Raghuveer Parthasarathy - So Simple a Beginning: How Four Physical Principles Shape Our Living World

So Simple a Beginning

So Simple a Beginning

HOW FOUR PHYSICAL PRINCIPLES SHAPE OUR LIVING WORLD

Raghuveer Parthasarathy

PRINCETON UNIVERSITY PRESS

Princeton and Oxford

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Library of Congress Cataloging-in-Publication Data

Names: Parthasarathy, Raghuveer, 1976 author.

Title: So simple a beginning : how four physical principles shape our living world / Raghuveer Parthasarathy.

Description: Princeton : Princeton University Press, [2022] | Includes bibliographical references and index.

Identifiers: LCCN 2021026503 (print) | LCCN 2021026504 (ebook) | ISBN 9780691200408 (hardback) | ISBN 9780691231617 (ebook)

Subjects: LCSH: Biophysics. | BISAC: SCIENCE / Life Sciences / Biophysics | SCIENCE / Life Sciences / Anatomy & Physiology (see also Life Sciences / Human Anatomy & Physiology)

Classification: LCC QH505 .P37 2022 (print) | LCC QH505 (ebook) | DDC 571.4dc23

LC record available at https://lccn.loc.gov/2021026503

LC ebook record available at https://lccn.loc.gov/2021026504

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British Library Cataloging-in-Publication Data is available

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Jacket Design: Jessica Massabrook

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Copyeditor: Jennifer McClain

In memory of my parents,
Kalyani and Sampath Parthasarathy

So Simple a Beginning

How does life work? This question may seem overwhelming, or even preposterous. How could any answer do justice to both a sprinting cheetah and a stationary tree, to the unique you along with the trillions of bacteria that live inside you? The experiences of even a single organism are breathtakingly varied: consider a chicks emergence from its egg, the first flap of its wings, the racing of its heart at the sight of a fox, and its transformation of food and water into eggs of its own. Could any intellectual framework encompass all of this?

The search for an answerfor some kind of unity amid the diversity of lifeis reflected in our ancient urge to categorize living things based on similarities of appearance or behavior. Aristotle partitioned animals into groups using attributes such as laying eggs or bearing live young. , including, similarly, manner of origin: those born from an egg, those born from an embryonic sac, those born from moisture, and those born from sprout. Modern taxonomy emerged from the eighteenth-century work of Carl Linnaeus, who systematized the naming of organisms and developed a hierarchical classification scheme based on shared characteristics that we continue to find useful. Classification in itself, however, is not very satisfying. We want to know the why, not just the what, of the commonalities that unify living things.

In this book, we look for that why through the lens of physics, revealing a surprising elegance and order in biology. Of course, this isnt the only perspective that offers deep insights into life. There is the viewpoint of biochemistry, with which we understand how atoms join together to form the molecular components of organic matter, how energy is deposited in and extracted from chemical bonds, and how the incessant flux of matter and energy in chemical reactions constitutes the metabolism of living things. But it is difficult to use chemistry alone to zoom out from the scale of molecules to the scales of the animals and plants around us, or even the scale of single cells, and make sense of shape and form.

Another all-encompassing perspective is that of evolution. Since the mid-nineteenth-century epiphanies of Charles Darwin and Alfred Russel Wallace, we can see the traits of living creatures as manifestations of deeper historical processes. Similarities, whether of visible characteristics of anatomy or more hidden patterns in DNA sequences, can reflect shared ancestry with which we can deduce a tree of relationships linking all of life together. Differences emerge due to random chance and the varied pressures on survival imposed by creatures environments; again, present forms reflect past history. Evolution provides a powerful framework for understanding life. It is not, however, one that we focus on in this book. In part, this is because there is already a large popular literature on the subject. More importantly, however, evolutionary principles alone dont illuminate the why as much as the how.

To illustrate what I mean by why, consider the swim bladder, a pair of gas-filled sacs possessed by many, but not all, species of fish. Comparing creatures both extant and extinct reveals this organs evolutionary history, with connections to the emergence of lungs in air-breathing animals that Darwin himself remarked upon. Understanding the function of a swim bladder, however, requires a bit of physics: the low density of the enclosed gas offsets the high density of bone in bony fishes, allowing the animal to maintain the same average density as its watery surroundings and thereby easily position itself at whatever depth it likes. A swim bladder is just one solution to the challenge of matching density. The fish might instead contain large amounts of low-density oil, or a skeleton composed of cartilage rather than bone, both of which are strategies adopted by sharks, which lack a swim bladder. The last common ancestor of cartilaginous and bony fish lived over 400 million years ago. Since then, the distinct evolutionary paths of the two groups have led to different solutions to the shared physical challenges of aquatic locomotion. We can state, with a point of view echoed throughout this book, that understanding the why of these anatomical features, related to control of density, highlights a hidden unity that fish share that transcends their evolutionary divergence. We should keep in mind, however, that the machinery of variation and natural selectionthe enhanced odds of survival that accrued over generations to those creatures better able to navigate their aquatic worldprovides the paths by which the forms we see arise.

There are other vantage points besides those of biochemistry and evolution from which to survey the breadth of life. Rather than list all the approaches we wont be exploring, however, lets turn to the one we will.

Ive already hinted at the view of nature the rest of this book expands upon, which I identify as biophysical. The term implies a unification of biology and physics. It encapsulates the notion that the substances, shapes, and actions that constitute life are governed and constrained by the universal laws of physics, and that illuminating the connections between physical rules and biological manifestations reveals a framework upon which the dazzling variety of life is built. The notion of universality is central to the utility of physics, and to its appeal. The same principles of gravity apply to an apple falling from a tree and to planets orbiting the sun, and current work aims to further expand this framework to encompass the strange behavior of the quantum world. Biophysics extends to the living world the quest for unity that lies at the heart of physics.