Is it a coincidence that life on Earth is based on carbon? Is this the only element capable of becoming the basis for life? Could there exist, somewhere in the Universe, a type of life different from ours, whose chemical composition is not based on carbon? On what atom or group of atoms could a chemistry of comparable complexity to organic chemistry be built, in theory?
There are 91 different chemical elements on Earth.
Others have been generated artificially in the laboratory, and it is suspected
that they can be produced in small quantities inside a giant star that becomes a
supernova, but their life is short, because they are very radioactive and
disintegrate quickly, transforming into more stable elements. Therefore, the
search can be reduced to the 91 natural elements. We will select from these
those that meet the following two conditions, essential to be able to be the
chemical basis of life:
- They must be capable
of establishing more than two covalent bonds with other atoms.
- They must be stable. That is, they must have at least one non-radioactive isotope.
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| Methane |
The first condition is necessary for the element to be able to bond with itself to form chains or rings. Two bonds are not enough, because in this case the chain or ring would not have free bonds to be used with different atoms. The second condition is logical, because a radioactive element would not give stability to chemical compounds based on it. Some of the atoms in the chains and rings would spontaneously transmute, and living beings would disintegrate.
The list of chemical elements that meet both
conditions is reduced to nine: boron, nitrogen, phosphorus, arsenic, antimony, carbon, silicon,
germanium and tin.
The first five can form three covalent bonds, and are not so good as the last
four, which have four bonds.
It seems reasonable to demand that the element on
which life can be based be fairly abundant. Of the nine mentioned, nitrogen,
carbon and silicon are abundant in the universe. Silicon, phosphorus and carbon
are the most common elements in the Earth’s crust. Carbon is the most common of
the nine elements in the hydrosphere. Finally, nitrogen and carbon are the most
common elements in the atmosphere. Boron, arsenic, antimony, germanium and tin
are much less abundant than the other four. We will leave them out, since it is
unlikely that any type of life based on them exists anywhere in the Universe.
We are therefore reduced to four elements: carbon,
silicon, nitrogen and phosphorus. Can we eliminate more? Yes. We must take into
account the stability of the resulting compounds. In nature, elements combine
in such a way that more energy is required to break down the most stable
configurations. This does not mean that there cannot be others whose bonds
require less energy to be broken, but these eventually transform into the first
ones. Although it would theoretically be possible to build chains of nitrogen
atoms, this configuration is much less stable than the molecule formed by two
nitrogen atoms linked together by a triple bond: N≡N. The energy of this bond is six times greater than
that of a single bond. This means that compounds based on chains or rings of
nitrogen will be unstable and decompose, giving molecular nitrogen. So we can forget
about this element.
We thus arrive at the following conclusion: among all the known elements, only three have a certain probability of being able to
serve as a basis for the construction of life: carbon, silicon and phosphorus.
We know that the first meets all the required conditions, since life on Earth
is based on it. But can we say the same about the other two?
Let us look at silicon, which is better than phosphorus;
it is more abundant in the universe, since it is produced in large quantities
in stars that reach the so-called red giant phase. It also can form four covalent bonds compared to the three of phosphorus,
and can therefore give rise to a much larger number of compounds. This leads to
a question: Since
silicon, in theory, could be the basis for the origin of life, why is life on
Earth not based on it, if silicon is much more abundant than carbon in our planet?
Carbon has smaller atoms, so the bonds it
can form with other elements and with itself are stronger, require more energy
to be broken, and are more stable. In particular, the energy of the
carbon–carbon bond is 1.5 times greater than that of the silicon–silicon bond.
Carbon will therefore be better than silicon to perform the role we are
interested in.
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| Structure of quartz |
Although silicon is capable of making chains and
rings similar to those of carbon, the energy of its bond with another silicon
atom is significantly lower than the energy of its bond with an oxygen atom. In
the presence of oxygen, silicon polymers will spontaneously break down,
transforming into more stable oxidized compounds. With carbon, it is the same,
but its affinity for oxygen is 1.13 times greater than the energy of the carbon–carbon
bond, compared to 1.66 times for silicon. This means that in the presence of
oxygen, carbon compounds are more stable than silicon compounds.
On the early Earth there was almost no free oxygen,
because when the Earth was formed most of this gas was combined with other
elements, especially silicon. More than 80 percent of the crust of our planet
is made up of various combinations of oxygen with silicon. Terrestrial life
could have been based on carbon because silicon had taken up most of the
oxygen.
It is true that almost all carbon was also oxidized
in the form of carbon dioxide. But silicon dioxide is solid up to high
temperatures: quartz melts at 1,780°C. It is also slightly soluble in water.
Carbon dioxide, on the other hand, is a gas soluble in water. On the early
Earth, silicon compounds accumulated in the solid mass of the planet, while
carbon compounds were mainly present in the atmosphere and hydrosphere, and
were therefore subject to the action of energy sources that caused the first
organic compounds to appear, useful for life.
This explains why carbon prevailed over
silicon as the basis of life on our planet. It also casts strong doubt on the possibility that
silicon could play that role on a different planet, as oxygen, the third most
common element in the Universe, will almost always be present in greater
proportion than silicon and will combine with it, making it incapable of
forming an organic chemistry.
The conclusion of these discussions may be
expressed in the words of Richard M. Lemmon and Wallace R. Erwin: It is
difficult for the modern chemist to imagine that life could be based on any
element other than carbon. It is difficult, but we cannot say that it is impossible, that there
may not be some place in the universe where a strange combination of
circumstances may have made possible the appearance of life based on silicon,
or phosphorus, or two or three elements at once. The universe is so vast, the number
of stars so unimaginable, that even if the probability of an event is small,
there may be some place where the right conditions have been met. Yet, for that
very reason, it is highly unlikely that we will ever learn of its existence.
Thematic Thread about Life in Other Worlds: Previous Next
Manuel Alfonseca
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