Breaking down the microbiology world one bite at a time
Meeting the ancestors
Planet Earth was formed about 4.5 billion years ago in the process of gravity pulling cosmic dust and swirling gases. Rather “quickly” after this, the first form of life appeared, and it was, of course, microscopic in nature. Scientists estimated that the very first sign of life was left by microorganisms on rocks about 3.9 billion years ago. As biological life continued to flourish in the most chaotic and incomprehensible ways, a very distinct single-celled organism appeared. It later became known as LUCA – last universal common ancestor. It is believed that LUCA is the latest ancestor to all current existing life on Earth (however, it is not the first form of life).
Human curiosity, as well as scientific and technological breakthroughs, kept unraveling the mystery of early life on Earth. Scientists wanted to figure out the pathway of evolutionary development of life. The question was: “What happened after LUCA and how did life develop to what we have today?” Microbiologists, evolutionary biologists, and geneticists proposed a model of three domains of life: Bacteria, Archaea, and Eukaryota (with the LUCA origin at the bottom).This was an established model for a long time, until the researchers decided to dig a little deeper…
It is known that Bacteria is a monophyletic domain, meaning that it had one clear ancestor. Therefore, there might have been a LBCA (last bacterial common ancestor). Joana C. Xavier and her colleagues were interested in learning more about LBCA, so they focused their research on a reconstruction of a lifestyle and a habitat of this very ancient bacterium.
The easiest thing was to identify whether or not that bacterium respired oxygen. Many life forms present on this planet today, including us, humans, cannot exist without oxygen. A simple molecule of O2 plays a vital role in the process of energy generation, which fuels all biological systems. However, Earth did not have atmospheric oxygen until ~2.4 billion years ago, which means that there were life forms that survived without this precious gas just fine. Microorganisms that do not require oxygen for functioning are called anaerobes, and it was concluded that LBCA was most likely one of them.
Many anaerobes have a set of distinct biochemical pathways that they are using to generate energy, and, as you might guess, these routes originated billions of years ago. Various lines of evidence suggest that the first LBCA cells were autotrophic. This means that the bacterium in question generated its own food using a very minimal set of elements. If you think about it, it makes perfect sense. At the time when LBCA existed, Earth was one big salty soup with millions of different atoms and minerals floating around. The planet’s crust just started to form, and continents were in their embryo state. So-called “atmosphere” was composed of only carbon dioxide (CO2), nitrogen (N2), and water vapor. What is interesting is that LBCA required only 9 compounds to complete its intermediary metabolism, which refers to all the processes in a cell that convert nutritional molecules (“food”) into cellular components (let’s call them “early organelles”). 9 seems like a very low number, but, nevertheless, this number was enough to ensure the bacterium’s survival. Can we call LBCA a primitive bacterium or was it smartly-equipped for the ongoing circumstances…It is up to you to decide!
What kind of energy source did LBCA generate? You will be surprised but it was a good old glucose (C6H12O6)! In order to make glucose, evolution gifted LBCA a very peculiar mechanism that even our bodies are using today: gluconeogenesis – formation of new glucose molecules from non-carbohydrate carbon substrates.
Finally, the researchers tried to find a modern organism that will be the closest relative of LBCA, and this microorganism turned out to belong to the class of Clostridia. Bacteria of this class are famous for their strictly anaerobic respiration, which is consistent with what Xavier et al. found about LBCA. This study was done by analyzing protein families and by calculating the evolutionary divergence of bacterial genes. However, the researchers emphasize that when it comes to bacterial gene lineages, they are usually highly intertwined due to the effect of lateral / horizontal gene transfer. Therefore, a traditional vertical perspective to bacterial evolution will not be correct here because genes were randomly transferred from one bacterial species to another for millions of years.
This research shows not only the power of reconstruction of evolutionary events, but also how closely related all life on Earth is. Remnants of ancient living forms, which existed billions of years ago, can still be found today and still have a significant impact on understanding of life. Who knows, maybe ancient microbes can, indeed, change the future of science.
Link to the original post: Xavier, J.C., Gerhards, R.E., Wimmer, J.L.E. et al. The metabolic network of the last bacterial common ancestor. Commun Biol 4, 413 (2021). https://doi.org/10.1038/s42003-021-01918-4
Featured image: https://pixabay.com/vectors/family-tree-genealogical-tree-6093805/?download and Biorender.com
*Note: Phylogenetic tree used as a featured image DOES NOT represent an exact evolutionary pathway of microbes placed in this tree. The only evolutionary-true fact in this tree is that LBCA was at the origin of microbial divergence.