Do Frogs Come from Tadpoles? Other Nature Institute Perspectives

Do Frogs Come From Tadpoles? Other Nature Institute Perspectives Extraordinary Lives: Disability and Destiny in a Technological Age by Stephen L. Talbott The Flexible Giant: Seeing the Elephant Whole by Craig Holdrege In the Belly of the Beast: Technology, Nature, and the Human Prospect by Stephen L. Talbott The Giraffe’s Long Neck: From Evolutionary Fable to Whole Organism by Craig Holdrege Do Frogs Come From Tadpoles? Rethinking Origins in Development and Evolution Craig Holdrege Evolving Science Association A collaboration of The Nature Institute and the Myrin Institute 2017 Copyright 2017 Craig Holdrege All rights reserved. Published by the Evolving Science Association, a collaboration of The Nature Institute and the Myrin Institute The Nature Institute: 20 May Hill Road, Ghent, NY 12075 natureinstitute.org The Myrin Institute: 187 Main Street, Great Barrington, MA 01230 myrin.org ISBN 978-0-9744906-4-9 Cover drawing by Martina Muller This book and other Nature Institute Perspectives can be purchased from The Nature Institute’s online bookstore: http://www.natureinstitute.org/store/index.htm or contact: The Nature Institute— [email protected] 518-672-0116 Contents Introduction 7 Chapter One. Do Frogs Come From Tadpoles? 11 The Life of a Tadpole 11 Metamorphosis 14 Thinking Development 20 But What About Genes and Hormones? Where Have all the Causes Gone? 22 Being Itself Differently 25 From Space to Time, From Thing to Process 29 Chapter Two. A Biology of Beings 31 Portraying a Frog 33 Scientific Portrayal 39 Chapter Three. Creativity, Ancestors, and Origins: What Frog Evolution Can Teach Us 45 The Double Nature of Life 46 Fossils and Life Past: Portraying or Explaining 49 Frog Fossil History 52 Forming a Picture of Evolutionary Transformation 62 The Problem of Ancestors 65 Of Origins and Creative Evolution 70 Acknowledgements 73 Notes 75 References 79 Introduction Introduction I think of this monograph as a meditation because it is concerned with heightening our understanding of facts already known. It involves seeing things in a new light. The illumina- tion comes from letting the phenomena of development and evolution reveal aspects of themselves that previous perspectives have hindered. For many years I have studied and taught about development and evolution, and always experienced them as fascinating and important topics. Theories about ‘mechanisms’ and ‘causes,’ however, have seemed inadequate to me. In fact, I have serious doubts about the value of speculative theories altogether. For this reason I have put much effort into trying to bypass theoret- ical frameworks so as to gain a more direct and unadulterated view of the phenomena themselves. As a result, much of my work has been concerned with studying development and evolutionary biology with the trust that the phenomena themselves, if I can just attend to them in a fresh way, have the power to help me break through mental barriers and awaken to new insights. This has been my aim in studying frog development and evolution. Many years ago, as a high school biology teacher, I began teaching about frog metamorphosis. I have remained intrigued by this remarkable transformation of little tadpoles into adult frogs that are such different creatures. A few years ago I returned to observing tadpoles and frogs in rural upstate New York, and also revisited the scientific literature describing the radical changes that oc- cur during metamorphosis. 7 Do Frogs Come From Tadpoles? Along the way, I had an “aha” experience: I realized that frogs do not come from tadpoles. I will explain what this apparently outrageous and erroneous statement means in Chapter One. My understanding of development was revolutionized, and from that point on all the details and the whole developmental process appeared to me in a new light. One central feature of my new understanding was to know in a concrete way that development has to do with agency, with the activity of beings. “Agency” and “Being” are terms that are strictly forbidden in mechanistic biology. But a veil had dropped. I realized that you can’t really do biology in a way that does justice to the phenomena unless you acknowledge and take into account that beings are active at all times in every as- pect of life. So in Chapter Two, I present some of the challenges and features of a biology of beings. I also realized that this new way of seeing development has huge ramifications for understanding the macro-developmental process we call evolution. So I decided to stay with frogs— who had revealed to me so much—and study their evolutionary history as it shows itself in the fossil record. Chapter Three presents the results of this effort so far. In the work of articulating and attempting to make under- standable the new perspectives, I experience two different kinds of hurdles. I confront my own difficulties in fully expressing what it is I’ve seen in a way that seems true and adequate. And I am also aware, given the habits of thoughts that inform so much modern thinking about development and evolution, that much of what I am trying to get at can easily be misunderstood. That’s a dilemma. Just over 100 years ago, Edmund Husserl was working to es- tablish phenomenology in philosophy as “a new way of looking at things … one that contrasts at every point with the natural at- titude of experience and thought.” He knew this would demand 8 Introduction that we “set aside all previous habits of thought, see through and break down the mental barriers which these habits have set along the horizons of our thinking.” This alone will allow us “to move freely along this new way without ever reverting to the old viewpoints, to learn to see what stands before our eyes…”1 This study has been such an adventure of breaking through boundaries. I hope you will experience some of that adventure and be stimulated by the glimpses into features of development and evolution that only a new way of looking can provide. 9 Introduction CHAPTER ONE Do Frogs Come From Tadpoles? Where does a frog come from? The answer seems obvious. It comes from a tadpole. But does it? Surely, without the tadpole the frog does not develop. But just as surely, nowhere do we find the frog in the tadpole. The frog comes into existence only as the tadpole disappears. We need to be keenly aware of what we mean, and what we don’t mean, when we say, “A frog develops out of a tadpole”—or a tadpole out of an embryo, or an embryo out of a fertilized egg, or an adult human being out of a child. As we will see, when we give careful attention to what is actually happening when a new phase of life develops out of a previous stage, there are large implications for our overall understanding of developmental processes. New and exciting questions arise about how we conceive of development— including that trans-species developmental process we call evolution. The Life of a Tadpole Most of you have probably seen tadpoles in ponds and vernal pools.1 With a thick squat body that abruptly tapers to a long, finned tail, a tadpole can be distinguished from a fish. And yet, tadpoles are fish-like in many of their characteristics. 11 Do Frogs Come From Tadpoles? Figure 1. Metamorphosis of the European common frog (Rana temporaria). Pictured to scale. (Photographs by Tim Hunt, reprinted with permission; http:// www.timhuntphotography.co.uk/) They remain submerged in water and breathe through their skin and gills. They have no limbs, and swim through water via undulating movements of a long boneless tail fin. Like fish, tadpoles have a lateral line organ, which runs along each side of the body and tail, through which they sense movements in water. A tadpole typically grazes on algae that grow on plants, rocks, or at the surface of the water. Tadpoles have a “beak” and rows of denticles in their mouth that function like rasps to scrape off the algae. The denticles do not consist of bone and enamel but of keratin—a protein substance that, for instance, also makes up our fingernails and hair. The tongueless tadpole sucks the algae into its throat, and the food enters the long intestine where it is digested. There is no stomach. The intestine can be more than ten times longer 12 Do Frogs Come From Tadpoles? Figure 2. Tadpoles (European common frog; Rana temporaria). (Photo by Friedrich Böhringer; Wikimedia Commons.) than the tadpole itself and is its largest internal organ, making up over half its body mass. Tightly coiled, the intestine takes up about half the space within the tadpole’s ovoid-shaped body and is visible through the translucent belly skin. Figure 3. Northern leopard frog tadpole (Rana pipiens) viewed from below (ventral; developmental stage 25 as given by Witschi). Note the coiled intestine visible through the belly skin. (From Witschi 1956, p. 80.) 13 Do Frogs Come From Tadpoles? How long a tadpole lives before it metamorphoses into a frog is dependent on the species and on outer conditions. A wood frog tadpole (Rana sylvatica), for example, usually metamorphoses into a froglet within two or three months after hatching in the northeastern United States. The time is shorter when there are higher water temperatures and ample food, and longer when there is colder water and little food. Bullfrog tadpoles (Rana catesbeiana) grow large—often around four inches (10 cm) long—and, depending on circumstances, can sometimes metamorphose into frogs in the fall (four to five months after hatching). But, more typically, they live as tadpoles for two to three years before metamorphosing.

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