwin, species evolve by gradually accumulating small changes that lead to significant modifications over long periods of time. However, Symbiogenesis shattered our understanding of the way evolution works by suggesting that it can also proceed in sudden leaps. Ridiculed by the scientific community at first, the theory was eventually proven true after mitochondrial DNA sequencing showed that mitochondria are indeed descendants of an ancient bacterium that got stuck inside an archaeon!

What makes Symbiogenesis more remarkable is that it has only occurred once in the history of our planet: no prokaryote has ever engulfed another one to create complex life anew in the 2 billion years since the first bacterial/archaeal symbiosis*. This goes to show that this event in our origins was a genuinely one-off evolutionary accident, an accident that we should be very thankful for.

*The acquisition of chloroplasts by the ancestral cell to plant life, despite also being an example of endosymbiosis, isn’t an event of the same magnitude as the first symbiosis because it was a eukaryotic cell that engulfed a cyanobacteria.

Further reading:

1. Archibald, J. (2015) “Endosymbiosis and Eukaryotic Cell Evolution”, Current Biology, 25(19), pp. R911-921. https://doi.org/10.1016/j.cub.2015.07.055

2. Eme, L., Spang, A., Lombar, J., et al. (2017) “Archaea and the origin of eukaryotes”, Nature Reviews Microbiology, 15, pp. 711-723, https://doi.org/10.1038/nrmicro.2017.133

3. Koonin, E. (2009) “Darwinian evolution in the light of genomics”, Nucleic Acids Research, 37(4), pp. 1011-1034. https://doi.org/10.1093/nar/gkp089

4. Lane, N. (2015) The Vital Question: Why is life the way it is? London: Profile Books Ltd.

5. Sagan, L. (1967) “On the origin of mitosing cells”, Journal of Theoretical Biology, 14(3), pp. 225-274. https://doi.org/10.1016/0022-5193(67)90079-3

6. Woes, C., Kandler, O. and Wheelis, M. (1990) “Towards a Natural System of Organisms: Proposal for the Domains Archaea, Bacteria and Eucarya”, PNAS, 87(12), pp. 4576-4579. https://doi.org/10.1073/pans.87.12.4576