Metazoa

The Taming of Charge

The taming of electric charge was a pivotal event in recent human history. In the nineteenth century, electricity went from being a mysterious, often dangerous force—encountered most directly in lightning—to an element in technologies that soon formed the modern world. If you are reading this book under electric light or on a computer, the act of reading is electrically sustained. This modernizing electrical advance was the second of two. Charge was also tamed billions of years before, during early stages of life’s evolution. In cells and organisms, electricity is the means by which much of what happens is done. It is the basis for brain activity—our brains are electrical systems—and also a great deal else.

What is electricity? Even many physicists find this question elusive. Electric charge is a basic feature of matter. Charge can be positive or negative. Objects with the same charge (positive and positive, for example) repel, and those with unlike charges (positive and negative) attract. The stuff of ordinary objects contains both. Any atom is a combination of even smaller particles, some that are positive (protons), others that are negative (electrons), and in most cases, other particles (neutrons) with no charge. Usually, an atom will contain the same number of electrons as it has protons, so the atom itself will have no net charge, as the positive and negative charges within it are exactly balanced.

The electrical tendency to attract and repel is strong. Here is the inimitable Richard Feynman, in his Lectures on Physics.

Matter is a mixture of positive protons and negative electrons which are attracting and repelling with this great force. So perfect is the balance, however, that when you stand near someone else you don’t feel any force at all. If there were even a little bit of unbalance you would know it. If you were standing at arm’s length from someone and each of you had one percent more electrons than protons, the repelling force would be incredible. How great? Enough to lift the Empire State Building? No! To lift Mount Everest? No! The repulsion would be enough to lift a “weight” equal to that of the entire earth!

In the mix of charged parts that comprises ordinary matter, electrons, the negative particles, are on the outside of atoms, while protons (along with neutrons) are on the inside. Electrons on the outside can sometimes be gained or lost, resulting in an ion. An ion is an atom (or sometimes a molecule that combines a few atoms) that has unbalanced its charged parts through such a loss or gain, and hence has an overall charge of its own. When many chemicals dissolve in water, they produce ions that then drift around. Salt water is water with dissolved ions. Any droplet of seawater will contain countless ions, interacting with each other and with the water molecules, attracting and repelling.

An electric current is a movement of charged particles, either positive or negative. In a metal wire, a current takes the form of a movement of electrons, with the rest of each atom making up the wire remaining in place. The electric currents used in technology (lights, motors, computers) mostly work in this way. But a current can also be a movement of whole ions. If some positive or negative ions in water, for example, can be induced to move in a consistent direction, that is an electric current. It does not make a current flow; it is one. Any container of salt water can contain such a current, if you can somehow get an overall pattern of movement of ions of the right kind to occur. In living systems, unlike human inventions, most currents take this form.

Charge is not life-like or mental in itself. It produces much of what happens in the inanimate world as well as the animate. But living activity runs on charge, especially by the corralling, pumping, herding, and unleashing of ions.

A cell’s membrane keeps many things either outside or inside, but it contains channels that selectively let some material through. Many of these are ion channels. Sometimes a channel will passively allow ions to move from one side to the other, perhaps under specific circumstances; in other cases, the cell pumps the ions across the membrane.

Ion channels are shared, with variations, across all kinds of cellular life, including bacteria. The reasons for bacteria to build elaborate ports and passageways for ions are often not entirely clear. Channels may have arisen initially just to enable cells to adjust their overall charge in relation to the outside—tuning as well as taming their charge. Whenever there is traffic across a living system’s boundaries, though, it tends to take on further roles. A flow of ions can function as a minimal form of sensing, for example: suppose contact with a particular external chemical opens a channel and lets in ions. Those charged particles can set new events in the cell in motion.

The next consequence of these ion flows is related to that to-and-fro traffic, but is a larger, more wholesale change to the cell. This next step is excitability. Channels control the flow of charged particles, and these channels can themselves be controlled: they can be opened or closed. This can happen through chemistry, or physical impact, but it can also involve charge itself. Voltage-gated ion channels are channels that open as a response to electrical events that they, the channels, are exposed to. This makes possible a chain reaction; a flow of current creates a greater flow of current, one that spreads over the cell membrane.

This might not seem much of a step, and it has less of an obvious ring of usefulness than the arrangement I described above, where the flow of ions is sensitive to chemicals the cell encounters in its travels. But voltage-gated ion channels are the basis for another innovation, the action potential. This is a moving chain reaction of changes to the membrane of a cell, especially in our brains. Positive ions flow into the cell at one point, affecting ion channels at adjacent points, which open and allow more ions to come in, and so on. A wave of electrical disruption travels along the membrane like a pulse. An action potential is the zap-like event that is described as a brain cell “firing.” That zap happens by means of voltage-gated ion channels.

In a voltage-gated ion channel, a controller of current is affected by charges it is exposed to; the flow of current is electrically controlled. This is the principle of a transistor. At the start of this section I mentioned the nineteenth-century advances that brought electricity into the realm of human technology. Another such advance occurred in the twentieth century, with the invention of the transistor. The silicon chips in computers and smartphones are collections of tiny electrical switches of this kind. The transistor was invented around 1947 at Bell Laboratories in the United States—or invented once then, anyway. The first Bell Lab transistor was an inch or so in size, and it has been continually refined and shrunk since then. The same device was invented billions of years ago in the evolution of bacteria.

If bacteria invented transistors, what were they doing with them? Why did they need to control electricity with electricity? As far as I can tell, no answer to this question is widely agreed on. Bacteria might have been using them as part of the electrochemical upkeep of the cell. They might have been used in the control of swimming. Channels that sense external chemicals may be incidentally sensitive to charge, and bacteria that form colonies in “biofilms” signal from cell to cell using ions. But bacteria don’t have action potentials—the zap-like chain reactions in our brains—and the situation does seem quite odd to me. Several billion years ago, nature invented the fundamental hardware device in computer technology—a complicated and costly device, too—and did so in bacteria, but bacteria do not seem to have been doing much computing with it.

Regardless of why it arose, the voltage-gated ion channel was a landmark in the taming of charge. These channels do not have a single obvious use, I said above. In a sense, neither does a transistor, and in both cases, that is part of their importance. A transistor is a general means for control, a device for making events here affect events there in a reliable, rapid way. The events controlled can be multifarious—whatever might be useful. When they enable action potentials, voltage-gated ion channels also make it possible for a cell’s activity to have a “digital” quality; a neuron either fires or not, yes or no. Not all animals have neurons with these zap-like firings, and nervous systems can work with milder kinds of excitability, but this digital feature is certainly a useful one. It is remarkable that this control device was invented so far back, when most of the uses it has now were not even glints in evolution’s eye.

In the days of ubiquitous computers and AI, it is natural, almost inevitable, to ask about the relationships between living systems and these artifacts. Do organisms and computers do essentially the same thing with different materials? Similarities between the two do arise, often unexpectedly, but it’s also important to recognize dissimilarities. One difference is that much of what a cell does, its main business, is something a computer never has to do. A great deal of the activity in a cell is concerned with maintaining itself, keeping energy coming in, keeping a pattern of activity going despite decay and turnover in materials. Within living systems, the activities that look like the things computers also do—electrical switching and “information processing”—are always embedded within a sea, a mini-ecology, of other chemical processes. In cells, everything that happens takes place in a liquid medium, subject to the vicissitudes of the molecular storm and all the chemical digressions that living systems engage in. When we build a computer, we build something whose operation is more regular and uniform; we build something that will be distracted as little as possible by the undirected ruminations of its chemistry.

This relates to a broader point. Often in these early chapters, I’m trying to describe a tangle of parts and processes inside cells and simple organisms. A natural word to use at many stages is “machinery”—we’re looking at the machinery of sensing, the machinery of excitability. I type the word “machinery” and am never sure whether to delete it. In a broad sense of the term, yes, voltage-dependent ion channels are bits of machinery, and so are nerves and brains. To deny this is to suggest a move toward dualistic (soul + body) or vitalistic (“life force”) views. So, I say to myself, don’t delete the word. But the contrasts between machines and living systems are also important. In cells, the processes of life involve imparting order upon a molecular storm and the imperfect herding of ions. This is nothing like what goes on in any machine we’ve built. We generally build machines to be predictable and restricted in their activities, even if we might then use them to simulate more chaotic goings-on. To describe the intricate materials in cells as “machinery” is right in some ways and wrong in others.

There is one more thing I want to emphasize in this inventory of features of life that were in place before animals. This one has been touched on a few times above, but I now want to put it at center stage for a moment. That feature is traffic, a to-and-fro between living systems and their surroundings. This traffic includes the flow of ions described above, also the taking in of raw materials and elimination of waste. Cells are bounded, but they are not closed to the world. I am emphasizing here the windowed character of cellular life.

This traffic has a metabolic side—a side that involves getting energy and using it to stay alive—and also an informational side. Some incoming influences are important in their own right (as food, for example), while others are important for what they predict and portend, for what they indicate about something else. The metabolic side of this to-and-fro is unavoidable if life is to continue. Living activity itself is a pattern that exists embedded in an energetic flow, one that begins and ends outside of the organism. My colleague Maureen O’Malley expressed this well; combining some chemical jargon with an image from a different source, she said that being alive requires learning how to exist “on a redox rollercoaster, perpetually giving and receiving.” (A redox reaction is one involving a transfer of electrons between two molecules.) A consequence of this, and part of what O’Malley wanted to emphasize, is that living systems are inherently sensitive to changes and events outside. They don’t have the option of being windowless, but are open to the world out of energetic necessity. Once open to their world in this way, they will be affected by what goes on. And once they are affected by those goings-on, evolution will tend to put this sensitivity to use—the organism will often find a way to react to events in a manner that furthers its projects, simple as these might be. All known cellular life, including tiny bacteria, has some ability to sense the world and respond to it. Sensing, in at least the most basic forms, is ancient and everywhere.

Metazoa

Those ideas complete one of the two themes of this chapter. Living cells are physical objects, but unlike any other object we are familiar with. They build membranes to contain and shape storms of activity. They are bounded, but forever dependent on traffic across those boundaries. Self-defining, self-maintaining, cells are selves. The next step in the story takes us to a new kind of unit, a new kind of self: animals.

When we think of animals, we usually first think of animals like us—other mammals, dogs and cats, perhaps birds. But animals extend much further than this. Animals—the Metazoa—form one large branch of the total tree of life, the genealogical network that links all life on Earth. The term “Metazoa” was introduced in the late nineteenth century by Ernst Haeckel, the German biologist of Chapter 1. He contrasted Metazoa, multi-celled animals, with Protozoa, single-celled animals (with “zoa” as in “zoo” and “zoology”). The Greek prefix “meta” originally had meanings like after and beside, then took on the connotation of higher, and now often means about—looking down on. Haeckel probably had in mind some mix of higher and later. But protozoa are no longer considered animals at all, so the “zoa” part of their name becomes misleading. The animals are now just the Metazoa.

Animals are made up of many cells living as a unit; beyond that, they live in a huge variety of ways. They include corals as well as giraffes, wasps that are smaller than some single cells as well as whales at fifty tons. Some look almost entirely like plants. In biology now, the word “animal” refers to any organism found on a particular branch of the genealogical tree, regardless of how it lives or what it looks like. A coral is as much an animal as a wolf is. This is not the only meaningful way the term “animal” might be used, but it is unambiguous and clear, unlike various other uses.

Animals do not form a scale from “lower” to “higher,” though the habit of talking about them in this way seems hard to break. On the genealogical tree, some animals are lower in the sense of earlier, but insects that are alive now are not lower than us; everything alive now is at the top of the tree. So there’s no sense in talk of an evolutionary “scale” or “ladder”; animal life has a different shape from that. Some animals are more complicated than others, in various ways (more parts, wider range of behaviors, more complicated life cycle …), but biology has no room for an overall scale from lower to higher, of the kind that seemed natural before Darwin.

The genealogical network that animals are part of—the “tree of life”—is not always tree-shaped; in many places it is more tangled than that. For simplicity, I will keep referring to it as a tree. This tree links all known life on Earth by relations of ancestry and descent. It is old now, but continues to grow. This growth occurs through evolutionary processes operating over huge spans of time. Populations or species occasionally split into two. Each side then evolves on its own path and acquires its own peculiarities. Extinction is always likely, but any segment—a new species—that doesn’t go extinct may later split again. From a single initial fork, we then have several branches, each with a collection of species rather than just one.

Many years ago, when the tree was younger and smaller, an outgrowth protruded: a new twig. The twig survived, branched repeatedly, and became particularly far-flung and diverse. The organisms on that part of the genealogical tree are the animals. Evolution is open-ended, and there is no telling where future branches might extend, both within and outside the animal part of the tree. But though animals have lived in a great variety of ways, there is a general style of living seen in animals, a way of life invented on the animal branch of the tree.

Animals arose from a particular kind of single-celled organism, larger and more internally complex than bacteria. These cells, eukaryotes, have special devices for handling energy—mitochondria—and an elaborate internal skeleton (the cytoskeleton). This is an internal network of filaments and tubes that can move in relation to each other, enabling the cell to control its shape and motion.

Well before animals arose, the cytoskeleton had initiated a new regime of mobility in single-celled organisms, including active hunting. This apparatus made possible a shift from an existence based primarily on chemical processing, as seen in bacteria, to one based partially on behavior: motion and manipulation. All of these sound like animal characteristics, but we are still talking about single-celled organisms—protists. Some of them have grown large. Members of the genus Chaos, for example, hunt not only bacteria but, in some cases, small invertebrate animals.

Plants are another branch of the genealogical tree, another long-term multicellular experiment, and they too are collections of eukaryotic cells. So are fungi. A recurring theme in evolution is the formation of new and larger units by the collaboration of smaller ones. The eukaryotic cell itself came to exist in this way, through the engulfing of one simpler cell by another. The engulfed cell gave rise to the mitochondria that eukaryotes use as powerhouses.

In the events that gave rise, separately, to animals and plants, another kind of coming-together occurred, this one not an engulfing but a juxtaposition. Suppose a single cell divides, and rather than going their own way, the two daughter cells instead stay stuck together, as the result of a mutation that affects their chemistry. When those cells divide, their daughters will stick together as well. The initial result is just a bigger living object. This object cannot act as a whole, and has no obvious way to reproduce, as opposed to getting bigger. But this is a step toward a new kind of life.

Multi-celled beings of this kind have evolved from one-celled forms repeatedly. On the animal line, this might have happened about 800 million years ago (with a good 100 million years of uncertainty). There are no fossils of the earliest forms, but we can picture the first stages: a ball of cells in the sea, formed by a succession of cells refusing to separate from their sisters.

Where to from there? One tradition of speculation imagines a cup, or a hollow sphere with an opening, as a likely next stage. The ball of cells folds in on itself and becomes hollow. This possibility was also first sketched by Ernst Haeckel.

One reason the cup hypothesis is tempting is that this form is seen in the early stages of individual development—development from egg to adult—in a wide range of animals. The hollow form is a gastrula. It is a mistake to think that something seen early in individual development must also have been there early in evolution (as Haeckel did suppose), but the cup form seems so ancient and widespread that it might be a clue. Haeckel christened this hypothetical animal the “Gastraea.”

The Bathybius affair of Chapter 1 was not Haeckel’s finest hour; the Gastraea was better. It is still a live possibility for a very early animal form. The open sphere might have been the beginnings of a gut; the first animal might have come to exist by forming around its stomach. In that enclosed environment, it could trap food and release digestive enzymes, without having them drift away.

A human gut holds our food. In addition, our guts contain countless living bacteria, from which we benefit greatly as long as things stay in balance. This kind of collaboration is extremely common in animals. It may have also been part of the early stages in animal evolution. That idea was not part of Haeckel’s original version of the view, nor part of most thinking since then. It is a newer idea, informed by the realization that normal animal bodies are homes for large colonies of bacteria that help them process food as well as playing other roles. The recognition of ubiquitous tight associations between our bodies and accompanying microbes has been a significant shift in how biologists think of animal life, and perhaps these associations go very far back. Remember also those engulfing events in the history of cells, the events that produced mitochondria, and also chloroplasts within plants. In those meetings, a metabolic ally was brought inside a cell—or first brought in and then tamed. Here, in comparison, we build a home for collaborating microorganisms without having them enter our cell bodies; instead we build a pen for them. A diverse digestive ecology might be at the beginning of animal life.

This open-sphere idea, with or without collaborating microbes inside, is like a second iteration of the evolution of cells. In the first case, we had the formation of a boundary, with channels across it, forming a unit that controls chemical reactions. Here, we have many cells and they form a hollow sphere, another object with an inside and an outside. Individual cells are each now pieces of the sphere, and they control traffic in and out of this larger unit.

From there—or somewhere—early animal bodies gained more shape. For those trying to work out the next steps, the fossil record is still frustratingly silent as I write these words. But we do have clues in some present-day animals. These clues are easy to misread; the present-day animals are not preserved ancestors, but distant cousins. They have been through as many years of evolution as we have. But some of them might have stayed in a form that resembles old forms, in some ways, or at least indicates something about them.

The animals that contain these clues are a trio: sponges, comb jellies, and placozoans. They are quite different. A sponge, once it has settled, does not move as an adult. It lives fixed in place like a plant. Some sponges also grow very large. Placozoans, in contrast, are tiny, flat, crawling creatures with little definite shape. You need a microscope to see one clearly. Both sponges and placozoans have no nervous system. Comb jellies, as the name suggests, resemble jellyfish, but may be a considerable evolutionary distance from them. They do have nervous systems, and they swim using cilia, tiny hairs along the side of the body that beat in rhythm. So of these clues, one is a piece of motionless undersea furniture, one crawls nervelessly and microscopically, and the third is transparent and swims.

Why are these, among animals, the clues about early forms? First, they are simple animals in various ways. They have few parts and not many kinds of cells. Second, they are genetically very far from us. On the genealogical tree, they are on lines that branched off from our line very early.