![]()
Part I
Being
Of what is a human body made? How does it appear when viewed through lenses of deep space, deep time, or a microscope? What makes a body alive, or dead? How big or complicated do bodies need to be for social life to emerge? How much of a human body is actually human? The essays in this section take up these questions at levels of organization from cosmic to subcellular and on time scales from geological to momentary.
![]()
1 The Matter of Life and Death
How Humans Embody the Universe
Iris Schrijver and Karel Schrijver
The human body and the network of terrestrial life that enables its existence are always in flux, creating and exchanging countless molecules, made mostly of elements forged inside stars or in the explosions that end their existence. Captured from interstellar space as the solar system took shape, these essential elements are shared by all life on Earth, shattering the notion of independent existence and highlighting universal interconnectedness.
The unfathomable vastness and the staggering hierarchy of scales in the Universe defy immediate experience, complete understanding, and even our imagination because astronomical numbers reach beyond any sense of practical reality. Yet, they fascinate. They spark scientific research as much as philosophical discussion and compel us to consider how human life fits in. An exploration of the myriad connections between life and the Universe leads to a startling, perhaps unsettling conclusion: We are nothing even as we encompass everything. But how, exactly, is that manifested?
All measurable time originated 14.6 billion years ago with the Big Bang that hurled the Universe into existence. The ensuing inferno of expanding and collapsing gaseous clouds eventually gave birth to uncounted galaxies, of which we can see only a trillion or two. In each of these, in cycle after cycle of successive generations, billions of planetary systems took shape around stars that sustain nuclear fusion until they fade away or explode. The Universe may seem ageless, but the countless individual objects contained in it, from entire galaxies to individual rocks, have materialized in ongoing processes of formation to obliteration. By current estimates, our own Milky Way Galaxy alone contains 100–400 billion stars and a multiple of that number in planets. Out of those, perhaps ten billion might have Earth-like characteristics, with some potentially harboring life. All are located lightyears beyond the Solar System and remain out of reach for humanity.
At 4.6 billion years old, the Solar System is the home of Earth and seven other planets. Earth is a mid-sized planet, currently supporting about 7.7 billion people and the only place, anywhere, known to sustain life. Based on microfossils of the earliest single-celled organisms, life on Earth began at least 2.5 billion years ago, but ancillary evidence suggests that life emerged around 3.9 billion years ago. Quite possibly on a random Tuesday afternoon. Earth orbits a sole star, the Sun, which is a middleweight star projected to continue its nuclear fusion for another five billion years. Given its life-sustaining energy, that should come as a relief! Apart from the Sun and the planets, the Solar System includes other heavenly bodies such as comets, which are frozen time capsules made of gas, dust, and rock from interstellar space. Comets date back to the formation of the Solar System and are made from the same materials. Although usually unnoticed, comet matter continues to sprinkle onto Earth at a rate that is currently about 260,000 pounds per day.
As inhabitants of Earth, human beings have a body that mirrors the complexity of the Universe. With a ballpark figure of 50 trillion cells per average body, the cells in a single human body outnumber the stars in our Galaxy 500 to 1 and Earth’s population around 10,000 to 1. Within each cell reside approximately the same astounding number of atoms, a fraction of which make spiraling clusters that contain our DNA, an extremely thin string some two yards long so tightly coiled up that, if unfolded, it could wrap around its cell thousands of times. Analogous to events elsewhere in the Universe, human existence is characterized by cycles of arising and passing – not only in the literal sense of birth and death but also in terms of what happens in the body during a lifetime. The tissues and their constituent cells that shape human bodies enable countless astonishingly coordinated functions. However, the stresses within the functioning body and those imposed from outside frequently cause cells to fail. In order to keep things running smoothly, entire cells and their building blocks are replaced and regenerated in a process that involves millions of cells every second, although this renewal becomes less efficient over time as evidenced by the aging process.
Hidden in the background of each lived life, the inner workings of the human body are always in motion, taking care of a continuously occurring vast number of adjustments. The hundreds of different cell types have life spans ranging from days to years but overall are replaced about every seven years. This cell turnover amounts to the weight of an entire human body each year! Cell contents turn over with an even greater frequency. Take water, for example, which makes up the largest bulk of body weight and volume and is completely exchanged in a matter of weeks, at most. Water is but one of an immense collective of molecules found in every cell that includes the four major classes of organic compounds: carbohydrates, proteins, lipids, and nucleic acids. These facilitate functionality, enable energy usage and storage, and balance all of life’s chemical reactions. But the human body does not make all of these itself, and those that it does make require building blocks and energy to be put together. That energy is supplied by combining the oxygen we breathe with the food we eat, but not directly. Between ingestion and use sits digestion that is enabled by the microbiome: Human beings are colonized by hundreds of species of bacteria. These organisms, on surface areas outside and inside the body, exist with us in a symbiotic relationship. Astonishingly, they outnumber the human cells of the body. And whereas bacteria are most prominently known for their disease-causing capacity, the microbiome categorically deserves more positive press. Not only does it process our food at the molecular level, it affects mind-states such as mood and helps strengthen our immune defenses. Ultimately, it critically contributes to the maintenance of health.
The energy in our food is stored there by other life forms that take it from yet another source: Almost all life on Earth depends on electromagnetic energy that is generated by nuclear fusion in the core of the gas ball that is our Sun. This energy ultimately escapes the Sun as warming, energizing light. Sunlight increases the planet’s temperature well above the deep-freezing background of the Universe and supports the growth of plants, thus sustaining the animals that use oxygen and consume food. Plants use the Sun’s energy to change electromagnetic bonds in organic compounds that, among other things, enable the growth and ripening of fruits and vegetables. Only weeks after leaving the Sun, the energy may be harvested and enjoyed as food.
The human body is always in a profoundly transient state of being. Accordingly, it is more similar to a relatively constant pattern that allows the perception of a degree of continuity than it is to a static entity with any measure of actual permanence. What outlasts the constant changes is a collection of energy and matter, all of which is steadily replicated to preserve the characteristics of an individual during the span of a lifetime. It will be evident by now that anyone’s chronologic age is a practical concept useful for daily life but in fact amounts to nothing more than an illusion. All of this provokes the question: What is the essence of the human body, and indeed of life? All life on Earth is integrated with replacement and recycling processes on multiple levels. On the level of the organism, the chemical matter consumed by eating, drinking, and breathing may become integrated for a while to (re)build and power the body, only to be discarded and replaced in due time. As amazing as that is, it represents a narrow view. There is a much larger perspective on life that begins to take shape as we consider the energy on which life depends.
The human body is not solely composed of stardust bound by energy coming from the Sun: It exists by the grace of elements that make up, and are made by, the Universe. Only the body’s hydrogen is as old as the Universe. All other elements originate from nuclear fusion inside stars and from explosions that ensued upon the end of their existence. Eventually, gravity trapped enough matter to configure our corner of the Universe. The matter that forms us participates not only in cosmic events but also in organic and inorganic Earth cycles such as continental drift, the water cycle, and the nitrogen cycle. A small amount of matter is added whenever comets hit the Earth or when invisible showers of ultrafast particles enter the atmosphere, and some matter is lost because the atmosphere slowly escapes into space. All life on Earth is composed of elements that are billions of years old, and one of these (hydrogen) has been around as long as the Universe has existed.
It seems a cliché to say that we are made of stardust as if it were a platitude only useful for the lyrics of the 1970s song Woodstock by Crosby, Stills, Nash, and Young. But dismissing this notion would negate that human beings are a very small piece of an unfathomably large puzzle, in which the elements that build our bodies are inexorably intertwined with other animals, plants, single-celled organisms, general biological and geological processes, and with the Solar System, the Milky Way Galaxy, and all of the Universe that stretches out to infinity. The diversity in the connections and all their processes is achieved by variation among molecules and the particular assembly of the millions of atoms from which these molecules are made. Life on Earth is based on the elements that are accessible in a form that supports chemical reactions and that are most readily available. Therefore, hydrogen, oxygen, carbon, and nitrogen comprise almost all of our body weight. What the human body is made of, the origin of its components, how it is maintained yet always changing, and how it is intimately connected with everything around it and with the history of time and space may remain obscure most of the time, yet human beings are never separate from a magnificent, all-encompassing, universal ecology. We embody the Universe in a literal sense.
Becoming conscious of the profound interconnectedness of life and the Universe is not inconsequential: That awareness makes it difficult if not impossible to maintain a position along the lines of a strict “self” versus “other,” or even “humans” versus “nature.” It affects the consideration of other human beings as much as it impacts the views held about other species. It challenges the notion of human existence separate from nature and highlights the futility of unquestioned human superiority. The pretense and hubris of human existence during its relatively minimal presence in the Universe have been revealed for what it is and, if life matters, then actions matter. Life is determined by Earth’s climate, which is undergoing unprecedented changes as a result of human activity. During common astronomical and geological cycles, the biosphere has been able to adapt to change by way of stabilizing feedback loops on time scales beyond human lifespans that supported gradual adaptation. At present, however, profound and rapidly unfolding environmental changes may not be curbed prior to irreversible effects. Our own existence is threatened as much as that of the life forms and biological networks that nurture us. Does it matter? The Universe will continue its cycles and recycling, its violent outbursts and subtle changes, with or without human beings. What will it ...
