Peter S. Ungar - Evolution’s Bite: A Story of Teeth, Diet, and Human Origins

EVOLUTIONS BITE

Evolutions Bite

A STORY OF TEETH, DIET, AND HUMAN ORIGINS

PETER S. UNGAR

PRINCETON UNIVERSITY PRESS

PRINCETON AND OXFORD

Copyright 2017 by Princeton University Press

Published by Princeton University Press,

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In the United Kingdom: Princeton University Press,

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All Rights Reserved

ISBN 978-0-691-16053-5

Library of Congress Control Number: 2016960239

British Library Cataloging-in-Publication Data is available

This book has been composed in Garamond Premier Pro

Printed on acid-free paper.

Printed in the United States of America

1 3 5 7 9 10 8 6 4 2

For DIANE, MAYA, & RACHEL

CONTENTS

ACKNOWLEDGMENTS

Much of what I have learned from my mentors, colleagues, and students has found its way into this book. Thank you all. I am grateful to those who read bits and pieces of text, and especially to those generous enough to share their stories. These include Leslie Aiello, Henry Bunn, Thure Cerling, Andrew Cohen, Alyssa Crittenden, Fuzz Crompton, Raymond Dart, Peter deMenocal, Dean Falk, Ann Gibbons, Ken Glander, Fred Grine, Kristen Hawkes, Lyman Jellema, Cliff Jolly, Rich Kay, Joanna Lambert, Leo Laporte, Clark Larsen, Julia Lee-Thorp, Daniel Levine, Peter Lucas, Mark Maslin, Scott McGraw, Andrew Moore, Dani Nadel, Mike Plavcan, Rick Potts, Melissa Remis, Mike Richards, Philip Rightmire, Jerry Rose, Pat Shipman, Becky Sigmon, Matt Sponheimer, Christine Steininger, Bob Sussman, Mark Teaford, Francis Thackeray, Phillip Tobias, Elisabeth Vrba, Alan Walker, Malcolm Williamson, Richard Wrangham, and Barth Wright. I also thank Richard Alley, Henry Bunn, Thure Cerling, Jennifer Clark, Peter deMenocal, John Fleagle, Fred Grine, Alain Houle, Julia Lee-Thorp, Scott McGraw, Andrew Moore, Dani Nadel, Dolores Piperno, Rick Potts, Jerry Rose, Matt Sponheimer, Bob Sussman, and James Zachos for their kind permission to use figures or data used in generating imagery for this book. I thank the two anonymous reviewers for their thoughtful comments and suggestions, Susan Ungar, for help with proof reading, Maia Vaswani for her excellent work copyediting this book, and Alison Kalett, Jenny Wolkowicki, and the rest of the staff of Princeton University Press for their support and encouragement throughout the process.

EVOLUTIONS BITE

INTRODUCTION

We hold in our mouths the legacy of our evolution. Nature has sculpted our teeth over countless generations into tools adapted to chew the foods our ancestors had to eat to survive. And paleoanthropologists, those of us who study human origins, spend a lot of time thinking about them. Teeth are our bridge to the past. They allow us to track changes from one species to the next to trace our evolution. Yes, we have fossilized skulls and skeletons to work with too, but teeth are special. They are essentially ready-made fossils that have remained virtually unchanged for millions of years. More important, they are the most commonly preserved part of the digestive system, and the key to unlocking the diets of our ancestors. We look to tooth size, shape, pattern of wear, and chemistry to work out details of the foods eaten by long-gone species.

Because we can use teeth to reconstruct diet, they are also the key to unlocking an extinct species place in nature. In Love and Death, Woody Allens character Boris Grushenko described nature as big fish eating little fish, and plants eating plants, and animals eating an He continued, Its like an enormous restaurant the way I see it. I prefer to think of nature as a buffet: animals can pick and choose from among the living things in whatever part of the biosphere they inhabit. Items on this biospheric buffet are constantly being swapped in and out with changing environmental conditions. For example, fruits and leaves are replaced by grass roots and tubers as forest gives way to savanna when and where the climate becomes cooler and drier. Different habitats mean different options, choices, and relationships between a species and its environment. In short, a species choices help define its relationships with other organisms, both eaters and eaten, and its place within the larger community of life that surrounds it.

This book tells a story of teeth, diet, and human origins. My goal is to show that we can use teeth to understand the diets of our ancestors, and, by extension, our place in nature and how we came to be the species we are today. Central to this story are the effects of climate and environmental change on our ancestors, a story only now coming into focus as scientists begin to understand that our success as a species is due, in no small measure, to how our ancestors dealt with an increasingly variable and unpredictable world in the distant past.

When we bring together these new insights from Earth system science and paleoclimate studies, along with new approaches to how teeth work and new discoveries in paleontology, primatology, archaeology, and other fields, we arrive at a more complete view of life in the past and how it changed over time. The story used to be simpler. The spreading savanna coaxed our ancestors down from the trees, and the challenges it brought made them human. It is now becoming clear, however, that environmental conditions actually swung back and forth between wet and dry in the past. This fluctuation winnowed out the pickier eaters among us, leaving only those flexible enough to find something with which to fill their plates from an ever-changing biospheric buffet. We are the most versatile of primate species, able to find something to satiate us no matter where we roam. That explains how we came to take over much of the world. Climate change provided the motive, and evolution offered the opportunity to make us human.

I decided to write this book to help me see the big picture in human evolution, and to share what Ive learned from being involved in the work for the past three decades. But as I began to think about it, it became clear to me that theres much more to the tale than the science itself; theres also the passion, ingenuity, and determination of those who gave us the knowledge we have. They make the story compelling and bring it to life. And so, in the chapters that follow, we travel around the world, visiting with my colleagues and other scientists along the way. We look over their shoulders as they make their discoveries and chart new paths to understanding the past.

We begin with teeth, how they work and how they are used. If we assume that nature selects the best tools for the job, animals with different diets should have teeth to match. Understanding how teeth work is the first step in figuring out relationships between dental form and function. And working out those relationships is fundamental to using teeth to reconstruct diets of fossil species. But its not enough. We know from watching living primates in their natural habitats that food choice is about much more than what an individual is capable of eating. The difference between how teeth work and how they are used is key to understanding diet, place in nature, and ultimately evolution. This is our point of departure from business as usual in paleontology, the assumption that animals are fated to specialize on the foods to which their teeth have evolved. Yes, every species is limited by its anatomy in what it can eat, but its equally important to remember that the dishes on the table vary from place to place, and that they get swapped out from time to time. In other words, food choice is not just about dietary adaptation but also about availability. When the options change, so too does diet and, along with it, the relationship between an organism and its environment.