However, in life, those small mutations at the molecular scale in the DNA are the source of its variety. From these alterations in the code, a chance transformation in a base pair of DNA caused by the mischievous wiz of an errant high-energy particle, variant codes are produced. The code within life appears to give it a persistent purpose—“Life will find a way.” However, this sense of purpose is an illusion. Life persists because differences in the code produce, in every successive generation, individuals, each with its own many idiosyncrasies. With so many variants, it is likely that some of these will survive and succeed in an environment, replicating with aplomb and filling an environment as far as is possible using the available resources, while elsewhere its kith and kin might perish. This process of selection gives the illusion of a sort of purpose or tenacity of life, a determination that somehow animates the living form. This character of life, the behavior that emerges from its code, does give it a special feature, but not one that categorically separates it from physics. This feature just makes life a particular embodiment of physical process, a coded process.
People are wont to imbue this chasm between life and nonlife with some sort of mystic unfathomability. Within this departure in the behavior of life from the rest of the cosmos, perhaps some see an opportunity to seize again that age-old desire for vitalism. Some people might hope to escape the nasty conclusion that life is just an interesting branch of organic chemistry, a particular, albeit fascinating, set of physical principles given expression in a special collection of molecules. Sadly, for those who dream of segregation, the difference is not that astounding.
One of the most fascinating things we can do in biology and physics—indeed, exactly what we have done in this book—is to take a journey from the sociobiology of ant nests to the atoms that make up life. Every level of the hierarchy, it is safe to say, is occupied by a different group of people (peruse the citations at the back of this book, and you can see this for yourself). Yet if you survey this literature, a common theme runs through it. Independently, many groups keep returning to the same idea. Look at life’s choice of amino acids, and you find it rooted in the physical properties of those molecules. Observe the folding of proteins, and you find an almost infinite possible number of amino acid chains collapsed into just a few forms. Examine the structure of life, and cells are universal. Survey the forms of animals and plants, and simple relationships confine their shapes like rivets. Marvel at the arrangement of birds, ants, and fish, and within their bewildering hordes, simple rules are at work. From the most diminutive parts of life to whole populations, physical principles have been shown to hold life captive, to corral it into a small set of possibilities. For the last several decades, a triumph of biology has been to reduce vast complexity and ostensibly unfathomable diversity to apparent simplicity, aided by a newly formed convergence between biologists and physicists.
These groups of scientists, like so many laboratories working on different floors of a high-rise building, each occupying and researching a different level of the structure of life, seem to conclude the same thing. Life is confined by rules that are surprisingly, maybe shockingly, narrow. As we improve our grasp of how these rules govern biological systems, we may even predict, if not in fine detail at least in general, the pillars and girders of life. Synthetic biologists have entered the twilight zone of predicting the result of new forms of life with their own designer genetic codes, even creating these new forms too.
If biology and physics are thus united, is there any room for contingency in all this, for the quirks and fancies of history, the unpredictable leaps and bounds of evolution from previous forms that lurch into new territories of biology? Although I have suggested that these opportunities are rather limited, others have notably taken a very different view.
Stephen Jay Gould famously took sides with contingency, at least at the scale of whole organisms. For him, chance was everything in evolution. He recognized the underlying laws of physics, but believed adamantly that although these principles operate in the background, everything interesting in evolution was ultimately the result of contingency, the success of particular animal body plans, the rise of mammals, or the emergence of intelligence. These might have been thwarted had the dice rolled differently. His view lay in his experience with the Burgess Shale, a 508-million-year-old fossil deposit buried in the slopes of the Canadian Rocky Mountains. He elaborated on this perspective in his book Wonderful Life. Within these sheeted rocks are the imprints of some of the earliest animals, one of the best-preserved early experiments in multicellular, complex life. Repeated in similar units around the globe, the fossils bear witness to life’s prolific steps beyond their three billion-year microbial past. Some of the strange arrangements of the segments, legs, tentacles, and other appendages in these animals seemed to suggest a biological landscape of contingency. From these unfamiliar forms, so Gould argued, the survival of our own lineage was a chance toss of the coin, a fluke.
I do not doubt that for any scientist who spends time analyzing the fine anatomical detail of these fossils—a bifurcated tentacle here, a segment strangely placed there, a leg doing something odd over there—their variety elicits sheer surprise, an example of contingency at work. However, as an outsider who has not studied the intricacies of invertebrates and become beguiled by them, but who did spend four days in a library poring over the reconstructions of the Burgess Shale monsters, I was struck not by the immense potentiality, but by their extraordinary uniformity. At that moment in history, there was the potential for an unlimited experiment; an open ocean (literally) existed for these newfangled animals to explore and exploit. Yet within their forms, they share a tedious similarity. Most are bilaterally symmetrical like you and me. Most have a mouth on their front and an anus on their rear. These animals have their detail and their bizarre contorted shapes, yet they also share page after page of gross similarity. They seem to be testament to an experiment so desperately trying to be unfettered, yet ultimately so unimaginative when confronted by the laws of hydrodynamics, diffusion, and a few other requirements besides. Contingency is there for sure, but it is none too stunning. The similarity in the products of this unique moment of unmuzzled opportunity was much more striking than any daring expeditions in biological form. Indeed, since Gould’s paean to contingency, scientists have come to understand that many (if not all) of the Burgess Shale animals are related to modern groups of animals.
We must be careful to delineate two things. There is the contingency in the details of living things: the number of membranes that envelop a cell, the pattern of a moth’s wing, the curves on the jaw of a dinosaur. In some refinements, if selection pressures do not vigorously impose themselves on the distinction between success and failure at reaching reproductive age, then contingency may have its way, a mixture of chance and some history. This very fact leads to the sheer enormity of variation in life on Earth.
For those who like the fine details, the variety and color of life, then I grant that contingency is everything. The historical nuances and developmental constraints of prior form may well in many features disallow the possibility of predicting the exact outcomes of evolution. However, on a deeper biological level, these are trivialities against the underlying physics that circumscribes life: the cell membrane, the aerodynamic forces that converge the wings of animals, the structure of jaws made for crushing food.
There is another realm in which contingency may play a role: in the major evolutionary transitions in life, the monumental changes that made a categorical difference to the capacities of life on Earth. Those transitions are not mere detail. Rather, they have shaped the edifice of life.
Some transitions seem to bear the mark of inevitability. The emergence of cellularity was required for biochemistry to be enclosed and then released within these comfortable chambers into the wider world. Without this step, life would never have emerged beyond a few localized self-replicating molecules in rocks or maybe a hydrothermal chimney.
Eve
n in transitions that we could demonstrate are inevitable, their timing may be uncertain. Since an asteroid delivered that fateful blow to the dynasty of the dinosaurs 66 million years ago, the mammals have mutated from shrews to radiotelescope-building apes. Yet after about 165 million years of animal dominion over land, sea, and air, the dinosaurs had remained stuck in a reptilian state of intellectual dormancy. Not a space program in sight. At the very least, even if these animals would eventually have built dinosaur space agencies, the timing of some of the major events of evolution, perhaps intelligence, may well contain contingency, the chance imposition of the right selection pressure to drive forward cognition.
In the animals of the Burgess Shale and their fossil predecessors, Gould saw a specific moment of such transition-scale contingency at work. For him, the new forms of animals that so puzzled the first paleontologists who examined them postdate the potential for vastly different trajectories. Travel a little further back in time, and you arrive in the Ediacaran period, named for a picturesque location in Southern Australia. Here in the Ediacara Hills, the remains of the earliest animals known are preserved. All of them were soft-bodied, and most were flat, frond-like, quilted and pancake creatures. They precede the great “Cambrian explosion” that gave rise to the myriad of forms preserved in the Burgess Shale.
Why the flat animals? Animals, like cells, rely on a high surface area to take in nutrients and food and to exchange gases. In our own lineage, this feat is accomplished with internal organs. Lungs and intestines increase the effective surface area of each individual animal. Your lungs, with their networks of intricate tiny tubes, cover an area of about 75 square meters. Your gut, with all its coils and protrusions, absorbs food over a capacious 250 square meters, about the area of a tennis court. Yet in the Ediacaran, confronted by the same laws of physics, the need for food and gases to diffuse in and out, the solution was different. Instead, the creatures took on flattened forms so that no part of the animal was far from its surface. Gould asserted that if these Ediacaran faunas had inherited the Earth and outcompeted the solution of growing larger and using internal organs to bring life’s substance into the body, the empire of the animals revealed in the gray slabs of the Burgess Shale might have been little more than a fandango of flattened forms. Contingency, the chance leap that evolutionary processes might have taken toward a different body plan when they reached that fork in the road, might have led to a very different world.
Can we say that the continued domination of the Ediacaran solution would have spelled doom for any further development of animals? Surely, one of the many lineages to have emerged from this experiment would sooner or later have developed an invagination within its body more capable of grabbing food and gathering oxygen? From that increase in surface area, such a form would have been at an advantage, perhaps allowing for more-complex and perhaps more-competitive animals?
Speculations are entertaining, but we cannot rerun this experiment. Whether there are contingencies, historical accidents of such import they could consign an entire biosphere to an ignominious eternity of pancakes, is an open question.
The science of evolutionary developmental biology and its revelations of both the hierarchical modularity of life and the way in which development can produce quite radical changes suggest that transitions of great magnitude, at least in whole organisms, can be made. The whale’s pelvic bone, a vestige of its belligerent decision to write off the experiment of land-living and return to the oceans, is also the evidence of the extraordinary ability of life to dash from ocean to land and back again, flipping and changing to take advantage of, and adapt to, the equations of life. Perhaps the pancakes would eventually have become bloated and sprung forth legs.
Are there other contingencies between the first self-replicating molecule and a spacecraft-building civilization with the power to halt the complexity of the evolutionary experiment? Like the whale’s indecision, the genetic code and metabolic pathways are apparently mutable. A growing compilation of evidence suggests that the inherent flexibility of even life’s core processes allows it to escape frozen accidents and explore new possibilities, optimizing and improving under selection, even with the odd vestigial part remaining here and there. But what about the emergence of multicellularity or complex life?
Even if we can demonstrate the existence of such contingent events in the story of life, one or two major transitions that may well have hinged between two versions of the biosphere, possibly one world less complex than another, these to me are interesting, even startling moments. But they are mere solos, moments of singular diversion in a symphony of music, the music of physical laws. Their significance lies in that these contingent moments could, if they exist, determine whether intelligence is rare in the universe by deciding between two evolutionary paths, one of which leads to complex multicellular life and eventually intelligence, the other not. Such moments of chance that decide how quickly life reaches beyond its microbial beginnings, or whether it stretches beyond these limits at all, may have importance for the distribution of different levels of complexity of biospheres and their attendant life forms through the universe, if life exists elsewhere. However, these alternative worlds really only interest an intelligent onlooker who values intelligence, a species that finds these differences significant.
We could equally look on biospheres altered in their complexity by contingent events, maybe even in their possession of intelligence, as similar to the great variety of butterfly wings. The myriad possibilities evident in life on Earth and maybe elsewhere represent grand experiments in evolution, the phenomena of life. What is extraordinary about this branch of collected matter, the ideas we have surveyed in this book, is not what is contingent in it. Instead we marvel at how such a diversity and efflorescence of capacities, in a tiny bubble of physical and chemical conditions to be found in the known universe, can be so restricted.
The detail of life is titillating and a joy to gaze upon, but understanding the narrowness of the channels that ultimately hem life in is precisely what circumscribes a myriad of questions about life. Is carbon-based chemistry and the use of water as a solvent the only choice for a living thing? What makes metabolic pathways and the genetic code the way they are, and could they have been different? Why do cells look like they do? How does life adapt to new environments, and what limits are there? Why do animals have some solutions and not others, such as legs instead of wheels? How do physical limits shape the perimeter of the extremes of the biosphere itself? And beyond Earth, would other life, if it exists, look like terrestrial life? Endlessly onward.
Implicit in these lines of inquiry is whether contingency has a role to play. Although we cannot easily repeat evolution and observe contingency at work, we can study the factors that shape life using scientific observation and experimentation. Today, we can even modify the genetic code and explore its alternatives. Maybe one day, in exploring distant worlds, we will have another evolutionary experiment entirely to derive stronger ideas on the universality of biology. Life’s predictabilities—its common features, its limits and boundaries—are what should enchant us.
Research that attempts to better establish how much physics constrains life and where chance may play a part, not merely in particular parts of life, but more synthetically across its whole hierarchy, has enormous potential to tighten the fusion between biology and physics. We can examine the physical principles that operate as information flows upward in the hierarchy from the genetic code to whole organisms. We can deepen our ability to describe evolution by more thoroughly investigating the principles that operate as environmental influences head downward, particularly in the form of selection pressures acting on organisms going about their lives and thence on the genetic codes that make it through to future generations.
Different levels of the biological hierarchy need not be linked to others. We need to define the physical principles that operate on life at a given scale independently of its properties at other scales to establish more
exactly what is predictable about life. For instance, the fusiform, sleek body of a dolphin is sculpted by the action of hydrodynamics operating through natural selection on the whole organism. These selection pressures ultimately act through, but largely independently of, how that organism is assembled from the cellular level downward. Similarly, a fictitious alien mole-like creature shaped for effective burrowing would look similar to the tunneling creatures we are all familiar with, even if we were to imagine the animal assembled from a silicon-based chemistry. Many contingencies and inevitabilities at different scales are separate from those at lower or higher structures of organization. This understanding might greatly simplify our ability to predict or at least identify the physical principles at play in different parts of living things.
By expressing the physical principles within life in equations, we have the enthralling possibility of pushing ever more convincingly into the ability to predict structures and outcomes in the evolutionary process. As efforts intensify to expand this activity into an ever greater range of biological processes, so will the physical contours that define the nature of self-replicating, evolving systems of matter become more clearly resolved and more amenable to modeling and study.
The Equations of Life Page 29