The question "how exactly did life begin?" is one of the greatest mysteries of modern science. While most scientists believe that all life forms evolved from a common primitive ancient microorganism, the details end there. What kind of genes did this life form possess and where did it live? A new study published in Nature Microbiology sheds light on the origin and development of this ancient organism.
Experienced scientists interested in the origin of life usually tackle this problem in two different ways. One of them is a bottom-up approach, where they try to imagine how long ago life began, and then recreate the main stages of its origin in the laboratory. An alternative top-down approach is to analyze and "chop" off modern cells to simplify them and deduce key steps in the evolution of cell complexity.
Computer scientists who are trying to solve this issue are exploiting the huge amounts of data that have emerged as a result of the revolution - DNA sequencing. It has flooded scientists with information about the genomes of organisms, from bacteria to humans. They may hide information about the DNA sequences of primitive cells - the first cells on the planet to use the modern genetic code - that have been passed down through billions of generations.
The "last universal common ancestor" is hypothetically one of the very first cells from which all life on Earth originated. The relationship between this ancestor and modern organisms is often visualized as evolutionary trees, the first known examples of which date back to Charles Darwin.
DNA sequencing provides an excellent and highly quantitative measure of genetic connectivity that permeates all biology. Almost all organisms on the planet use the same code of four bases A, C, G and T. Therefore, in principle, it could be used to build evolutionary trees of all life. We know that certain genes existed at the dawn of cellular life and were inherited by all subsequent life forms. Over four billion years, copies of one small 16S rRNA gene, for example, have gradually changed in the course of random mutations in individual lineages that have led to different forms of life. It follows that each of them has a characteristic sequence, which will be similar in newly developed organisms, but more and more different in pedigrees.which appeared earlier on the evolutionary segment.
The first analyzes of these “universal” DNA sequences, carried out about 30 years ago, led to significant changes in our assessment of the diversity of life on Earth, and especially the diversity of unicellular organisms without nuclei (prokaryotes). They also singled out a whole new domain of prokaryotic life, which is now called archaea.
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Attempts to develop truly universal trees that will determine the origin of all modern cells from their last universal ancestors have been limited by a number of technical problems. One problem is the large number of groups that have separated from each other from the very beginning of life. What's more, bacteria can also exchange genes with each other, making it harder to determine their origin.
Hydrogen eaters?
In the new study, the researchers used a clever, cutting-edge method to organize sequenced prokaryotic genes into families. Then they looked for similarities and patterns across all bacterial groups and found a small set of genes that were present in both archaea and bacteria. Scientists were able to show that these genes were most likely inherited directly from a common ancestor and were not obtained through exchange.
This result is significant because it identifies the specific groups of bacteria (clostridia) and archaea (methanogens) that carry early versions of these genes, and indicates that they are very ancient and may be similar to the very first organisms that gave rise to separate lineages of bacteria. and archaea.
More importantly, the nature of the genes that survived tells an amazing story about the environment in which their last ancestor lived - including how he received energy. Research shows that the world inhabited by these organisms four billion years ago was very different from ours. There was no available oxygen in it, but if you believe the genes, the common ancestor received energy from hydrogen, produced, apparently, by the geochemical activity of the earth's crust. "Inert" gases, including carbon dioxide and nitrogen, provided the basic building blocks for the production of all cellular structures. Iron was available in abundance, and the lack of oxygen did not turn it into insoluble rust, so this element was used by enzymes in the first cell. Several of the genes are believed to have been involved in adapting to high temperatures,which suggests otherwise: organisms evolved in a hydrothermal environment - similar to modern hydrothermal vents or hot springs, where bacteria still live with pleasure.
Unfortunately, without a time machine, we cannot directly verify these results. But such information is of great interest, especially for scientists who are trying to recreate the forms of primitive life. It’s scary to think that our first ancestors (the very first) did without oxygen.
Ilya Khel