Evolution: Have we found a key missing link in the story?

Obviously evolution is real. It’s not a theory, it’s not a belief system, it’s an irrefutable fact. Now that the scientific disclaimer is out of the way, how did higher beings such as Homo sapiens (humans like you and me) really come to be? How did humans evolve from single celled organisms whose physiology and cell architecture is rather primitive compared to ours? At this juncture it is important to state another disclaimer: I’m not an evolutionary biologist. I just find evolution endlessly fascinating. 

Life forms are broadly classified into two groups: prokaryotes and eukaryotes. The prokaryotes appeared first in the infant Earth and include all the various bacteria. Eukaryotes evolved from them about 1-1.5 billion years later and are far more complex. Humans are essentially multicellular eukaryotes. If we put a single prokaryotic and eukaryotic cell next to each other and just study their architecture, there are several striking differences. One of these is the presence of several tiny organs or organelles in eukaryotic cells that have diverse functions. The mitochondrion, more popularly known as the powerhouse of the cell, is one such organelle. Through oxygen based metabolism, they generate a molecule called adenosine triphosphate or ATP which is the energy source that drives cell function. Mitochondria are curious organelles because they seem to function like an independent organism. They also have their own genetic material (DNA), that encodes some of their components. These observations have led to the theory that modern eukaryotes evolved from a symbiotic relationship between two or more prokaryotes; essentially bacteria living within larger cells.

Based on current data, it is thought that eukaryotes may have evolved from an ancient lineage with the best possible name. The Norse mythology / Marvel comics nerd in me loves the idea that we might be evolved from an organism that belongs to the group called Asgard archaea. Members are named after popular characters such as Thor, Loki, Odin, Hela and Heimdall. Asgard archaea are thought to be the most likely ancestral candidates due to their eukaryote- like genomic features. However, in spite of such compelling evidence, it has always been difficult to clearly understand the evolutionary transition from Asgard archaea to eukaryotes. The main problem has been the lack of a pure sample of Asgard archaea which could be genetically analysed. All the data to date were unreliable due to sample contamination and variations in protocols. It is possible that this problem has now been overcome and we might be getting answers to this baffling question.

A group in Japan spent nearly 12 years attempting to isolate an Asgard archaeon related to Lokiarchaeota. As an aside, can I just marvel at how the Japanese are willing to fund basic research for 12 long years? This group isolated the archaea from deep sea sediments found 2533 m below the sea surface. Trials, tribulations and several cool electron micrographs later, they identified round shaped (cocci) archaea with an average diameter of 550 nm. These organisms were christened Prometheoarchaeum syntrophicum strain MK-D1. This will be referred to as MK-D1 from now on. By isolating a pure sample, they were able to achieve a ‘closed genome’, allowing them prove that MK-D1 represents the closest archaeal relative to eukaryotes. A key feature of MK-D1 was its dependence on at least one other type of archaea (Methanogenium in this case) for sustenance via the catabolism (break down) of amino acids. This phenomenon called syntrophic growth is the cornerstone of this evolution theory. Genetic, biochemical and morphological analysis of MK-D1 in association with Methanogenium led to the ‘Entangle- Engulf- Enslave’ evolutionary model. According to this model, when a Asgard archaea ancestor was challenged by a rapidly oxygenating Earth, it might have been forced to syntrophically interact with an oxygen scavenging alphaproteobacterium (PA). Through protrusions on its cell surface, the archaea might have established a close physical interaction with PA, ultimately leading to its engulfment. This might have eventually led to the enslavement of PA by entrusting it with ATP generating metabolism. This would have caused this new fused organism to switch from amino acid catabolism to ATP transport from PA to the cytoplasm for sustenance. Hence, PA is essentially a primitive mitochondrion. 

This research might seem insignificant to most, but it is a huge step forward in our understanding of how we evolved. It is not the end point though, this research needs to be refined before we have a definitive understanding of how eukaryotes evolved. One of the biggest questions that needs to be answered is: How did other cell organelles evolve? MK-D1 did not have any cell organelles; so how did those appear? Additionally, it is important to understand how the enslaved PA evolved into the modern mitochondrion. Was it just natural selection? Now that we have a clear starting point for research, I’m really looking forward to finding out the answers. Maybe 12 years from now?

Reference: Imachi, H. et al.Isolation of an archaeon at the prokaryote-eukaryote interface. bioRxiv726976 (2019). doi:10.1101/726976Reference:

Published by The Very Curious Biochemist

I am a protein biochemist by training, with a keen interest in new and fascinating science. I am passionate about communicating and discussing science and life as a scientist. As I belong to the rare species that actually enjoys writing, I thought I'd start this blog. I'm currently a postdoctoral scientist, so this really offers me an excellent distraction from the rigours of research. I hope you lovely visitors enjoy the material on offer!

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