A favored trope among science-fiction writers is to imagine any number of extraordinary life forms inhabiting other planets and to contend, therefore, that we are limited in our imaginations and, consequently, that we cannot make sensible predictions.
As early as 1894, in a Saturday Review article about alien life, science-fiction writer H. G. Wells reflected on earlier suggestions that silicates (the silicon-containing materials that make up rocks and minerals) might do interesting chemistry at high temperatures: “One is startled towards fantastic imaginings by such a suggestion: visions of silicon-aluminium organisms—why not silicon-aluminium men at once?—wandering through an atmosphere of gaseous sulphur, let us say, by the shores of a sea of liquid iron some thousand degrees or so above the temperature of a blast furnace.”
He is not alone. In 1986, Roy Gallant wrote Atlas of Our Universe, a well-known exposition of the possibility of the limitless potentialities of life, for the National Geographic Society. The book contains a wonderful plenitude of imagined life forms in our Solar System. The Oucher-pouchers are large bags of gas that prance around on the surface of Venus and cry “ouch” every time they hit the surface, baking at 460°C. Their counterparts on Mars are the Water-Seekers, long, slender creatures like extended ostriches that sport vast furry ears with which they can enclose themselves in the cold Martian nights and winters. A giant carapace over their heads protects them from ultraviolet (UV) radiation, and with their long proboscises, they dig deep into the Martian subsurface to find water. The imagination reaches far beyond these two worlds. The Plutonian Zistles are intelligent ice cubes on Pluto (the National Aeronautics and Space Administration [NASA] New Horizon mission, perhaps glinting briefly overhead, presumably changed their culture for good), and the Stovebellies of Saturn’s moon Titan combust material inside themselves to maintain warmth at a chilly −183°C. They propel themselves through Titan’s hydrocarbon-rich atmosphere by the unedifying means of emitting bursts of gases from their rear ends.
None of Gallant’s creatures have ever been observed, and that is an interesting fact. Assuming (and this is a big assumption) that life would emerge on other planets if the conditions were right, these novel biochemistries and creatures are, not insignificantly, absent in our Solar System—life forms that would have merely adapted to the different conditions found on those worlds. On most of these worlds, the conditions are so extreme that, according to our knowledge of the limits to life on Earth, we would predict that none of these planets and moons could today have complex multicellular life on their surfaces. That is what we observe. What we see on Venus, for instance, fits our predictions based on our knowledge of the boundaries of growth of terrestrial life—boundaries established by physical laws.
We do not yet have another example of life with which to test whether our biosphere is exceptional. Consequently, many observers might say that the question of whether life on Earth represents a universal norm can be nothing more than speculation, unbounded conjecture of the type that makes interesting conversation at the coffee table, but little more. However, this observation is inaccurate. The principles provided to us by physicists reveal the foundations of what is possible. Observations of the universe from astrophysicists can tell us about the preponderance of elements such as carbon and of molecules such as water; this information can yield insights into how common the chemical building blocks of life may be throughout the cosmos. From chemistry laboratories, our extensive knowledge about the reactive potential of elements in the periodic table and their ability to form complex structures can tell us about how universal the chemistry of life might be.
The biophysicists have much to tell us about molecules that have evolved independently in many organisms on Earth and allow us to question how universal the rules for doing chemical reactions in cells might be. The microbiologists’ knowledge of life in extremes informs us about the physical boundaries of life and whether these are likely to be universal. From the paleontologists, we are given a vista across the life forms of the past. How similar or different are they from those alive today, and what might explain these observations? Planetary scientists collecting information about other worlds tell us whether, with their cameras and other instruments, they find conditions potentially supportive of life. We can compare the biological status of these worlds with our expectations.
From these disciplines, we can gather an abundance of information to build a hypothesis about the nature of life. In this book, in investigating the link between physics and life, I also explore the idea that life is universal at all levels of its hierarchy. By this, I do not necessarily mean identical. Ladybugs may not be the same on other worlds as on Earth, but the solutions used by living systems to proliferate on the surface of a planet might be broadly similar, from the way they use a subatomic particle, the electron, to gather energy right through to the behavior of their populations. We will know life if we eventually find it, and it will be recognizable as very akin to life on Earth.
It is apposite to give Charles Darwin the last word of the first chapter. In the finale of his Origin of Species, he summed up his feelings by declaring that he could see a certain grandeur in the evolutionary view of life. We might also remark that there is a beautiful simplicity. As physical laws, unyielding and unswerving, work their way through every form of life, extinct and extant, there is a breathtaking similarity in the products of evolution, a resemblance molded by the very laws that have shaped our universe for over thirteen billion years.
CHAPTER 2
ORGANIZING THE MULTITUDES
WHEN I WAS A boy of about eight, I was a typical daydreaming child. I would sit on the Victorian stonework, leaning against the black iron railings, and with my small magnifying glass, I would focus the rays of the sun on an oblivious ant going about its tasks. With my death ray, I would chase the little creature across the irregular pitted surface until I caught it in the glare and it spat and fizzled.
Ant chasing was an extracurricular activity at a typical English boarding school, and it was preferable to some of the others, including learning more Latin. I dare say it remains a macabre pastime for inquisitive, slightly destructive children to this day. In my juvenile unpleasantness to these innocent insects, I was party to their miniature world. I saw on many occasions the long, regular lines of ants tramping back and forth across the stones, some moving slowly, others fast, some with pieces of food, and a few with the carcasses of their fallen comrades. Every now and then, two of the scurrying forms would clamp heads as if exchanging instructions and then part, running off with hurried intent in opposite directions. What were they saying? The social activities of these ants fascinated me, and more often than not, I would prefer to just sit diligently and watch them.
But I saw something else. In my preadolescent activity, I witnessed the delicate nature of life. By merely taking the natural light from the sun and magnifying it just a few times, I could transform a living, intricate machine of organic matter into a blazing inferno. Life really was tenuous, poised at the boundary of physical extremes that, with a mere change in their magnitude, could define the difference between life and death. These creatures, like all of us, lived in a world at the mercy of hard physical limits.
Nevertheless, within these constraints, the ants went about their business. Watching them coalesce, exchange information, and organize could convince anyone that what was at work here was nothing short of social organization. A vast society of insects, merely on a scale smaller than us, worked toward their goal of constructing their nest and ensuring that they had enough food to perpetuate their colony. For many years, this top-down society was how it seemed to scientists. The queen ant, safely ensconced in a chamber within the nest, was further proof that this incredible collective effort was under the control of a monarch, a figurehead that directed and controlled the many instructions that must be needed to coordinate hundreds, thousands, and sometimes millions of ants toward a single, unambiguous task.
It is easy to see
how this phenomenon led many to question how such a tiny thing, the queen ant, even at her often-bloated size, could possibly contain, let alone process the astonishing amount of information needed to operate the ant society. Ant civilization attracted the attentions of many biologists and animal behaviorists, such as American scientist E. O. Wilson, whose work from the 1970s on insect societies helped found the field of sociobiology.
A fascination with insect societies and the draw to understand what managed their multitudes caused a new group of scientists to become engaged with ant organization. Physicists, who are wont to avoid the dizzying complexities of things so unconstrained as ants, took an interest in the creatures. A collaboration emerged between biologists and physicists. They asked some different questions: Are these societies really so complex? Are they fashioned by flows of information and instructions beyond the realms of our computers? Are they under the whims and dictates of their queen, a leader we may never fully fathom?
What they found was remarkable.
Ant nests are complicated structures whose extent and detail can reach colossal proportions. A metropolis of nests containing an estimated three hundred million worker ants and a million queens was found on the coast of Hokkaidō, Japan, in 2000. Not a single nest, this labyrinthine construction of forty-five thousand nests was connected by shafts and tunnels and covered an area of over 2½ square kilometers. If such a city were to be built by humans on our own scale, we would require numerous architects musing, deliberating, and planning with someone who could oversee the whole project and could be relied on to keep the entire enterprise on track.
Yet among the ants, it seems, such vast empires can be constructed using the simplest rules.
Deep underground, an ant carefully and gently removes one soil grain at a time, dragging it away and dumping it to one side. Quietly and with seeming intent, it starts on a job too big for a single ant, so it releases some chemicals, pheromones, that attract a neighbor to help. Now two ants are busy working away, removing grains and starting the task of building a new chamber. They too need help, so they recruit two more ants, and those four ants recruit four more. Now there are eight. Quickly, we have what is known as a positive feedback effect, a near exponential growth in the number of ants now steadfastly dragging away grains of soil. At last, we have some significant effort and the chamber begins to grow at a sensible rate. Over minutes and hours, the new home takes shape.
But there is a problem: there are not an infinite number of ants available. Other chambers are being built, adding to the pressure for workers across the expanding empire. As the chamber grows, the ants working on it become more dispersed across its surface. The recruitment of ants slows down, and as it does so, a negative feedback sets in. Fewer ants mean reduced pheromone emissions and therefore still fewer ants. Building of the new chamber grinds to a halt. But no worry, because next door, another ant has started a new hole next to a tunnel full of ants. And so the process repeats: in little holes across the nest, brand-new chambers are formed. Now with all this fresh space available, the nest can accommodate more ants, so as the volume of the nest grows, the total population of the colony will also swell, keeping pace with the volume of the nest.
Take these simple ideas of positive and negative feedback effects between individual ants meeting and greeting each other in their fossorial wilderness, and write them into a computer program. Now you can recreate the chamber-building activity of ants and even predict the growth of the whole colony.
Remarkably, no architect is needed for this task, no grand designer to draw the ant nest on a board and to direct and supervise the workers on the job. Despite the overpowering wish to draw some sort of parallel between the impressive scale and collective effort these insects master and the building of the Egyptian pyramids, there could not be a bigger difference. The ant nest can be predicted with nothing more than simple rules operating between individual ants. The queen provides the focus of the nest, the source of eggs and new workers, but the everyday tasks of building the nest are the product of basic interactions between lots of busy ants.
The consequence of this order is that some of these antics can be written down in relatively straightforward equations. Often in the natural world, in physical, chemical, and biological systems, a power law explains the relationship between things. Put simply, it means that one item we might be measuring, such as the volume of an ant nest, changes in proportion (as a fixed power) to something else, perhaps the number of ants, with the simplest expression being:
y = kxn
where x is one thing we might measure (say, the volume of the nest), y is what we want to know (say, the number of ants), and n is the number (the power, hence, a power law) that scales the relationship between them (which can be measured). For example, for the ant species Messor sancta, the value of n is 0.752. The value k is another proportionality constant that can be worked out for any given process.
Power laws come about because of some inherent link between two things that are being measured, and often that link is rooted in a physical principle. For our ant example, the more ants there are, the more grains of sand or soil they can move. Since the collected three-dimensional grains essentially amount to the total volume of the chambers in the nest, it perhaps is not surprising that all other things being equal, the number of ants is related to the volume of nest they build.
Not confined to ants, power laws scatter through biology from the largest to the smallest scale. The laws appear in other places as well, since their ubiquity underscores the regularity in life. In quite different places, we find the same mathematical relationships. The laws of the ants are written in the same formula as other features of living things.
Perhaps best known among power laws is Kleiber’s law, named for Max Kleiber, a Swiss-born physiologist. He measured the activity of a variety animals and found a simple relationship between the metabolic rate, essentially the energy the creature is burning, and its mass:
Metabolic rate = 70 × Mass0.75
This equation tells us that large animals have greater metabolic needs than do smaller ones. A cat has a metabolic rate about thirty times that of a mouse. This relationship makes sense, since a large animal has more mass to keep going. However, the power law also tells us that smaller animals have a higher metabolic rate for each part of their volume than do larger animals. Smaller creatures tend to have a higher proportion of “structure” such as muscles than fatty reserves than larger animals have. They also have a high surface area relative to their volume and so they will tend to lose heat more easily, burning up more calories per unit of weight than larger animals burn.
The exact physical underpinnings of Kleiber’s law and many other so-called allometric power laws that relate the size, physiology, and even behavior of living things are becoming better understood. Their elevated status to “laws” would make many physicists wince. Most of these mathematical observations do not express some fundamental law like Newton’s laws of motion; rather, they are general relationships. However, these intimate links, like many other power laws in biology, speak to us of the underlying order in the biological world, the interconnectedness, from populations of ants to the size and physiology of living things, that ultimately must conform to real physical laws. Many fixed relationships between the features of living things such as metabolic rate, life span, and size of animals that conform to power laws can be explained by the network-like properties of life.
Within the phenomenon of the ant chamber, there is a beautiful example of how complexity can emerge from populations of organisms with simple rules. Put many ants together that interact, and the to-and-fro of their exchanges will lead to patterns. At their core, the interactions are elementary, but mixed and matched, they lead to variegated behaviors.
Attempting to reduce the tangled complexity of populations of organisms, from ants to birds, to more tractable physical laws has fallen under the realm of a part of physics sometimes called active matter. This field strive
s to fathom how matter behaves when it is far from equilibrium, when it has not settled down into a stable and sometimes inactive state. For most of us, being “out of equilibrium” is synonymous with disorder and imbalance. Yet, physicists have found that when systems are far from being in a settled state, rather than disorder, ordered patterns sometimes emerge, and this order can drive biological processes.
In a landmark paper published in 1995, one early attempt to ignite the study of active matter, Tamás Vicsek at Eötvös Loránd University in Hungary developed a straightforward model of hypothetical particles bouncing around and occasionally meeting each other. He found that at low density, these virtual creatures, or blips of data, behaved randomly. Their concentration was just too small for anything noteworthy to happen. However, bring them together at high enough density, and now they move in a way that is influenced by the movements of their neighbors. Mutual interactions cause collective patterns and behavior to emerge. A shift, a phase transition, from one state to another dramatically occurs. These early beginnings in the field of active matter showed how grand things can happen from simple designs. A growing interest in self-organization in living and nonliving systems followed.
Biology is no doubt a special part of active matter. Living things have history, evolutionary quirks, behavioral specialisms even, that make them not mere particles bouncing off each other like atoms of a gas in a box, but more complex and, to some extent, unpredictable entities. Despite these idiosyncrasies, many features of the biological world at the scale of whole populations are successfully reduced to principles that are more transparent. From the swarming of bacteria to the flocking of birds, equations can be derived that help predict behaviors seen in the natural world. Vicsek’s elegant paper hinted at the physical underpinnings of collections of entities in evolution’s great experiment.
The Equations of Life Page 3