As with ladybugs and other organisms at the large scale, physical principles are manifested within life, but through mutations, evolution also offers possibilities that allow the organism to reach reproductive age. Physical principles are as much a means to expand the repertoire of adaptations that enhance the chances for survival as they are an ineluctable condition of life’s existence. These laws open up new space for invention. The cell wall opened up the vista of different shapes, each shape exploiting different principles that enhanced the chances of reproduction.
In this medley of microbes, do we see any chance events at work? If we reran evolution, we cannot say for certain whether all the shapes we see on the Earth would be explored. The giant Epulopiscium fishelsoni depends on being in an animal gut, and animals were not around until geologically recently. Like our ladybugs and moles, contingency allows for a range of details and modifications to be explored—a range that results in a lavish diversity in living things at the small scale. However, the range of forms is not endless, and those that exist seem to be channeled by some basic physical laws, which puts us in a position to make predictions. We can predict that on an alien world, cells would be small, and when starved, growing as a rod or filament might be one common way to circumvent the problem. If they are large, they will at least need to actively pump food in or make folds to maximize their surface area.
Where we might find contingency is in the structure of the membranes themselves. Since life first emerged, it has proliferated enormously and these munificent forms have explored numerous types of membranes.
A simple membrane surrounded by a cell wall is the choice of the Gram-positive bacteria, so named after Christian Gram, a Danish bacteriologist who first figured out a method of staining different bacteria according to differences in their membranes. The process makes bacteria easier to see in a microscope. Among the ranks of Gram-positive bacteria are the staphylococci, bacteria species of the genus Staphylococcus, a group of bacteria that can sometimes cause skin infections or food poisoning. They contrast sharply with the gram-negative bacteria, whose membranes are more complicated. Gram-negative bacteria have two membranes, with the cell wall between the two.
These two-membrane bacteria are everywhere and include the eponymous Salmonella. For many years, scientists have been intrigued by their strange double membrane—how did it get there? A possibility is that back in the days when bacteria dominated the Earth and were experimenting with their new planet, one bacterium got engulfed by another. By gobbling another bacterium without killing it, the host gained the benefit of the food that the bacterium might produce, maybe sugars or other products from its metabolism. The gobbled-up bacterium had the benefit of a safe home and maybe some other nutrients from its host. Think of a bacterium being engulfed. As the membrane from the host encircles it, it is now surrounded by two membranes: its own and the membrane of the hungry interloper. Once the prey is engulfed, we now have a double-membraned bacterium.
Neat though this story is, other people disagree and suggest that instead, the double membrane was a clever defense mechanism against antibiotics. These chemical products of microbial warfare are produced by many bacteria in the natural environment, and it is thought that they are used to fight for resources by killing or immobilizing their competitors. Because Gram-positive bacteria, with their less stringent single membrane, are generally more susceptible to antibiotics, the notion that the double membrane provided a more rigorous way of keeping antibiotics at bay seems to make sense.
In the membrane lipids themselves, the microbial world exhibits dazzling variety. In the archaea, a branch of the microbes that inhabit extreme environments, soils, and the oceans, the membrane lipids are chemically different from the bacterial membrane. In some archaea, the lipids are linked in the middle of the membrane, a position that is thought to give the membrane more robustness and integrity against high temperatures.
We could take long and discursive diversions to look at all the proteins found in cell membranes, the various sugars attached to them, and membranes’ functions in grappling to surfaces and communicating with other cells. Not least, the membranes themselves are often covered in slime and meshes of sugar strings that provide yet more protection from the outside world. Sometimes, membranes spare bacteria from the fate of desiccation by trapping water, which is important for microbes that occupy rocks in hot deserts. Within and around a simple membrane, there is an efflorescence of biochemical ingenuity and diversity.
Looking on this wondrous profusion, we might reasonably think that once the chassis of a membrane is built—a wall that will enclose cell components, concentrate them, and provide a channel of communication to the outside world—then it is likely that chance can play a much greater role in all the membrane’s details and accoutrements. Double membranes, links between lipids and slime layers—all these things and more can now blossom and experiment around that central cellular structure. Some of these developments might well be the result of random routes taken by evolution and less thoroughly channeled by physical requirements.
Even the cell wall may be a product of chance evolutionary changes. Does it have to be made with certain amino acids and sugars? We do not know if its composition is a mere random event, established and carried on by evolution. The wall might be made of a selection of chemicals that simply happened to provide a reasonable barrier and a rigid casing. Once the job was done, it got fixed into life. Perhaps variations on its theme are also possible.
If we were told about a distant alien world with life, would we be confident to predict double-membrane life forms or the exact structure of cell walls? I suspect that eventually, once we know more about the biochemical origins of many of these adaptations, some may seem inevitable, likely innovations. Others may be contingent quirks of evolution with adaptive value in particular environments—quirks that, once discovered, did the job required of them. Rerun evolution on another world, and many appurtenances and jangling ornaments of cell membranes may be different in biochemical detail, but the cell membrane as a fundamental structure of life would be found in these other places too.
These many microbes dominated the planet for several billion years. Yet during this time, they did not cover the Earth with many isolated cells going about their individual business. One cell’s waste could be another’s food. Under the innumerable forces of evolutionary selection, from food scarcity to avoiding predation, these cells cooperated, forming multicellular aggregates. Nowhere is this spectacular division of labor better exemplified than in microbial mats, structures made up of layers of different microbes. These structures, thick gloopy mats in browns, oranges, and greens, often grow around the edges of volcanic pools, hanging on at the edges of boiling cauldrons. You may have seen them in the less exuberant form of green films growing on the sides of old buildings.
On the top of the films are usually the green photosynthetic microbes, trapping sunlight and using its energy to convert carbon dioxide into sugars. These tasty organic compounds find their way into the layers beneath, where other microbes, denied the light and warmth of the sun, carry out chemical transformations of the sugars in their dark underworld. In this way, waste and food are caught up in a cycle of give and take as each microbe has its place in this society of the small.
Cooperation, forced on the multitudes by the rigors of life on and within a planet, creates these cities of microbes. It is not uncommon for people to distinguish between microbes and the macroscopic world of animals and plants. These latter forms comprise “multicellular” structures. Yet microbes rarely live on their own and instead are often found cohabiting with other cells. Like the case with ants and birds, put microbes together, and complex, self-organized behavior emerges, patterns and movements that produce order that is more than the parts. These patterns and order arise particularly among microbes that can move and swarm across a surface in a coordinated way like a pack of microscopic wolves in search of food. Equations can be used to predict the coordinat
ed behavior of the smallest cells. Here, in the cooperation of cells, we find mundane physical principles at play.
This cooperation is not mere chance, a fixed accident, but is an inevitable consequence of many cells inhabiting a planet. As they lock into a diversity of different nutrients and energy sources, it becomes a natural progression that the waste of one microbe might be used by another. An association is born. The sugars produced by a photosynthetic microbe will make it advantageous for a sugar-eating microbe to grow in close union with it. The biomass of this hungry little cell will become higher than it would become alone, in a sugar-poor pond. We need no superintendent to corral microbes together, to engineer their various collaborations and interactions. Place all this variety on a world, and cooperation emerges from the advantages this provides for each microbe. This self-organization can be modeled, predicted, and probed using equations. We would expect cooperation and aggregates—microbial mats, even—on any planet on which biological evolution has taken hold.
The key innovations of cells and their interactions leading to a world of cooperating microbes seem inevitable. So too their core metabolic capabilities. Those microbes at their cellular level show diversity in their membranes and their molecular clothing, but the cell as a discrete entity and the biochemistry that turned it into a self-replicating, metabolizing form is likely universal, forced by the laws of physics.
Before we close our tour of the principles underlying the cellularity of life, let us call into view one final question. This question is more complex, uncertain, and controversial but nevertheless explores an attribute of life that seems increasingly narrowed by physical principles. That question is how microbial life made the monumental transition to become those complex multicellular aggregates we call animals and plants. Was it inevitable and driven by the laws of physics?
Animals and plants are made of cells that are quite different from the cells of most microbes. The cells of the larger organisms are often lumped together as eukaryotic cells, their distinguishing feature being that their DNA is collected into a nucleus, a tiny subcompartment, or organelle, within the cell. They contrast to most of the cells we have been talking about so far, the prokaryotes, which lack a nucleus.
The domain of the eukaryotes, which, apart from animals and plants, include some single-celled living things such as algae, represents something of a revolution in the life of the cell. Eukaryotic cells tend to be much larger than the prokaryotes. Apart from their nucleus, the most noticeable difference between the eukaryotes and most prokaryotic cells is that eukaryotes have other organelles. Among them are the mitochondria, the power plants in most eukaryotic cells, including yours. Mitochondria make energy by burning organic compounds, such as the molecules from your lunchtime sandwich, in oxygen.
The eukaryotic cell is the child of a strange liaison in the early history of life. Engulfed perhaps by a feeding bacterium, the enclosed microbe would become the mitochondrion. This endosymbiosis was a curious internal agreement between the newly engulfed bacterium and its host, each giving something to the other. The engulfed bacterium gained the advantage of food and a homely environment, and the host got access to more efficient energy respiration using oxygen (aerobic respiration) provided by its new tenant. Many hundreds or thousands of mitochondria could generate energy in one cell, like a collection of power plants in a city, producing an energy revolution. This alliance released life from the restrictions of the energy-limited prokaryotes. Coupled with this, the growing genetic size and complexity of eukaryotic cells allowed for more-elaborate biochemical networks on which natural selection could act.
Thus, to make an animal, a concatenation of three extraordinary events had to occur. First, oxygen levels had to rise in the atmosphere to produce the gas that would unleash the energy-yielding capacities of aerobic respiration, the form of energy production used by you and me. Second, endosymbiosis had to happen. When a bacterium was engulfed, mitochondrial power stations could be made. They would multiply the capacity to make energy inside a single large cell, a feat needed to tap into the new energy reserves. Finally, those cells must have collected together into a single machine, to commit irreversibly to differentiate into various organs and at once to produce something that operates as one.
It seems that a baffling set of contingencies, unlikely ones, was instrumental in the rise of a biosphere that reached beyond the kith and kin of prokaryotes. But buried inside these events, could there have been a thread of inevitability?
The physical causes are easy to grasp. The rise in oxygen was necessary to release the power of aerobic respiration, which produces many times more energy than do other ways of making energy in oxygen-free environments. The new animals that tapped into oxygen produced more energy and more power. The rise of oxygen itself was the waste product of photosynthesis. Microbes that could grab the energy of the sun and use water as the source of electrons in this process of photosynthesis would instantly have within their reach every watery habitat on Earth with some sunlight. As a consequence, they would produce oxygen. The physical imperative and advantages to be gained by reaching into those possibilities were there. We might plausibly think that the rise of oxygen was an inevitable consequence of the evolutionary process exploring the different thermodynamically favorable energy-yielding reactions on offer on a planet.
Powering up cells by filling them with mitochondria was just physics, gathering up more power stations to produce more energy per unit volume, allowing for more energy to build more cells and more-complex structures. Any cell that evolved to be a home to more mitochondria would have had more energy available to grow and divide. This change might also appear inevitable. Endosymbiosis has happened many times during the history of life on Earth.
And what about the alliance between cells that grow to be different, an alliance that allows each cell to specialize and produce both efficiency and dedicated complexity in the tasks undertaken? This is itself easily understood when the efficiency and exquisite division of labor is translated into more successful competition by the whole organism to survive in the environment. Slime mold, an inauspicious-sounding fungus, lives a sedate life until it needs food. Then the cells gather and march in unison across the landscape, an often vivid yellow network of moving, veinlike tentacles writhing over a forest floor in search of sustenance. Over nine hundred species of these molds in far-flung corners of the eukaryotic empire show that multicellular behavior, combining to improve success, is by no means rare in the biosphere. We do not know the exact events that led cells to take up irreversibly dedicated roles for themselves, relinquishing their chance for unicellular autonomy, but even today we can see the pressure for combination and collaboration across the biosphere. On any planet where cells compete for resources and habitat, there is a good chance that multicellular behavior will eventually lead to the emergence of discrete multicellular organisms. Physical principles provide the impetus; cellular structures and their attendant genetic pathways provide the means.
Put simply, the rise of multicellularity, that is, the emergence of a complex biosphere of animals and plants, is based on uncomplicated physical principles. We can see why cells cooperated and harnessed more energy. This phenomenon would have been driven forward by competition. Large animals would have made better predators. Their prey would have evolved to be larger and thus less vulnerable. A biological arms race demanded yet more effective, and sometimes larger, machines. Once animals emerged, the vast diversity in the evolutionary experiment with multicellular creatures was assured.
There is an important distinction to be made between the claim that physical principles tightly constrain forms of life into predictable structures at all levels of its hierarchy—one that I have primarily explored so far—and the claim that the major transitions of evolution are inevitable consequences of physical principles. The second claim is one yet to be tested. Nonetheless, we can discern the reasons why life was transformed from single-celled microbes to multicellular, complex life a
nd the physical advantages and potentialities that lay open to, and may have encouraged, life to take that route. For now, however, reducing to a list of equations all the processes that we have just been discussing would be a formidable task. To write down equations that would clearly describe the transition of a microbe to an animal is a far more ambitious task than, for example, collapsing the temperature of a ladybug into a single equation.
Considering the inevitability of these momentous transitions, given enough time for life to evolve on any other planet, it may not be wild speculation to think that life would embark on the same journey, from early molecule-encapsulating lipids to lumbering leviathans. However far these forms of life get, riven throughout them will be the physical principles that lie within the cell.
CHAPTER 6
THE EDGE OF LIFE
A VISITOR STANDS ON the pier in the breezy and delightful Victorian seaside town of Whitby, watching the seagulls on the beach pecking at bits of bread or the odd chip dropped by a summer tourist. The view couldn’t be further removed from the clanking shudder of the dark cage hurtling its way down into the depths of the Earth just a few miles away.
Jump in a car, and drive north from the town, whose imposing ruined Gothic abbey was Bram Stoker’s inspiration for Dracula, and there on the left-hand side, a perplexing sight for the unfamiliar, is Boulby mine, a collection of gray, dusty buildings surrounded by brown and white piles of salt, between them the busy roadways of a working mine. Like a cathedral to the working man and woman, nestled in the eclectic mixture of processing plants and hangars are two giant gray cylindrical towers rising into the sky, the shafts that connect the surface world with the underground labyrinth.
The Equations of Life Page 12