Fundamental Science: Life arose in crystals

From the beginning of its history, the Earth already had all the ingredients to generate life. The elements involved in the formation of organic molecules, such as carbon, oxygen, hydrogen, nitrogen, sulfur and phosphorus, in addition to several other micronutrients, were already abundant in rocks, oceans and the atmosphere since they formed, about 4 years ago. 5 billion years. The energy needed to “cook” all these elements in the cauldron of the early Earth was also abundant, and it came from several sources: from the internal heat of the Earth’s mantle and from that expelled in oceanic fumaroles, from the decay of radioactive elements, from the great amount of impacts of meteors, and even lightning.

In 1953, scientists Stanley L. Miller and Harold C. Urey of the University of Chicago conducted an experiment that became famous. If all these ingredients were put in a closed jar and “cooked” by electrical discharges that simulate lightning, various organic molecules, such as amino acids, would spontaneously form. The experiment has been repeated countless times since, and it seems to confirm that organic molecules may have been a commonplace component of the early Earth.

Isolated organic molecules, however, are far from what we can call life. There are several definitions for this concept, but the most accepted involve some main properties: what is alive must be able to reproduce, carry genetic information from generation to generation, and use energy to carry out some type of metabolism.

The presence of organic molecules, the basis of what we call life, is a necessary condition, but it is far from sufficient. And that’s because, among other problems, the “soup” in which the organic molecules were created was probably too diluted, considering the gigantic volume of Earth’s primordial ocean. Another is that it is not enough to produce amino acids, lipids, sugars and other organic components: it is necessary to combine the right molecules.

Nature is, in fact, prodigious in the production of organic molecules: a meteorite, for example, can contain more than seventy types of amino acids. Life, however, is extremely selective: it uses about twenty types. And it is even more demanding: chains of identical chemical composition can present organization of their molecules to the left or right of the main chain, and this difference in symmetry makes the so-called “left-handed” chains behave differently from the chains of “right hand” and vice versa. For some reason, life almost exclusively uses left-hand chains.

It is necessary, therefore, some mechanism that can select and concentrate the molecules of the correct composition and geometry, so that a self-replicating structure capable of passing information from generation to generation can be built, that is, for the recipe of life to work. Luckily, the enormous variety of minerals that make up the Earth’s crust since its beginnings are capable of doing just that, aggregating and concentrating organic molecules on its surface. Furthermore, some crystalline faces of common minerals, such as quartz and calcite, show an ability to select only molecules of a certain geometry, that is, they are able to select both left-handed and right-handed molecules. In addition, some minerals can serve as catalysts for chemical reactions, perhaps for reactions important for the formation of organic molecules, their reproduction and metabolism. Could the crystalline surfaces of minerals, then, have functioned as the first substrate on which organic molecules organized themselves into something that can be called life?

Experiments in this direction have revealed that the idea is plausible. Mineral structures such as graphite and molybdenite, for example, have been shown to attract and organize crucial organic species such as adenine and guanine, two of the bases of nucleic acids, RNA and DNA, into interesting two-dimensional structures. Other scientists have even suggested that not only did the foundations of life arise on the surfaces of minerals, but that the first form of “life” was exactly a kind of clay mineral. This hypothesis, championed by chemist Graham Cairns-Smith of the University of Glasgow, returns to the question of what we call life itself, and whether there is any intermediate stage between the living and the non-living. For Cairns-Smith, some clay species carry a form of genetic information in their structure and chemical composition, with the most “survivable” clays being chosen in a selective process similar to that visualized by Darwin.

Whether as catalysts of biochemical reactions, a canvas where the history of life could be painted through the grouping and selection of the ideal molecules for its recipe, or even as the first beings that could somehow be interpreted as something other than the non-living. , the relationship between the minerals that make up the Earth and the life that developed on its surface is further evidence of how interconnected we are all in one great system.

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Fabrício Caxito is a professor of geology, principal researcher in the GeoLife MOBILE project and philosopher at UFMG.

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