We hardly think much about carbon, except perhaps in its most glittering form: diamonds. Yet, without an abundance of carbon, life on earth would be impossible, for carbon is the main element of DNA, sugars and proteins, fats and muscle tissues and just about everything in our bodies. Carbon is also an important link in the miraculous story of life, a story that began literally in the stars shortly after the Big Bang and ends in the nucleus of every atom. Luckily for us, carbon (from Latin: carbo “coal”) is the fourth most abundant element in the universe by mass (after hydrogen, helium, and oxygen). It is also the second most abundant element in our bodies by mass (about 19.5%) after oxygen.
The reason carbon is so special to life comes down to its atomic structure. Four electrons circle its outer orbit, allowing each carbon atom to form four chemical bonds to other atoms. In addition, the carbon atom has just the right size to fit in comfortably as parts of very large molecules. This allows many complex life-giving compounds such as amino acids and hence proteins to be formed.
The Origin of Carbon
Carbon didn’t just appear by itself; its formation required a universe full of stars. That process started eons ago. When the stars formed after the Big Bang start aging, they accumulated helium, two atoms of which form beryllium. But beryllium is unstable in that it almost immediately converts back into helium. As the stars aged further, their cores collapsed and they became white hot. It was in this furnace that beryllium was transformed into carbon. This process cannot produce enough carbon to take the next step towards life. As explained by physicists Stephen Hawking and Leonard Mlodinov in their popular book, The Grand Design, the critical point came when
“the sum of energies of a beryllium nucleus and a helium nucleus [was] almost exactly the energy of a certain quantum state of the isotope of carbon formed, a situation called resonance, which greatly increases the rate of nuclear reaction.” [1, 2]
To take the next step towards the formation of life, stars must explode, flinging trapped carbon atoms over the infant universe. At this point, the big picture gives way to micro forces played out at the subatomic level. A starring role is played by the strong nuclear force, one of the four fundamental forces of nature [3]. The strong force can’t be too weak or too strong. If it were only a bit weaker than what it is in nature, it would not hold atomic nuclei together against the repulsive force of electromagnetism. This would jeopardize the stability of all the elements essential to living organisms, especially proteins [4]. If the strong force is just a bit stronger compared to the electromagnetic force, two protons could stick together in spite of their electromagnetic repulsion. In this case, all the hydrogen in the universe would burn into helium and there would be no water or long-lived stars like the sun. Moreover, as Hawking and Mlodinov states:
“It turns out that it is not only the strengths of the strong force and the electromagnetic force that are made to order for our existence. Most of the fundamental constants in our theories appear fine-tuned in the sense.”
The fact that we are here, pondering on these things is because we are the recipients of a miracle, residing in a universe that is almost too good to be true [5].
Notes
[1] Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in mass number. All isotopes of a given element have the same number of protons but different numbers of neutrons in each atom.
[2] Stephen Hawking and Leonard Mlodinov, The Grand Design, Batam Book, 2010, p. 159. Quoted in Alan G. Gross, The Scientific Sublime, Oxford University Press, 2018, Chapter 6, parts of which this post is based.
[3[ The strong nuclear force is one of the 4 fundamental forces of nature. More than a hundred times stronger than electromagnetism and far stronger than the weak force and gravity, the strong force holds most matter together by (a) confining quarks into hadron particles such as the proton and neutron and (b) binding them to create the atomic nucleus.
[4] Proteins are one of the primary constituents of living matter. They consist of long chains of amino acids, bonded together by peptide linkages and thus called polypeptides. Although there are hundreds of amino acids in nature, only 20 so-called standard amino acids or proteinogenic amino acids are used by cells in humans and other mammals to form proteins. All 20 standard amino acids present in mammalian proteins are alpha-amino acids (except proline). They are comprised of an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom (-H) and a side chain (-R group) bonded covalently to a central alpha-carbon atom.
[5] See Barrow, J D and Tipler, F J, The Anthropic Cosmological Principle, Oxford University Press 1986.