Chemical elements and the emergence of life in the universe

By combining knowledge from various branches of science, we can often reach surprising conclusions that are difficult to conceive by other means. This article will be an exercise of this kind, and together we will learn the result.

Biochemistry and cosmogony

Contemporary biology, particularly biochemistry, has made it clear that living organisms are made up of a variety of chemical elements. Among them there is a small group that are in the majority in quantity: carbon (C), hydrogen (H), oxygen (O) and nitrogen (N), or briefly and as a mnemonic rule: CHON. In addition, there are other elements in much smaller quantities that are also essential for living organisms and that make possible the amazing phenomenon we call "life". Among them we can mention, using their chemical symbols: Na, K, Ca, Mn, Mg, S, P, Si, Cr, Fe, Cu, Zn, E Cl, L Mo and others.

It should be clarified that most of them are found as ions and not as elements, since in many cases the latter are very reactive. For example, the elements of the alkali metal family, such as Na and K, are explosive in contact with water. Obviously, it is not in this chemical form that they intervene in cell biochemistry, but as ions (or cations, to give them their more specific name) Na⁺ and K⁺.

The same can be said of some elements that give rise to ions of opposite sign (anions), such as fluorine (F). This is the most reactive element of all, it is an extremely irritating gas and its inhalation represents a danger. The F^– ion, called fluoride, has lost this reactivity, is soluble in water and is essential for cell physiology, mainly for use in teeth and bones.

Let us now turn to another branch of science: cosmogony, which is the branch of astronomy that studies the evolutionary behavior of the universe and the origin of its characteristic features. According to this science the universe had a beginning, which is located 13±2 billion years ago. That is to say, there is uncertainty as to the precise time, but there is a very high probability that it occurred in the time interval between 11 and 15 billion years ago, with the highest probability occurring in the vicinity of 13 billion years ago.

The theory that best explains this event is known as the "Big Bang" theory. It also states that 4 minutes after our rapidly expanding universe emerged, its chemical composition was 76% hydrogen (H, with atomic number one), 24% helium (He, with atomic number two) and negligible amounts of lithium (Li, with atomic number three). The atomic number indicates how many protons the nucleus of the atoms of each element has.

There was no other element. Only the first two positions, if any three, in the table of the periodic classification of the elements had been occupied. With this raw material, chemistry was obviously quite limited. There was not enough variety (diverse chemical elements) to constitute systems as complex as life.

Where then did C, N, O, Cu, Zn and so many other elements with atomic numbers greater than three come from and which, as we have already seen, are indispensable constituents of a living system? Cosmogony clearly answers this question, supported by nuclear physics, as we shall see below.

Nuclear physics and cosmogony

The explanation offered by cosmogony on the origin of the other elements involves the stars, which, according to their mass, produced in the course of their evolution (or life) the different elements heavier than H and He. The stellar conditions are such that they favor the realization of the various nuclear reactions that form the other elements of the periodic classification table that we know today.

Due to its very high temperature (for example, on the surface of our Sun the temperature is between 4 700 and 6 000 K and in its center at 20 million K) each star is an enormous plasma sphere. Plasma is the state of matter characterized by having the atomic nuclei devoid of all their peripheral electrons and waving at high speeds, just like electrons.

Under these conditions it is possible for the nuclei to collide with each other even though there are repulsive forces between them (because they are all positively charged). At lower temperatures, with less thermal agitation, nuclear reactions are not possible. The nuclei would be accompanied by their electrons and would simply not touch each other, they would be deflected by the repulsive forces between charges of the same sign (negative, of their respective peripheral electrons).

However, at high temperatures the nuclei do touch, collide and fuse together, like two droplets of water colliding and forming a larger droplet. These are the nuclear fusion reactions that give rise to the process of nucleosynthesis, i.e., the synthesis of new nuclei, of new (heavier) elements.

For a star like our Sun, by nucleosynthesis and starting from the mixture of H and He, the formation of carbon and oxygen could be achieved. Stars of greater mass are required to generate other heavier elements during their evolution. And still others (different) are synthesized in the final stages of the life of these stars more massive than the Sun, during explosive processes of unimaginable violence.

The material produced by nucleosynthesis in stars reaches to disperse through space, particularly that derived from those stars that are massive with an explosive and furious death.

In summary, the mixture of H and He of the early universe has been slowly changing thanks to the formation of stars of mass similar to or greater than that of our Sun. At present, the chemical composition of the universe is 75% hydrogen, 23% helium and 2% of all other chemical elements. Slowly, the space of the universe has been subtly enriched with elements heavier than H and He, in particular C, O and N, and also with others that are essential for the development of life.

That material enriched in elements heavier than H and He will disperse and, under the right conditions, a solar nebula may form, a protosol may ignite by nuclear reactions between H and He, and a sun may be forged. Perhaps, planets will also form to constitute a planetary star system, perhaps with characteristics similar to ours. But this story of the formation of the Planetary Solar System and the Earth must be told in more detail in another article.

Stellar physics and the emergence of life

In the preceding paragraphs I have implicitly classified stars according to their mass into two classes: those of Sun-like mass, and those of greater mass. There cannot be stars much smaller than the Sun; if the mass of the Sun had been only 9% lower, the temperatures required to initiate nuclear fusion reactions would not have been reached, there would be no Sun, and we would not be here to tell the tale.

It happens that according to the magnitude of its mass will be the duration of a star or, to put it another way, "the duration of its life" (since a star arises, burns or uses its fuel in nuclear fusion reactions, this sooner or later runs out, and therefore, without more fuel, the star goes out or dies).

All stars start their combustion with the primordial mixture of H and He. Massive stars consume their fuel much faster than Sun-like stars. In fact, they consume it a thousand times faster. This makes a big difference. Indeed, it is thanks to massive stars that the synthesis of elements of higher atomic number was possible.

This in turn introduces a much wider variability in chemistry, which surely makes the emergence of life more feasible. However, although suitable for producing diverse elements, massive stars are too short-lived to serve as a source of light and energy for possible planets surrounding them that could be the cradle of life.

On the other hand, smaller stars, such as our Sun, produce only light elements such as carbon and oxygen, and perhaps in that way the emergence of life would be less feasible or perhaps impossible because not enough complexity and variety of chemical functions would be achieved.

Nevertheless, because of their long life, these lower mass stars can be a reliable source of light and energy for a sufficiently long time to planets that could present the necessary conditions for the emergence of life. Our Sun is in the middle of its lifetime (its total duration will be approximately 10 billion years).

To put it simply, there is a kind of complementation of functions, for the purposes of the origin of life in the universe, between the two types of stars.

In our Planetary Solar System, we, on Earth, have been able to verify the existence of a great variety of elements, we have identified them and constructed a periodic classification table. We now know that these elements, of which the Earth and all its inhabitants are made, were created in previous generations of stars, from their remains or ashes. The solar nebula that gave rise to the Planetary Solar System must have already contained these heavier elements, remnants of stars that shone before our Sun.

Conclusions

After having flown over cosmogony, nuclear physics and biochemistry we can make at least one inference that has to do with the title of this article.

The emergence of life in the universe could not be an early event in its history. The appropriate chemical elements had to be present, which emerged after a long succession of events. It had to wait first for the first stars to form, perhaps a billion years after the Big Bang. Then, for them to burn in nuclear fusion reactions. Massive stars take about 10 million years to burn out. At least one generation of these stars had to burn out to begin dispersing new elements into the interstellar void.

Then, a planetary star system enriched in the new elements and containing a small star like our Sun had to emerge to provide a sufficiently long and constant supply of light energy (the Sun began to ignite in nuclear fusion reactions 4.6 billion years ago). Life arises on Earth (as prokaryotic unicellular life) 730 million years after the emergence of our Planetary Solar System, and only 4.6 billion years later, life adopts, among other multiple forms of life, the human form.

Taking these numbers into account, assuming the duration of the universe to be 13 billion years and considering a history similar in duration to that of the Earth, unicellular life could have arisen somewhere in the universe approximately 1.74 billion years (1000+10+730 million years) after the Big Bang. In other words, after a time equal to 13.4% of the present age of the universe.

The emergence of intelligent life like ours (thus qualified by us) would have taken much longer: 5.61 billion years (1 000+10+4 600 million years). That is, it could have appeared after a time equal to 43% of the present age of the universe, not before, not earlier.

Taking this into consideration, the flowering of our humanity has been late, very late in the history of the universe. We exist at the tip of time, at the tip of the present age of the universe. There has been plenty of time (57% of the current age) for intelligent life to flourish in other corners of the universe. In fact, just when our Solar Planetary System was being formed, there could already be sprouts of intelligent life in the universe.

Another aspect that never ceases to impress me deeply when I evoke this scenario, is the unimaginable violence and the enormous temperatures through which each and every one of the elements that constitute me, that constitute us, had to go through; to finally be thrown into the cold and black void of interstellar space... waiting for a new beginning.

Author: Guillermo Mosqueira Pérez Salazar, Source: Correo del Maestro, No. 32.