Biologists are still struggling with the mystery of the origin of life on Earth. It is necessary to understand how primitive bacteria and other life forms originated. Little is known about the progenitor organism, but genomics allows us to find out something about the most ancient creatures that inhabited the world at the dawn of its existence. "Lenta.ru" tells about an article published in the journal Nature, in which the authors are trying to answer the question of who LUCA (last universal common ancestor) was, Luca is the universal common ancestor of all modern organisms.
There were not yet three domains (super kingdoms) of life - bacteria, archaea and eukaryotes, but he already existed. This organism is an intermediate link between the inanimate environment of the early Earth and the first microbes that lived in rocks 3.8-3.5 billion years ago. It is not known what Luke looked like and in what conditions he lived. Scientists, like detectives, have reconstructed its basic features piece by piece. We proceeded from the following principle: since Luke is the ancestor of all living organisms, it means that they inherited some traits from him. Based on the biological characteristics inherent in every living being, biologists have created a portrait of Luke: a single-celled organism that resembles a bacterium.
A new study by German scientists made it possible to clarify the internal organization of the universal ancestor. Scientists have determined which genes could include Luke's DNA. To do this, they used a phylogenetic approach, in other words, analyzed the evolutionary relationships between different types of life on Earth. This was done in the following way. Having established which proteins are encoded by the prokaryotic genome, biologists selected those that met several criteria. First, the protein must be present in the higher taxa of both bacteria and archaea. Secondly, if we construct a phylogenetic tree - a diagram that reflects evolutionary relationships - then bacteria and archaea that possess this protein should form a monophyletic group, that is, have a common ancestor. The latter condition increases the likelihood that these same proteins were present in Luke,and from him were passed on to descendants.
In total, more than six million genes encoding proteins were analyzed and present in 1,847 bacterial and 134 archaeal genomes. From the total, scientists formed 286 514 groups (clusters), of which only about 11 thousand contained bacterial and archaeal proteins. When the phylogenetic trees were built and the protein groups were tested to follow the monophyletic principle, only 335 clusters remained that met the initial conditions. All proteins in the final sample, according to biologists, were present in the LUCA genome. It should be noted that these criteria do not exclude the possibility of horizontal gene transfer. Thus, a protein that first appeared in early bacteria could enter Archea and spread among representatives of each of the domains, although it was never present in Luke's body.
Biologists were interested in the genes that make up the "information core" in the cells of living organisms. We are talking about 19 proteins involved in the synthesis of ribosomes, as well as eight enzymes that play a major role in the formation of transport RNA (they move amino acids to the sites of construction of protein molecules).
Black smokers
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Luke's reconstructed genome suggests that it was an anaerobic (adapted to an oxygen-free environment) creature that received the energy necessary for life as a result of chemosynthesis - chemical reactions that oxidize minerals. Apparently, the universal ancestor lived near hydrothermal vents, like black smokers. This is indicated by the possible presence of gyrases in it - enzymes specific to thermophilic (thermophilic) organisms. Also in LUCA, most likely, there were enzymes that make possible chemosynthesis, in which carbon dioxide is the only source of carbon. In general, this organism could receive energy from gases such as hydrogen, carbon dioxide and nitrogen.
Some of the enzymes contain iron-sulfur (FeS) clusters, which are a group of cofactor molecules that specifically bind to proteins and determine their catalytic activity. This indicates that Luke lived in an iron-rich environment. Another group of proteins involved in sugar metabolism has been identified: glycosylases and hydrolases. These enzymes in modern cells are important for the synthesis of the cell wall, which may indicate the existence of a primitive cell wall in LUCA.
The Great Prismatic Spring is a typical Archaean habitat
Photo: Jim Urquhart / Reuters
The findings of the researchers confirm a number of important theses. FeS clusters, as well as transition metals in the composition of cofactors, are the legacy of ancient metabolism. The first living organisms arose in hydrothermal vents. Chemical reactions occurring at the border of the aquatic environment and rocky rocks created the conditions for the emergence of life. The first representatives of bacteria and archaea were autotrophs, dependent on hydrogen and using carbon dioxide as a terminal acceptor in energy metabolism (in animals and plants, inhaled oxygen plays this role).
The constructed phylogenetic trees did not make it possible to isolate the proteins characteristic of LUCA, which were involved in the synthesis of amino acids that make up proteins and the nucleosides that form DNA and RNA. Nevertheless, a universal ancestor could have formed from those components that were formed as a result of spontaneous chemical processes characteristic of the early Earth.
Interestingly, the results of German biologists contradict the findings of French scientists published in 2008. They attributed onion to organisms that prefer moderate temperatures (less than 50 degrees Celsius). It was believed that LUCA could not be a thermophile due to the fact that its proteins were not resistant to high temperatures. At the same time, the ancestors of bacteria and archaea could well have lived in a highly heated environment. The new work pays attention not to the immediate stability of enzymes, but to what environmental conditions these proteins are characteristic for.
Alexander Enikeev