by Caleb Scharf
THE SIMPLICITY OF COMPLEXITY
But these cellular machines also come and go. Human red blood cells live for some four months; skin cells live for a couple of weeks. Other cells, such as those in a human colon, live for just a few days. Even when you die, many of your cells will live on for hours or days. The cells of a multicellular life-form are as much about themselves as they are about their host.
Cooperative communities of life have existed on Earth for a long while—probably for at least as long as three billion years if you count the first simple colonies of cyanobacteria, with variants of multicellularity being “reinvented” many times as a biological strategy. Multicellular organisms with differentiated cell types—plants, animals, and fungi—began to appear between half a billion and a billion years ago. The cooperation and cohabitation of cells offers a variety of evolutionary advantages in the right environmental conditions. Multicellularity has resulted in organisms today existing with a range of masses spanning an astonishing twenty-two orders of magnitude (twenty-seven, if you count viruses as organisms). Life on Earth extends from the smallest microbes (10−16 kilograms) to the largest plants and mammals (106 kilograms).
A louse’s compound eye, and a grain of pollen on a whisker
Cooperation yields much more than evolutionary one-upmanship on other organisms. Together cellular units can build something far greater than the sum of their parts. Trillions of them can organize into fungi, plants, bugs, birds, and mammals. They can build humans like Albert Einstein, Ada Lovelace (the first computer programmer, and daughter of Lord Byron), Isaac Newton, and Marie Curie, or like Wolfgang Amadeus Mozart, Johann Sebastian Bach, Pablo Picasso, Frida Kahlo, Mary Shelley, and Leonardo DaVinci.
They can also create surprising simplicity from complexity. In animals, for example, there is an observed mathematical relationship between basal metabolic rate (how fast animals burn chemical energy while resting) and body mass. This relationship actually holds from bacteria to tiny shrews, and all the way up to enormous blue whales: the metabolic rate increases with body mass to the power of three-quarters. That’s a law that runs across the whole 1022 span of masses for living things.
A mouse and a whale contemplate each other across two orders of magnitude in scale.
Representation of a neuron cell body
Somehow, a regular and straightforward law emerges across all those assembled biological cells in radically different species and sizes. Other characteristics in biology also show surprising regularity. For example, the ways in which life spans, growth rates, and physical sizes scale with one another also exhibit simple mathematical relationships.
How can this be? It’s a fair bet that these are all manifestations of optimization, driven by natural selection favoring organisms that, putting it crudely, do the best with what they’ve got. But it works because a complex system of many parts can operate and cooperate across many scales.
The same is true in the collective behavior of multicellular organisms. They often form groups, herds, flocks, schools, or swarms. These gatherings behave like new types of entities with their own emergent laws. Their member creatures may each follow simple rules, but en masse they generate unexpected forms of cooperation as they move through the world.
Individual cells of a multicellular organism
Those cooperative patterns are recognizable across radically different species. For example, starlings in their great “murmurations” swoop and stream like a school of herring in the ocean. For both species, the advantage to this group behavior likely arises from creating a sensory overload for potential predators. But it also creates hazards, such as attracting the attention of those very same predators.
Humans are not excluded from this club. We are social animals, bound by language and by the almost unspoken advantages of working together. Research has shown that our cities also follow a number of quite simple mathematical codes. Lengths of roads and electric power lines, and number of gas stations, scale in specific ways with the total population of a city. Wages, violent crime, and disease scale with population too, although differently.
Natural collectives can also solve computational problems with elegant simplicity. For example, pheromone paths laid down by individual ants en route to food sources from their nest are reinforced faster when the round-trip is shorter. So over time more and more ants will follow that shorter path. Consequently, the colony “discovers” the quickest way to feed itself.
A calculus of cost versus benefit for group behavior works across all species. There are good reasons why some creatures are loners and some are best off in a swarm of millions. Our own societies, cultures, financial systems, and individual behaviors are certainly governed by these rules.
All these properties—from the assembly of cells into larger organisms to the emergence of new behaviors and capabilities—are rooted in complexity. But complexity itself has roots in a more fundamental property of the cosmos, something we call entropy—a measure of disorder. As our journey takes us to even smaller scales, we will discover that entropy and uncertainty are more and more important. They are part of the glue that makes you, your hands, and your cells possible.
8
THE UNDERGROWTH
10−6, 10−7, 10−8, 10−9, 10−10 meters
From 1 micrometer (micron) to 1 Angstrom (or 0.1 nanometer)
From the size of a bacterium (prokaryote) to the approximate diameter of a hydrogen atom
Early on in this grand journey through the scales of the universe, it took just five steps in order of magnitude to go from the scale of the observable universe to our local galactic collective. That was a major transition, from a size where the cosmos is, well, the cosmos—an encapsulation of all space, time, and physics as we know it—to the domain of a few scrappy galaxies, each with an individual tale to tell.
Brace yourself, because you’re about to go on a more mind-bending trip. Here, in another five orders of magnitude (from one micrometer to a tenth of a nanometer), is the shockingly brief passage from a reality we might recognize to the very strange places that exist not far beneath the range of our day-to-day senses.
Picture yourself taking a walk through a series of connecting rooms, right out of Alice’s Adventures in Wonderland. Each room is separated from the next by a narrow door that you open and pass through. Stepping through each door, your view of the world shrinks in scale by another ten times.
Open the first door. Here, filling the room ahead, coated in a sticky film of gunk, is a living bacterium. This single-cell entity is a concoction of humming biological machinery contained inside a translucent membrane, forming a structure much like a capsule. At one end of the capsule is a spinning and whipping tail, or flagellum. On the surface are countless tiny hair-like extensions called pili. Inside the capsule is a mess of genetic material: a lengthy DNA strand, small loops of double-strand DNA known as plasmids, and a crowd of smaller compounds and molecular structures.
This bacterium is a fully fledged biological entity. It’s a busy chemical factory, with compounds endlessly filtering in and out of its membrane covering. As alien as it appears, the bacterium’s actions also feel purposeful. It senses the world around it through chemical messengers and electrochemical variations, and responds to those senses.
A bacterium, with a flagellum (tail) and a peppering of viral invaders
Squeeze past this organism and move along to the door leading to the next shrinking room. Take a deep breath and pull it open. An even more alien and unsettling form fills up the space. Here is a single virus: a collection of molecular structures, wrapped up in a knobbly and pod-like casing. It’s a nub of tightly packed protein molecules arranged about a small loop of genetic material—the virus’s code. Is it alive? Perhaps not in the sense that you are familiar with, but it is certainly not inert: as you watch, it drills its way into the genetic substructure of some other helpless cell.
Something odd is also happening to your vision in this room. The virus is hard
to see distinctly. The light that you usually rely on to perceive the world is not interacting the way you expect with this tiny object. In fact, the wavelengths of visible light are larger peak-to-peak than the virus. These waves no longer reflect from or pass through this object in a consistent way. The virus doesn’t so much appear directly. Instead, it merely muddles the light. Squinting, you press on to the next scene.
You have to fumble and probe your way forward as you swing the third door open. Visible light is next to useless here. Instead, you feel the electrostatics of your own shrunken body interact with the thing lurking in this next room. It’s like wearing a blindfold, clumsily reaching for the way ahead, and discovering your hands on an undulating, bumpy, stringy something.
This is the room of giant molecules. The scary object is a ribosomal assemblage, a centrally important structure for the phenomenon of life—a station for synthesizing proteins. This intricate form moves and shape-shifts as you reach for it. The ribosome attaches to other molecular structures and works its way through a sequence of actions, as if part of an assembly line. Molecular arms and pivots gather and manipulate simpler molecules, such as amino acids, to join them into larger structures.
The process is graceful and precise, even as it is full of agitated movement and vibration. The ribosome takes mere minutes to accumulate and assemble vital pieces of cellular life’s components. You can feel new threads of proteins ticker-tape out from its maw, tumbling and folding as they do.
While it works away, you squeeze through, dodging past its electrically sticky protuberances, and continue on to the next drastically shrinking room. Through that fourth door, a busy molecular tumble is replaced by the quasi-regularity and symmetry of a sequence of thin, twisted DNA that has drifted past the ribosome.
A myovirus bacteriophage prepares to inject its genome into the cytoplasm of a bacterium.
At this scale what you actually sense is a space of possibilities, of ethereal electrostatic pushes and pulls. The closest comparison we can make to this experience is a blindfolded tasting of unknown foods and flavors. There is a menu of such sensations here, unique flourishes lined up end to end.
Here’s an entity we call a carbon atom. Here are ones called oxygen, nitrogen, hydrogen. They’re clumped together as other recognizable things, relatively simple molecules called nucleotides: adenine, thymine, guanine, cytosine, arrayed along a pair of sugar-phosphate rails that curve off into the distance in either direction.
But what any of these look like is no longer entirely meaningful. What is meaningful is the “state” of these entities, their electromagnetic energies, their vibrations and rotations, their still-intangible patterns of presence. Walking among them you are buffeted by a multitude of calls and entreaties in the form of attractions and repulsions, yet this seemingly disordered cacophony is shot through with regularity and information.
ACROSS THE THRESHOLD TO THE STRANGE
And now comes the fifth door, the next level down. Except this time, as you pass across the threshold, something very odd happens. You sense a change in yourself, a change that was hovering at the periphery of your awareness at the last doorway and is now almost all that you experience. It is as if you’ve suddenly ascended to a great altitude, exhaling and expanding.
The intricate, functional form of a ribosome
You are no longer quite the integrated thing that you were. You are spread out, simultaneously here, and there, and over there. And the structure in front of you, which happens to be a carbon atom, is similarly odd. You don’t feel this atom in any distinct way. Instead you mingle, sharing space and time. You mostly exist right before the atom, but you exist to the sides too. The atom reacts to your intrusion. Its electrostatic field is a dancing cloud of negative charge, yet the cloud has structure to it, areas of more or less charge. And lurking somewhere deep inside is a positive electrostatic charge that keeps a critical grip on everything.
Welcome to the quantum world. In truth, you haven’t been altered by passing through the doorway—you’ve simply been exposed to an underlying aspect of your nature that was there all along. Specifically, you’ve just been diffracted because of your shrunken state.
Normally, you exist at a scale where your body consists of a thousand trillion trillion atoms in an assembly ten billion times larger than a single atom. And you move around quickly, at meters per second. Any diffraction in passing from one scale to the next smaller power of ten has been beyond your perception, or that of any current measurement device. But now, at this tiny scale of a tenth of a billionth of a meter, you can’t waltz through a doorway between magnitudes and hope to be just fine. The particle-wave duality of matter—the deeper truth of reality—is in full force. On these scales the universe is a place of probabilities, of statistics, a dance of a multitude of branching pathways and curious relationships. That weirdness is at the heart of reality—it is what lets us exist.
THE CARBON COSMOS
Life, as we currently know it, is built around the element carbon. Why is this?
Carbon atoms just happen to be great at building all sorts of molecules. Carbon atoms are like your favorite pieces in a Lego set—the pieces that always let you complete your most ambitious projects.
Carbon atoms harbor six electrons. When these electrons are all nicely settled, four of them (called valence electrons) can easily be attracted to the nuclei of other atoms and share space with those atoms’ electrons. In quantum-speak, these valence electrons occupy zones of spatial probability around a carbon atom that let the atom associate with other atoms—or, in other words, form chemical bonds.
A strand of deoxyribonucleic acid (DNA)
For example, one of the carbon atom’s valence electrons can form a covalent bond with the valence electron of another atom (like hydrogen), where the two electrons are effectively shared between the atoms in a state of “quantum superposition.” (This is a piece of the quantum weirdness that we’ll encounter more and more as we descend in scale.) The new carbon-hydrogen molecule will have an overall lower energy state than the two separate atoms. Why does this matter? Because that lower energy increases the probability of the bond forming.
Of course there are plenty of other elements with four valence electrons. For example, the next heaviest elements with this electron complement are silicon and germanium. The difference is that carbon atoms are lighter and smaller, and the energy changes that go into making or breaking carbon bonds are more modest. The upshot is that carbon-based molecules are kinetically nimble, and simultaneously able to form, preserve, and break bonds at temperatures where other, inorganic chemistry happens. This is especially true in the presence of liquid water, which serves as an incredible solvent and enabler of chemical mobility. And to top it off, carbon atoms bond readily to other carbon atoms to make long polymers, or chains and branches, and other complex molecular structures.
Where carbon comes from in the universe is also a deep function of quantum physics. Like all the heavier elements, it is produced via nucleosynthesis (from nuclear fusion) in stars. But the high abundance of carbon (it comes in fourth in the count of atoms in today’s universe, after hydrogen, helium, and oxygen) relies on several key properties of the cosmos.
Most carbon forms through the triple-alpha process: the fusion of two helium nuclei into a beryllium-8 nucleus, followed by the fusion of the beryllium nucleus and another helium nucleus into carbon. This would be a horribly inefficient way to make carbon, except for some subtle coincidences. These coincidences are pretty technical, and may only be truly relished by nuclear physicists, but they’re worth knowing about because they can help you grasp the connections between fundamental physics and us.
The first coincidence is that in a star’s interior, the combined energy of a beryllium-8 nucleus and a helium nucleus can closely match that of an energized carbon-12 atomic nucleus. This “resonance” in energies is key; it greatly enhances the rate of the next fusion step—making carbon-12. The second coincidence is th
at the nuclei of beryllium-8 just happen to be stable for long enough for them to have a good chance of catching one of those helium nuclei as they buzz around. And finally, the new carbon-12 nucleus is not efficient about immediately fusing with any spare helium nuclei to make a heavier oxygen nucleus—the carbon doesn’t get gobbled up into oxygen, and lives to build your DNA a few billion years later.
The probability-density cloud of electrons bonding a carbon atom to four hydrogen atoms
Another way of looking at this is to say that our existence, and the existence of life as we know it, is acutely dependent on these esoteric bits of physics. Some thinkers take that as a sign of deep connections between life and cosmology—an “anthropic principle”: the universe had to be “just so” for us to be here to observe it. For others, it’s just how things happened to turn out—especially if the cosmos is just one part of a multiverse. You may have your own thoughts on whether or not the universe was compelled to produce life. In either case, it’s good fodder for late-night arguments.
These nuclear tales and puzzles, and the shrinking rooms of this stage of our journey, are just a taste of what’s coming next. Now that we’ve dipped a toe into the quantum universe, let’s dive deeper down into the roots of existence itself.
Inside the atom
9
THE EMPTINESS OF MATTER
10−11, 10−12, 10−13, 10−14, 10−15 meters
