The Equations of Life
Page 1
Copyright
Copyright © 2018 by Charles S. Cockell
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Library of Congress Cataloging-in-Publication Data
Names: Cockell, Charles, author.
Title: The equations of life : how physics shapes evolution / Charles S. Cockell.
Description: First edition. | New York : Basic Books, [2018] | Includes bibliographical references and index.
Identifiers: LCCN 2017056455| ISBN 9781541617599 (hardcover) | ISBN 1541617592 (hardcover) | ISBN 9781541644595 (ebook) | ISBN 154164459X (ebook)
Subjects: LCSH: Evolution (Biology)—Philosophy. | Physics—Philosophy. | Exobiology.
Classification: LCC QH360.5 .C63 2018 | DDC 576.801—dc23
LC record available at https://lccn.loc.gov/2017056455
ISBNs: 978-1-5416-1759-9 (hardcover); 978-1-5416-4459-5 (ebook)
E3-20180501-JV-PC
CONTENTS
Cover
Title Page
Copyright
Preface
Chapter 1 Life’s Silent Commander
Chapter 2 Organizing the Multitudes
Chapter 3 The Physics of the Ladybug
Chapter 4 All Creatures Great and Small
Chapter 5 Bundles of Life
Chapter 6 The Edge of Life
Chapter 7 The Code of Life
Chapter 8 Of Sandwiches and Sulfur
Chapter 9 Water, the Liquid of Life
Chapter 10 The Atoms of Life
Chapter 11 Universal Biology?
Chapter 12 The Laws of Life: Evolution and Physics Unified
Acknowledgments
About the Author
Also by Charles S. Cockell
Notes
Index
P=F/A.
Image of Lesser Mole-Rat (Nannospalax leucodon) by Maksim Yakovlev.
PREFACE
SOME OF THE MOST fascinating questions whose answers still remain obscure to science lie at the interface between traditional fields. Of course, disciplines are not real entities. They are artificial constructs made by people. Collecting scientific questions into neat disciplinary boxes is administratively useful but artificial and sometimes intellectually counterproductive. The unguided processes of the cosmos do not recognize these neat divisions, either. There is just the universe, about which a civilization can ask questions.
This book explores one line of thinking that tries to make sense of diverse areas of science that straddle the living and the nonliving, the indefeasible links between physics and evolutionary biology. The connections reflect the reality that life is just a form of reproducing, evolving matter in a universe with many interesting and distinctive types of matter.
The reader should know from the outset that this book is not a sterile attempt to demonstrate that evolution is an utterly predictable product of physics. Historical quirks and chance do play a part, and that point is indisputable. They result in the remarkable plethora of detail and the kaleidoscope of forms that we observe in the great evolutionary experiment occurring on our planet. Travel to the Indonesian islands of Lombok and Bali, and despite their similar size, location, and a mere thirty-five kilometers between them, the fauna of each island is distinct. Life on the islands is the evolutionary progeny of that invisible Wallace Line that carves through the deep waters of the Lombok Strait, placing Bali within the historical trajectory of Southeast Asia’s particular evolutionary journey. Bali’s forests echo with the calls of Asian woodpeckers and barbets, while Lombok, alive with the shrill cries of cockatoos and honeyeaters, lies within the fold of Australasia’s evolutionary umbrella. But lurking within this riot of evolutionary experimentation are unyielding principles of physics. It is those that concern me in this book, principles that have increasingly explained many facets of biology, from the subatomic scale to whole populations—biological observations that were previously assumed to be flukes of history and beyond prediction.
Which features of life are deterministically driven by physical laws and which are mere chance, contingent events decided by a metaphorical role of the dice? This question remains one of the most cogent and interesting puzzles about life and its evolution. I do not intend to provide a definitive answer to this question; I’m not convinced anyone currently has the knowledge to do so. However, I do intend to shed light on the growing understanding of the principles that channel life at all levels of its structure and how this expanding corpus of work shows that life is firmly embedded within the basic laws that shape all types of matter in the universe, much more so than a cursory glance at the menagerie on Earth might suggest.
From this view of life emerge conclusions that some people might find sobering, others might find frightening, and still others thrillingly comforting. For people who share with me a fascination for life on Earth, there is something reassuring about our increasing ability to demonstrate that the apparently fathomless profusion of living things on this planet conforms to simple principles that apply to all other types of matter. For those people who also enjoy speculating about what life on other planets might look like (if it exists at all), a conclusion might be that at all levels of its structural hierarchy, alien life is likely to look strangely similar to the life we know on Earth.
As we let go of many of our ancient traditions of thought that have separated life from inanimate matter, we may find that our fear that this will dangerously consign people and other creatures to what is often viewed as the blandness of physics that determines the fate of most matter in the universe is unfounded. Instead, the unity of evolution and physics brings a new richness to our view of life, an appreciation that within the simplicity of rules that govern and limit the forms of living things there is remarkable beauty.
CHAPTER 1
LIFE’S SILENT COMMANDER
A SHORT WALK THROUGH the Meadows in Edinburgh would leave most people in little doubt that life on Earth is a remarkable anomaly in a universe of bland conformity. Trees of various shades of green rustle in the wind, birds take to the air in gymnastics that left the ancients aghast at the agility of these heavier-than-air flying machines, and along the ground run all manner of animals; the smallest ladybugs land on picnicking tourists, while domestic dogs leap and cavort across the grass.
Compare this spectacle with the velvet black emptiness of space seen with some of our best telescopes. Images of galaxies colliding in astrophysical violence, the light of these long-since-dead encounters collected after traveling billions of years through an empty void almost unimpeded. In this vast, infinite vacuum, punctuated by a collection of stars, planets would materialize. And on one planet, tou
rists would be swatting flies in a meadow below a castle. What could be more of a contrast between the apparently simple laws that govern the gravitational rotation of a collection of stars—mere balls of fusing gas in the rarefied expanse of space with their attendant planets—and the unpredictable leaps and bounds in something as complex as a pet Labrador, the phenomenon of life?
I once heard a distinguished astrophysicist declare that he was glad to be studying stars, since a star is much easier to understand than an insect.
As we peer into outer space, we can certainly find merit and empathy in this view. Even at the intimidating scale of a massive star, we find simplicity. As the star burns, fusing gaseous elements and releasing energy, so these building blocks successively grow in atomic mass. We begin with hydrogen atoms, the universe’s most abundant element, which joins with other hydrogen atoms to form helium. Other helium atoms combine to form carbon and so on, until successive layers are formed—oxygen, neon, magnesium; the atomic mass of the elements grow as the products of each new round of fusion are formed. In the center of the star is iron, which cannot fuse to form any other elements. Iron is the last stage of fusion, and the result of this sequence of relatively simple atomic additions is layer upon layer of heavier and heavier elements from the surface to the core of the star. Elements heavier than iron are formed by other means, such as in the catastrophically energetic explosions of stars that herald the spectacle of a supernova.
Compare the onion-like arrangement of elements in a massive star, over a million kilometers in diameter, to our ladybug (a ladybird in many places in the world), just seven millimeters long, sitting on the thumbnail of a sleeping tourist in the Meadows.
The little oval ladybug, a beetle, is just one insect among many species that inhabit the planet. (We do not know how many species there are. About a million are known, and there are likely to be many more still undiscovered.) However, this unassuming creature is full of complexity and comprises eight major parts. Its head is one discrete portion and contains its mouth. The ladybug has antennae and eyes for sensing the world around it, with the antennae considered a separate part. The pronotum, a tough protrusion behind the head, protects it from damage. Behind this are the thorax and abdomen, the body section to which the wings and legs are attached. Finally, this complex machine has elytra, wing cases that protect its vulnerable and delicate flying apparatus.
Yet like the star, each of these features is molded by physical laws. The power of flight conferred by wings means the ladybug must observe the laws that govern aerodynamics, as must all flying creatures. And as for its legs—well, why not wheels? Like all land animals, besides snakes and some lizards that lack limbs altogether, ladybugs evolved legs instead of wheels. There are physical reasons for that, rooted in the relative effectiveness of legs in navigating the irregular terrain of our planet. In protecting their gossamer wings, the elytra must observe rules that pertain to the behavior of tough materials, such as abrasion resistance and flexibility.
In all segments of the ladybug, we can identify the principles of physics. The apparent complexity of the ladybug compared with a star lies merely in the greater number and variety of principles embedded within the insect and which it uses to live its life. Evolution is simply a very good process for assembling different principles, which we can represent as equations, into an organism. Any natural environment usually presents multiple challenges to survival. If a physical process leads to a living thing’s development of a characteristic that makes the living entity less likely to be eliminated before it reproduces (reproduction being the measure of evolutionary success), then creatures will evolve over time and can be thought of as containers manifesting diverse physical laws.
The menagerie of life, impressive enough even on a casual walk in the Meadows, grows in variety as you explore its forms through time. Equipped with every scientist’s favorite improbable toy, the time machine, we might revisit the Meadows 70 million years ago. Then, we would find forms of life very different from today. Like modern birds, the reptilian pterodactyls had mastered aerodynamics, some with ten-meter wingspans. Feathered dinosaurs and strange insects roamed the land, and in ponds or lakes, reptiles, slender and long, achieved mastery in their watery habitats. If we hop back into our machine and return to about 400 to 350 million years ago, we would find Edinburgh in the site of immense volcanic activity, thick mat-like conglomerations of microbes growing here and there. Across the land, between the volcanic cones, the earliest land plants, the Cooksonia, stake their claim to the new habitat. Scurrying among their short knobbly stems of just a few centimeters’ height were early insects and the now extinct eight-legged beetle-like trigonotarbids. Return just a few tens of millions of years later and you would have seen four-legged Pederpes, a forerunner to modern land vertebrates, awkwardly shifting its one-meter-long slithery body through the undergrowth, peering this way and that with its triangular head to chart its way through.
But impressed though we may be with all the wonderful life forms we have observed in our excursion through time, there is a strange familiarity about all these living things. Their shapes and forms, although different, share fundamentally the same types of solutions to living as we see in modern forms. These similarities are not merely an artifact of evolutionary descent. The growth of early plants against gravity, the size of bones that hold up a dinosaur, the sleek shapes of water-bound animals, and the features of wings that allow a pterodactyl to fly lead to evolutionary forms similar to modern-day organisms faced with the same unyielding laws.
The complexity and sheer diversity of life in time and space could convince anyone that life represents something quintessentially different from physical processes, a divergence of form that transcends the simple principles that seem to fashion the apparently predictable structure of the inanimate world.
Yet physical laws restrictively drive life toward particular solutions at all levels of its assembly. The outcomes are not always predictable, but they are limited. It does not matter at what level those requirements are operating, from the subatomic to the population scale: the results are various, but not boundless.
Even at the smallest scale, we can see these narrow channels of evolution. Consider molecules, such as the proteins, from which the eclectic mixture of monsters on Earth is assembled. Like life at the larger scale, proteins do not display untrammeled potential. Rather, in observing the limited ways that proteins (including enzymes, biological catalysts) can be folded together, some scientists have argued that these configurations reflect a set of forms analogous to the perfect and unchanging forms of things espoused by Plato. To some people, such a view seems to contradict the Darwinian view of life, which emphasizes natural selection’s tendency to produce apparently unlimited variety.
Science can sometimes be unnecessarily polarized, and, of course, challenging the Darwinian view is always popular, edgy, and controversial. In the synthesis I present here, there is no challenge to Darwin’s basic precept that natural selection can fashion an extraordinary range of creatures or even proteins. I merely illustrate how this process is limited in the basic pattern of its products, not just at the level of the organism, but in any part of its construction, from populations down to proteins and down to the atomic level. I gather evidence from what has become an impressive corpus of work by many researchers to show how physical principles sharply narrow the scope of the evolutionary process at all levels of life’s structure.
My view is underpinned by a simple proposition. Evolution is the process by which the environment acts as a filter to select units of organic mass in which a mosaic of interacting physical laws are optimized sufficiently to allow for reproductive success. The environment in this context can include a whole range of challenges—from weather phenomena such as storms to the appetites of predators—that might prevent an organism from reproducing. Evolution is just a tremendous and exciting interplay of physical principles encoded in genetic material. The limited number of these principl
es, expressed in equations, means that the finale of this process is also restrained and universal.
Equations are merely a means to express in mathematical notation the physical processes that describe certain aspects of the universe, including features of living things. The phrase equations of life is shorthand for this growing capacity to use physical processes, and often their mathematical formulations, to describe life at different levels of its hierarchy. Throughout this book, I will provide some examples of these equations, but I do not expect you, the reader, to have to understand their nuances and details or how to use them. I show them to illustrate how physical principles that underlie evolution can often be expressed in these conveniently shortened mathematical forms.
That the laws of physics should bound life is hardly a controversial statement. The ladybug in the Meadows does not exist outside the same principles that govern the formation of the Sun that warms it on a sunny day. Life is very much part of the universe: it cannot operate outside its rules. Yet although this observation seems trite, we are often remarkably unwilling to accept the extent to which life is tightly constrained by physical laws. When observing living things, we can easily forget that the rules that govern their architecture set a harsh perimeter; the extravagance of life can appear, to the unreflective observer, to be limitless in its variety.
For me, what is surprising about the journey through the different scales of life’s structure, accompanied by our growing knowledge, is that life is much more amenable to description in terms of physical processes, and thus simple mathematical relationships, than once was thought, and that these principles are now being elaborated at many different levels of its hierarchy. These insights also suggest that life is much more narrowly circumscribed than those who favor the role of chance and history would like to think. Accordingly, life is more predictable and potentially universal in its structure than is sometimes assumed.