Where Do We Come From, Literally?

Sometimes we are gazing at the stars, scintillating reassuringly across the sky, as if we are yearning for a long-forgotten speck of what once felt as home. An enigmatic melancholy, barely noticeable, yet ever so present, falls upon us.

Would that nostalgic state of mind not make more sense if we knew that the body we inhabit is composed of indiscernible parts that originate from the place we are staring at?

Let us cosmically retract the steps along our physical journey of creation and see what precisely makes up you and me.

Our Chemical Self

Zooming in close enough, we come to witness that the overwhelming majority of the molecular structures in our body are assembled from merely 7 chemical elements: oxygen, carbon, hydrogen, nitrogen, calcium, phosphorus, and potassium.

Chemical elements are the most fundamental building blocks in the realm of chemistry. Meaning that during chemical reactions these basic entities will not be altered or broken down in any way. Atoms, in contrast, are individual units of a particular chemical element, and various atoms — from the same or different elements — can join together to form stable structures, i.e. molecules.

The periodic table lists 118 of these chemical elements, out of which half of them, i.e. 59, are needed to design a human body, according to Bill Bryson or Herbert Zeng. It turns out that roughly 21 of those vital elements are classified as essential to human life, whereas the remaining 38 do play some, though not always entirely understood, biological role.

For an individual with a mass of 70.0859kg, the above chemical categorization translates after some calculations into the following: the 38 non-essential elements collectively amount to approximately 0.0027kg (0.0038%), while the essential elements conjointly give a mass of 70.0832kg (99.9962%).

If we now cap the percentage of essential elements at 99.5%, we learn that only one third of the 21 essential chemical elements suffice to compile 99.5% of the human body. That is, we consist mainly of 7 fundamental chemical constituents: oxygen (43kg), carbon (16kg), hydrogen (7kg), nitrogen (1.8kg), calcium (1kg), phosphorus (0.8kg), and potassium (0.1kg).

Fig. 1. 99.5% Chemical composition of the human body for an individual of an average of 70.1kg. (Source: raw data available from Ed Uthman, MD).

One final reflection before we delve into these 7 elements separately: How do we actually expand in volume over time? Given that atoms do not change size or multiply — by and large physics prohibits that mass is either created or destroyed — the predominate ways of amassing atoms is by means of eating and drinking. In other words, by taking up the required nutrients, and thus increasing the number of atoms present in our bodies, we have been able to physically grow over the years.

On to the next two questions: Where exactly do these atoms of the 7 elements originate from in the first place, and how are they helpful to us?

Cosmic Breakdown

1. Oxygen

Relevance to the Human Body

Broadly speaking, oxygen is used for two key functions: breathing and metabolism. Around 21% of the air that we inhale contains oxygen, the rest being mostly nitrogen. As the oxygen permeates through the lung membrane into the bloodstream — this takes place in the epithelium of the alveoli sacs — red blood cells transfer this life-sustaining element across our body where it can be employed to perform metabolic duties.

Circa 20% of the total oxygen consumption is taken up by our brain, where the oxygen underpins both neuronal activities and energy metabolism.

Metabolism refers to a gamut of important cellular actions: constructing and repairing tissue, converting food into energy, removing waste products (including carbon dioxide, urine, and sweat), and reproduction of cells.

Basically, the source of our energy can be traced back to the process of oxidation. That is to say, we obtain energy when cells burn nutrients — technically, the act of burning is called oxidation — which occurs with the support of enzymes, i.e. protein molecules that speed up chemical reactions.

In this way, certain biomolecules (think of adenosine triphosphate (ATP)) can carry around that newly available chemical energy to other places within these cells to fulfil different cellular tasks.

Almost all our molecules hold oxygen, bar a few exceptions, such as carotenes (antioxidants). By mass, the largest fraction of oxygen is found in carbohydrates — for instance, glucose, lactose, and starch — which are storage molecules for chemical energy.

Origin in the Universe

The earliest time and place that oxygen has been spotted in the Universe is 600 million years after the Big Bang, the birth of the Universe, which took place 13.8 billion years ago, within interstellar dust of the galaxy A2744_YD4.

Having said that, the greater part of heavier elements, Joel Bregman et al. argue, are nevertheless thought to have been created outside of galaxies, i.e. intergalactic space. As a case in point, the research by Jesper Sommer-Larsen and Johan Fynbo indicates that merely 20–25% of the total oxygen stems from galaxies.

The genesis of chemical elements heavier than (and equal to) carbon materializes exclusively within the inner core of stars — a process termed stellar nucleosynthesis whereby the nuclei of lighter elements are merged together into heavier ones.

For elements up to iron (this comprises oxygen), nuclear fusion releases energy, whereas nucleosynthesis for elements beyond iron expects an inflow of energy to get the job done. This is why heavier elements are usually produced in energy-rich explosions, i.e. supernovae.

What is more, the mass of a star influences which elements can be fashioned, bearing in mind that the more massive a star the warmer its core, and, therefore, the higher the amount of energy at its disposal to fuse heavier elements.

With regard to oxygen, it is massive stars (stars with at least 8 times the mass of our Sun) that are primarily responsible for crafting this essential element. Overall, most of the oxygen throughout the Universe, as reported by Grażyna Stasińska et al, is made during the non-explosive nucleosynthetic phase of burning helium — in the context of stellar nucleosynthesis, burning refers to nuclear fusion.

Fig. 2. Production of oxygen through burning of helium. (Source: adapted from University of Jyväskylä).

2. Carbon

Relevance to the Human Body

The 16kg of carbon in our body is put to use as foundational ingredient for carbohydrates (e.g. fructose, also known as fruit sugar, which is found in fruits, some vegetables, and honey), proteins (e.g. myosin describes, together with the protein actin, muscle contraction in muscle cells), and fats (e.g. cholesterol, which helps with cell creation and digestion).

Just as with oxygen, carbon is essential during cell metabolism: cells transform the absorbed glucose (a carbohydrate, which holds carbon) into chemical energy (ATP) through a series of oxidative biochemical reactions — a process referred to as cell respiration. The body then drives out part of the carbon as CO₂, a waste product of cell respiration.

Not only that, besides hydrogen, oxygen, phosphorus, and nitrogen, carbon is instrumental in the formation of nucleic acids, i.e. deoxyribonucleic acid (DNA), and ribonucleic acid (RNA). These large biomolecules store our genetic information, contain instructions for cell replication, and build proteins.

Origin in the Universe

Carbon is present under multiple disguises in the void between the stars within a galaxy (the interstellar medium): both in atomic form (C) and in molecular arrangements (e.g. CH₃OH, or CH₄). But taken collectively, the majority of carbon is observed in intergalactic space — as was already hinted at in the section on oxygen.

After the finalization of hydrogen burning via either the proton-proton chain or the carbon-nitrogen-oxygen (CNO) cycle, the inner core temperature of stars with a mass above 0.5 solar masses rises sufficiently to initiate the burning of helium. As indicated in Fig. 2, this marks the start of the triple-alpha procedure, resulting in the production of carbon.

Nonetheless, recent research suggests that the greatest birthplace of carbon are white dwarfs with a minimum initial mass of 1.5 solar masses. For lower-mass stars, the stage of white dwarf is the endpoint in stellar evolution; nucleosynthesis has come to a complete halt. When stars run out of fuel, approximately 97% of them turn into white dwarfs. The white dwarf disseminates carbon throughout space by shedding its carbon-rich outer mantle.

Fig. 3. Schematic overview of the formation, evolution, and death of stars. The white dwarf follows the upper path. (Source: University of Waikato).

3. Hydrogen

Relevance to the Human Body

Apart from the fact that hydrogen is available in carbohydrates, proteins, and fats, and plays a role in cell respiration, it also is an integral part of all body fluids.

As such, hydrogen keeps the joints lubricated, enables the transport of cells and proteins throughout the bloodstream, flushes out toxins, and adjusts the body’s internal temperature.

Moreover, the element not only enhances the body’s immune system, but also acts as an insulator to shield the brain, the organs, the spinal cord, and the fetus from shocks. And it furthermore safeguards that both proteins and DNA take on the appropriate shape for them to function properly.

Origin in the Universe

Unlike the two previous elements, hydrogen — the first element of the periodic table — was fashioned shortly following the birth of our Universe. In astronomic terms, short here means 380,000 years after the Big Bang.

It was at that time that the Universe had cooled down enough for neutral atoms to appear: electrons settled in a stable manner around protons to develop hydrogen.

However, the hydrogen’s nucleus, i.e. one proton, came about much earlier — between a millionth of a second and one second after the Big Bang. At that moment, fundamental subatomic units, named quarks, tightly coalesced to make a proton.

To our current knowledge, hydrogen is also the most common chemical element: scientists estimate that 92% of the total number of atoms in the observable Universe are hydrogen atoms.

4. Nitrogen

Relevance to the Human Body

Nitrogen enters the human body largely via plants, for instance soybeans and peas, but any protein-bearing food will provide us with this element, such as meat, eggs, poultry, fish, and nuts.

The 2.6% of nitrogen that our body holds is indispensable for our survival, as it regulates the health of our tissues (e.g. hair, muscles, and skin), makes up our DNA and RNA, and ensures that enzymes sustain metabolic processes.

Plus, it has an influence on the workings of neurotransmitters within our central and peripheral nervous system — neurotransmitters are molecules in our nervous system that relay messages between nerve cells, i.e. neurons. For one, we have acetylcholine (C₇H₁₆NO₂), whose effects on the body encompass a lower heart rate, less elevated blood pressure, and better digestion.

Fig. 4. Another example of a nitrogen-containing neurotransmitter, nitric oxide, which plays a key role in the modulation of pain. (Source: Yara Cury).

Origin in the Universe

According to L.S. Pilyugin and Danielle Berg et al., both intermediate-mass stars and massive stars account for the production of nitrogen. Even so, the die is not yet cast: some scientists, including Elisabeth Vangioni et al., contend that intermediate-mass stars are the main reason for the presence of nitrogen in the interstellar medium, when at the same time others, take for example Arpita Roy et al., put forward the hypothesis that also rotating massive stars are responsible for introducing nitrogen (with the help of pre-supernova winds).

One thing that we take from these discussions is that nitrogen is not being dispersed across the Universe by way of exploding stars — supernovae — but it happens in the event that stars cast off (part of) their outer shells called planetary nebulae, which consist of glowing ionized gas. Ionization occurs in the situation where neutral atoms or molecules become electrically charged through the gain or loss of electrons.

Be that as it may, it remains a fact that for low-mass stars — like the Sun — the proton-proton chain is the prevailing source of energy creation, while for heavier stars the CNO cycle gains relevance.

More to the point, it is the CNO cycle that explains the development of nitrogen.

Fig. 5. In massive stars, the carbon-nitrogen-oxygen (CNO) cycle is the main source of energy creation. (Source: adapted from University of Oregon).

5. Calcium

Relevance to the Human Body

The 1kg of this most common mineral in our body is for the greatest part stored in our bones and teeth, as its principal duty comes down to growing and cultivating a healthy and robust bone structure.

It is vital to consume calcium-rich food (e.g. milk, cheese, tofu, and almonds), as our body does not generate this chemical element by itself. We need calcium because our skeleton is continuously being broken down and replaced by new bone material — in technical terms, this is designated as remodeling.

Even more noteworthy is the fact that the replenishment of bone substance winds down after the age of 40, which is why it is crucial to maintain strong bones to minimize corrosion, not least since bone loss carries a risk of osteoporosis, i.e. heightened bone fragility.

The remainder of calcium in the body undertakes a wide scope of tasks, ranging from taking care of proper muscle functioning, making sure our blood clots at the point of injury, and lowering the blood pressure to controlling hormone secretion, and optimizing nerve impulse transmission between the central nervous system and body muscles.

Origin in the Universe

When nuclear fusion grinds to a halt in massive stars — at this point ordinary stellar nucleosynthesis has reached the element iron — the gravitational pressure gets the upper hand, leading to the contraction of the inner core, upon which the star violently explodes, i.e. a Type II supernova (see Fig. 3).

Upon the explosion, all the chemical elements assembled prior to the blast, including calcium, get flung out into space, as what happened for instance with the supernova Cassiopeia A (Cas A) some 300 years ago.

About half of the available calcium originates from bursting massive stars, such as Cas A, whereas the remaining half arises out of exploding white dwarfs (Type Ia supernova).

An additional yet related way of creating this silvery, soft metal is during a Type Ib/c supernova itself — dubbed calcium-rich supernovae. In other words, as a result of extremely high temperature and pressure within the final stages of the dying star, the blast itself is able to produce considerable amounts of calcium in a very short time span, as was the case with the supernova SN 2019ehk.

Fig. 6. An X-ray image of various elements, including calcium, present in the supernova Cassiopeia A. (Source: Chandra X-Ray Observatory).

6. Phosphorus

Relevance to the Human Body

Nearly the entire 1.1% of the mineral phosphorus is in one way or another attached to the most abundant element in our body, oxygen. The combination of the two is then referred to as phosphate. Roughly 85% of that combined substance resides in our skeletal system with the aim, in concert with calcium, of building and preserving both bones and teeth.

The residual 15% is found in tissues across the body where the chemical compound not only supports cell respiration (ATP), muscular contraction, hormone balance, neural and cardiovascular activities, and digestion, but also constitutes the basic components of cell membranes and DNA.

What is more, the levels of phosphate and calcium within our bloodstream are inversely intertwined: the parathyroid hormone reduces (pushes up) the phosphate concentration if the share of calcium becomes too prominent (small). In addition, our kidneys are tasked with clearing any excessive quantity of phosphate in our blood.

Origin in the Universe

Either by nuclear fusion towards the massive star’s last moments, but before the supernova, or by supernova nucleosynthesis — the birth of new elements during a supernova — is how this essential element sees the daylight, by figure of speech.

Concretely, nucleosynthesis teaches us that the element is forged by the burning of neon and oxygen, at temperatures reaching between 1.7 and 2.4 billion Kelvin.

Fig. 7. The nucleosynthetic process of producing phosphorus through oxygen burning. (Source: adapted from paper S.Woosley, T.Weaver, and A.Heger and Prof. Richard Pogge).

Based on the study of two supernovae, i.e. Cas A and Crab Nebula, the yellowish mineral seems to be sparsely distributed, at least within our galaxy (the Milky Way). Notwithstanding the scarcity, Olga Zamora et al. affirm the existence of phosphorus-rich stars in our Solar System, acting as potential primary sources for the element’s presence on Earth.

7. Potassium

Relevance to the Human Body

Next to chloride and sodium, potassium is a blood mineral — electrolyte — and 98% of the 0.1kg of this chemical element is stored in our cells. Overall, it helps sustaining a satisfactory water and acidity level in the blood and tissues as well as supporting the body to strive for a regular heartbeat, a healthy nervous system, and adequately operating muscles.

Importantly, a delicate balance is struck between potassium and sodium; a high-sodium and low-potassium diet negatively impacts the cardiovascular system. To that end, a nutritional regime that is rich in potassium and low in sodium can keep the blood pressure in check.

The intake of potassium can be boosted by eating, among other things, bananas, apricots, potatoes, spinach, broccoli, cod, and whole grains. And mind you, alcohol, caffeine, and sugar are capable of decreasing the potassium quota in our bloodstream.

Other than hypertension (high-blood pressure), a lack of the mineral can furthermore cause fatigue, depression, or cardiac arrhythmia (irregular heartbeat). In more severe cases, a significant deficiency may adversely affect the nervous system, substantially throw off the heartbeat rhythm, notably enhance the risk of bone fragility, or dramatically hamper muscle functioning.

Origin in the Universe

Typically, we can trace back the birthplace of this silvery-white alkali metal to the phase of supernova nucleosynthesis within massive stars. To be more precise, potassium emerges from the star’s oxygen and silicon burning shells.

In essence, the elements silicon and sulfur, obtained during oxygen burning (see Fig. 7), are input products for the silicon burning process, which ensues at temperatures between 3.3 and 3.8 billion Kelvin.

Fig. 8 does not portray the actual reactions that silicon burning entails — they involve magnesium, neon, oxygen, carbon, phosphorus, silicon, and sulfur.

However, given that silicon burning releases protons and 𝛼-particles (helium nuclei), what Fig. 8 does show is how these 𝛼-particles are further used in reactions, giving us, inter alia, argon.

Then, finally, the chemical element argon sets in motion the synthesis of potassium in massive stars, for temperatures exceeding 170 million Kelvin.

Fig. 8. Schematic overview of potassium synthesis based on silicon-burning. (Source: adapted from paper C.Iliadis et al. and University of Toledo).

Coming Back Home

Quite literally, we are connected to the Universe, in the sense that we are physically made from imperceptible substances with a cosmic origin.

And in 7 or 8 billion years, we will give back to the Universe all the atoms that we received from the stars and the Big Bang, when the Sun inflates to the size of a red giant — a next phase later in the life of a lower-mass star (see Fig. 3). At that moment, the Earth inevitably vaporizes away, allowing all the atoms on a free course to their new destinations.

That is still a long time from now. So perhaps it is sufficient for today to wonder about the dizzying fact that 7⨯10²⁷atoms from across the Universe have come together to form the person that you are today.

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