In their details, living things do show apparently illimitable embellishments. The vastness in the details of living things has probably caused a divergence between the biological and physical view of the universe. Yet if we return to our facile observation that biology operates within the laws of physics, then we should be able to more comprehensively reconcile this division.
When I joined a university physics department a few years ago as an astrobiologist, I was asked to teach an undergraduate physics course called Properties of Matter. For me, with a background in biochemistry and biology, a semester of this material would be unpalatable without some biology, so I set about modifying my task by using biological examples to illustrate the physical laws and ideas I needed to teach. The inclusion of some biology improved my own motivation, and I also thought that doing so would be interesting for undergraduates.
It was not a difficult task to find these examples. At the molecular level, the van der Waals forces that hold molecules together—these feeble forces from the inherent polarity in molecules make the molecules behave like little bar magnets (even the unreactive noble gases such as neon can behave like this)—can be illustrated with a gecko. These agile desert lizards have an abundance of tiny hairs on their toes; the hairs allow the combined van der Waals forces on all four feet to hold the creature fast on a vertical surface, allowing it to run up a shiny glass window with ease.
The two strands of the genetic material DNA, the molecule that encodes the information in your cells and all other cellular life, are held together to make the familiar double helix by links called hydrogen bonds. The forces involved in these links are just enough to hold the strands together and maintain the integrity of the molecule, but just weak enough that the two strands can be easily unzipped when the cell is dividing in two and the information in DNA must be copied. The replication of DNA and the architecture of its multiplication can be understood as the forces between atoms.
At higher levels of its hierarchy, biology still came to the fore. In explaining phase diagrams (graphs that show the state that matter adopts at given pressures, temperatures, and volumes), I found some illustrations from the world of biology useful. The fish that swim unmolested by predators and in the comparatively warm water trapped beneath the ice on a frozen wintry pond take advantage of the negative gradient of water’s melting curve on a phase diagram. Put simply, when water is frozen, it becomes less dense and floats. Fish that remain active in the winter have evolved to cope with living in the habitat under ice—their behavioral evolution is constrained by some simple facts about the behavior of water that can be manifested in a phase diagram.
Even at the macroscopic scale, physics both explains biological systems and constrains their operation. When clarifying how large creatures travel through water, we are confronted with questions such as why fish lack propellers—what physical laws make a flexing body a better way to get through the ocean and away from a shark than a propeller, the solution of choice to human engineers? The behavior of fluids and the objects that travel through them provides extraordinarily tight constraints on the organisms that can evolve and the solutions they find to live within these constraints.
After teaching this course, I was surprised not by how we could find biological examples of physical laws in action, but by how deeply simple physical rules fashion and select features in life at all levels of its hierarchy, from an electron to an elephant. I was well aware of how physics can shape whole organisms, but I was awed by the sheer pervasiveness of the reach of physical principles, like tentacles, stretched through the entire fabric of life. And despite the inherent uncertainty swirling around subatomic particles in the quantum world—uncertainty that might reasonably make a cautious physicist wonder about how confidently we can bring biology and physics together—the shape and chemical composition of Schrödinger’s cat and the height of Werner Karl Heisenberg himself are highly predictable, convergent features of physical principles operating in biology.
Sometimes, scientists use the oceans as an analogy to evolution. Different animals represent islands of biological possibility, where solutions to successful adaptation to the environment are constrained by what is physically possible and what an organism already has on hand: its history. Between these islands, there are vast oceans of impossible solutions that life must navigate between to find new islands of possibility. Its seems extraordinary that life manages to home in on these islands and that living things seem to arrive at the same haven, like a party of separated shipwrecked seafarers that find themselves marooned on the same deserted outcrop in the middle of the Pacific Ocean. How is it that two animals, such as a bat and a bird, home in on the same functional solution to flight? This convergence cannot be easily explained by a common ancestor, since their ancestors lacked wings, a fact borne out by the very different wing anatomy in the two creatures. However, there is nothing uncanny about life’s ability to land on the same solutions. Impossible solutions are impossible solutions, which means that the ocean of impossibility does not exist at all.
We might instead try to visualize the physical aspects of evolution as like a chessboard. Each square is a different environment, a different set of physical conditions to which life must adapt. When a living thing moves across the board, it automatically finds itself in another space to which it must adapt using a range of well-defined physical laws. For example, the laws of hydrodynamics that enforce certain forms in a fish would be replaced by the dominance of new rules when it crawls onto land. Limbs that allow movement against a more dominant influence of gravity and equations that determine the rate of evaporation as the midday sun mercilessly tries to desiccate our new denizen of the landmasses become some of the shapers of evolution. But there is no intermediate ocean of impossibilities. There are only physical principles seamlessly operating together in different combinations and magnitudes in different environments. Life moves from one environmental condition to another, those laws operating all the time to select successful conformists to physics, while the environment or competitors ruthlessly eliminate the forms whose adaptation to the unwavering requirements of these laws fails to allow them to reproduce.
There is a distinction worth making here. The ocean analogy works rather better when we think about how effectively creatures are adapted to their environment. In the extreme example, an insect born with a missing wing is likely to be severely handicapped in its ability to succeed in the evolutionary game. The idea of organisms occupying a vast landscape where islands and the peaks of mountains represent organisms best adapted to their environment and the plains and oceans between as the organisms less well adapted and less likely to succeed forms the basis of the concept of adaptive landscapes. However, there is nothing strange about life’s ability to find similar evolutionary solutions to environmental challenges. There is no empty space to explore. Living things just move from one place to another; when the physical laws confront them, they must adapt to reproduce. If they do not, we never see them again. Those physical laws often demand similar solutions.
In this book, I do not expect the reader to be surprised that biology and physics are inseparable, that physics is life’s silent commander. Instead, I intend to illustrate the wonderful simplicity of life from population to the atomic scale. I also suggest that these laws are so ingrained, from the atomic structure of life to the social behavior of ants, that life elsewhere across the universe, if it exists, will show similar characteristics.
Surely, though, we might say, “Life cannot just be about physical principles. What about the cheetah that chases the gazelle? Not merely a physical effect on the gazelle, but a true biological interaction.” The cheetah that races across the African savanna to catch the hapless gazelle for its next meal is exerting a selection pressure on the gazelle, and this pressure is, at the level of the biological response, physical. The gazelle will survive this encounter if it can outpace the cheetah. Whether the gazelle can escape depends on how quickly it can release energy i
n its muscles or how deftly it can twist and turn as it seeks to evade the oncoming predator. This ability is itself a product, among other things, of the forces that the knees of the gazelle can endure and the torsion that its leg bones and muscles can accept as it seeks freedom. These factors ultimately are determined by the structure of muscles, bones, the acuity of eyesight, and so on. Either the gazelle will survive to get closer to reproductive age, or it will not. This selection pressure cares not that the cheetah is another biological entity. It could just as well be a fast-running robot built in a physics laboratory at the University of Edinburgh programmed to run across the savannas of Africa, randomly intersecting and killing gazelles. The only matter of importance is whether the biological, and ultimately physical, capabilities of the gazelle allow it to survive the cheetah and what adaptations in muscular properties, bone strength, and other factors allow its offspring to be the successful successors.
The points I make above apply equally to the evolutionary changes that occur in organisms not just from selection restrictions in the environment, such as predation, but also by new expansive opportunities provided, for example, by unexplored habitats and food resources. Many of these changes, both in the short term and ultimately in evolutionary terms, projected onto living things in their environment may be caused by fellow biological travelers on Earth. However, the adaptations required to ultimately survive or exploit the changes in the environment or other organisms are often tightly channeled by physical principles.
All these adaptations are, of course, bounded by the restrictions that may be imposed by the prior shape and form of the organism’s ancestors or in its developmental patterns. These historical architectures and limits in how living things can develop and grow are in themselves boundaries set up by previous evolutionary selection, and these boundaries merely constrain how an organism can respond to the full set of physical laws theoretically available to it and imposed on it. They narrow the field of play further.
There is a question that might be lurking in the mind of reader. You might be wondering, “But what is life?” After all, the preceding discussion has rather assumed we agree on what life is. The question of what defines life has occupied the minds of many good people for a long time. But for the purposes of this book, I do not need to advance that discussion. For simplicity’s sake, I take as implicit in this book a convenient working definition of life, which is essentially that living matter is material capable of reproducing and evolving, consistent with a definition made by biochemist Gerald Joyce, that life is a “self-sustaining chemical system capable of undergoing Darwinian evolution.” The capacity to evolve, that is, Darwinian evolution, is the feature of life that allows organisms to change over time and become better adapted to their environment. On a more easily understood level, on the Earth, this capacity includes almost all the familiar life forms, including the eukaryotes, the domain of life that embraces animals, plants, and many other organisms such as fungi and algae, and the prokaryotes, within which the bacteria and archaea (another branch of single-celled organisms) reside.
We could argue that the word life is merely a human categorization, something that will never yield to concrete definition. Life might just be an interesting subset of organic chemistry; it is a branch of chemistry that broadly deals with lumps of carbon compounds that happen to behave in complex ways. Its capacity for reproduction leads to evolution as environmental forces act on this reproducing material. Life’s apparent persistence on the planet is a product of the evolution of a genetic code within the reproducing material; this code allows for modification and variation in many reproduced units of that matter. Selection pressures act in different environments to whittle the variants down to the successful ones that are subsequently reproduced and distributed into new conditions.
However, whatever we decide about life, whatever the definition or concept we choose, any of these possibilities is entirely consistent with simple physical laws. In his engaging 1944 book, What Is Life?, Nobel Prize–winning Austrian physicist Erwin Schrödinger famously described life in physical terms as possessing the attribute of extracting “negative entropy” from the environment, a slightly unfortunate phrase as it has little formal meaning in physics. However, it was a phrase he chose to capture the idea that life seems as if it is working against entropy, which is the tendency for energy and matter to be dispersed and dissipated into thermodynamic equilibrium. Entropy is a basic attribute of matter and energy encapsulated by the second law of thermodynamics, which recognizes this tendency for things to achieve such an equilibrium. In many cases, this attribute equates to things becoming more disordered. In Schrödinger’s view, life was in a struggle to fight entropy.
Life tends to create order in a universe ultimately prone to disorder. This attribute perplexed Schrödinger and has seemed mysterious to generations of thinkers. When a lion cub grows and eventually reproduces, all the new matter bound up in that adult lion and its offspring represents more ordered, less randomly dissipated energy than when the lion was a small cub nipping at its mother’s heels. Indeed, for a long time, it was something of a challenge to biologists and physicists to explain why life seemed to be doing something apparently in violation of the laws of physics. However, when we look at life in another way, rather than viewing it as something anomalous and almost fighting the laws of physics, we can instead see it is a process that accelerates disorder in the universe—very much in line with physical processes that describe the cosmos. The best way to explain this idea is to use my lunch sandwiches.
If I place my sandwiches on a table, provided they are left alone, it will take a very long time for the energy in their molecules to be released. Indeed, the energy in the sandwich may not be released until it ends up in the Earth’s crust from the movements of the continents during plate tectonics, the sandwich crashing down into the depths of the Earth, heated to great temperatures in the far future, when its sugars and fats will be broken down into carbon dioxide gas. However, if I eat the sandwiches, within about an hour or two, their contained energy will be released as heat in my body and some carbon dioxide exhaled in my breath, with some portion of it being used to build new molecules. In essence, I have accelerated, greatly, the dissipation of the sandwich into energy. I have enhanced the rate at which the second law of thermodynamics, which drives the universe toward disorder, has had its way with the sandwiches. Of course, if I leave my sandwiches on my table, they will go moldy and be eaten by bacteria and fungi that land on them—these organisms will have merely beaten me to it in dissipating the energy of the sandwich into the rest of the universe. Mathematical models show that this idea is not mere whimsy, but that the process of life and its tendency to grow, expand in population, and even adapt can be described by thermodynamic rules.
Living things show extraordinary local complexity and organization, but the process they are engaged in is accelerating the dissipation of energy and the rundown of the universe. Local complexity in organisms is an inevitable requirement to construct the biological machines necessary for this dissipative effect to occur. As the physical universe favors processes that more rapidly dissipate energy, then life is contributing to the processes resulting from the second law, not fighting it. At least, that is one way to view the phenomenon of life. Seen from this perspective, it is easier to understand why life is successful.
Ultimately, of course, when there is no more energy to dissipate or when environmental conditions become unsuitable for life as the oceans all boil away in the searing sky of a more luminous Sun a couple billion years from now, these local oases of complexity that once seemed to defy the second law will do so no more. They too will be destroyed.
This apparent detour relates to us simply because it underpins the idea that life is very much a physical process at work. Living things are collections of molecules behaving in a way that is consistent with, and encouraged by, the laws of physics. We would expect it to be elsewhere across the universe. Within this overarching
behavior, the living things carrying out this process are themselves subject to the laws of physics. In these pages, I am less concerned with prolix and otiose deliberations on the definition of life and more interested in the universality of reproducing and evolving matter that we choose to call life.
The more we learn in physics, chemistry, and biology, the more we are confronted by the simplicity of rules that govern the universe and their unexceptional character. It has been something of a theme through the history of science that major paradigms have overturned the exceptionalist view of our place in the universe. The Earth as just one planet circling the Sun and the descent of humans from apes are two of the most traumatic conceptual changes to our worldview in the last few hundred years. These ideas replaced the geocentric view of the universe—the Earth at the center of our Solar System, populated by people very special and separate from the rest of the animals.
That biology conforms to physical laws raises fundamental questions about the wider universal view of biology: If life exists elsewhere in the universe, will it be like life on Earth? Is the structure and form of life unexceptional as well? At what level of organization could life elsewhere be the same? Is the choice of elements in the ladybug’s leg the same in another galaxy? What about the molecules the atoms come together to form—would the molecules that build and shape the ladybug leg be the same? And what about the ladybug itself? Are there other ladybug-like creatures in another galaxy? Could it be that ladybugs, at all levels of architecture, are unique to Earth?
If physics and biology are tightly coupled, then life outside Earth, if such life exists, might be remarkably similar to life on Earth, and terrestrial life might be less an idiosyncrasy of one experiment in evolution, and more a template for much of life in the universe, if it exists elsewhere. Such an assertion would imply predictability, the hallmark of a good scientific theory.
The Equations of Life Page 2