The Monkey's Voyage
Page 1
The
Monkey’s Voyage
The
Monkey’s Voyage
Alan de Queiroz
New York
Copyright © 2014 by Alan de Queiroz
Published by Basic Books,
A Member of the Perseus Books Group
All rights reserved. Printed in the United States of America. No part of this book may be reproduced in any manner whatsoever without written permission except in the case of brief quotations embodied in critical articles and reviews. For information, address Basic Books, 250 West 57th Street, 15th Floor, New York, NY 10107-1307.
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Library of Congress Cataloging-in-Publication Data
De Queiroz, Alan.
The monkey's voyage : how improbable journeys shaped the history of life / Alan de Queiroz.
p. cm.
Includes bibliographical references and index.
ISBN 978-0-465-02051-5 (hardcover) -- ISBN 978-0-465-06976-7 (e-book) 1. Animals--Dispersal. 2. Plants--Dispersal. 3. Biogeography. I. Title.
QH543.3.D42 2013
570--dc23
2013036248
10 9 8 7 6 5 4 3 2 1
For Tara, Hana, and Eiji
CONTENTS
Introduction: Of Garter Snakes and Gondwana
SECTION ONE:
EARTH AND LIFE
Chapter One: From Noah’s Ark to New York: The Roots of the Story
Chapter Two: The Fragmented World
Chapter Three: Over the Edge of Reason
Chapter Four: New Zealand Stirrings
SECTION TWO:
TREES AND TIME
Chapter Five: The DNA Explosion
Chapter Six: Believe the Forest
SECTION THREE:
THE IMPROBABLE, THE RARE, THE MYSTERIOUS, AND THE MIRACULOUS
Chapter Seven: The Green Web
Chapter Eight: A Frog’s Tale
Chapter Nine: The Monkey’s Voyage
Chapter Ten: The Long, Strange History of the Gondwanan Islands
SECTION FOUR:
TRANSFORMATIONS
Chapter Eleven: The Structure of Biogeographic “Revolutions”
Chapter Twelve: A World Shaped by Miracles
Epilogue: The Driftwood Coast
Acknowledgments
Figure Credits
Notes
References
Index
Introduction
OF GARTER SNAKES AND GONDWANA
Science must begin with myths, and with the criticism of myths.
—Karl Popper
I recently put up a large map of the world in our house, ostensibly for our daughter and son, ages five and two, although to this point I’m the only one who’s looked at it much. As something of a map hoarder, if not exactly a connoisseur, I appreciate a map made with care and some measure of creativity, like this one. It’s a standard Mercator projection (the type of map that makes Greenland appear the size of Africa), but beyond that there is hardly anything conventional about it. The continents show no political boundaries and are colored in pale earth tones that blend into each other, the transitions having only the vaguest correspondence with the boundaries of actual biomes. Glass-like fragments depicting sea ice fill the Arctic region, with the smaller pieces cascading southward as if raining down on the rest of the world. The oceans, so often represented on maps as featureless blue expanses, are here pleasingly filled with the topography of the sea floor—the ridges and valleys, the broad plateaus and deep trenches, the gently sloping continental shelves. These characteristics make the map feel dynamic, chaotic, and alive, complementing its most obvious feature, namely, that it’s populated with the painted images of dozens of wild creatures, from iguanas and sperm whales to water buffaloes and birds of paradise.
The map is entitled “The World of Wild Animals,” but, more accurately, it should be “The World of Wild Vertebrates,” and, even within that restricted scope, the coverage is decidedly mammal-centric. Nonetheless, it can serve as an introductory lesson for the budding biogeographer, for the student of how living things are distributed across the Earth. Perusing the map, a fundamental fact of biogeography immediately jumps out: different regions have distinct faunas. That, in fact, is presumably the main intended message of the map. Lions, a giraffe, and an elephant are stacked in a column in Africa; kangaroos hop toward a duck-billed platypus and a frilled lizard in Australia; a family of tigers and a family of pandas cozy up to each other in Asia; penguins are scattered across Antarctica, while the frozen seas of the far north carry puffins and auks, black-and-white birds that look a bit like penguins but aren’t. These sorts of connections between animal and place are known even to small children. (Our five-year-old can recite at least a few of them, even if she can’t consistently identify Africa or Australia on a map.) In time, those children (hopefully) will learn that it is evolution, the great overarching theory of biology, that makes sense of these differences between faunas; the sets of animals are distinct because they have evolved in isolation from each other. The separate landmasses are like different worlds, with long (unimaginably long) independent histories of descent with modification.
There are exceptions to this grand pattern, however, and it is a large part of the business of biogeography to explain these anomalies. On the “World of Wild Animals” map, for instance, we find that both northern North America and northern Eurasia have wolves, moose, and elk, among other shared creatures. These facts do not fit the rule of separate landmasses having distinct faunas, but they’re exceptions that are easily explained: North America and Eurasia were connected at various times in the recent past (most recently some 10,000 years ago, during the last ice age) via the Bering Land Bridge, so the histories of those regions are not as independent as their current separation would suggest.1 Just a moment ago in geologic time, wolves, moose, and elk could pass on solid ground between North America and Asia.
Our children’s map raises other questions that are not so easily answered, however. That’s especially true if one focuses on the landmasses of the Southern Hemisphere. For instance, on our map we see four kinds of flightless birds in the group known as the ratites: a rhea in South America and an ostrich in Africa, facing each other across the Atlantic, and, thousands of miles from these, a herd of emu in Australia and a kiwi poking at the dirt in New Zealand. These four species are clearly distinct from each other, yet, in the grand scheme of things, they are fairly closely related, so how did they end up in these far-flung places, separated by wide stretches of ocean? Similarly, on the map we see a mandrill in Central Africa staring across the Atlantic in the direction of another monkey, a South American capuchin. Again, these species are obviously different, but they are also obviously part of a fairly tight evolutionary group. And again, they present the puzzle of how closely related species can end up on landmasses separated by oceans. Furthermore, in both of these cases, the seafloor topography artfully depicted on our map indicates that the landmasses in question are separated not by shallow shelves, but by deep ocean. This fact adds to the mystery, because it means we cannot invoke movement across a Bering-type land bridge to explain these piecemeal distributions.
As it turns out, the ratites and monkeys are just the
tip of the iceberg. There are southern beech trees in Australia, New Zealand, New Guinea, and southern South America. There are baobab trees in Madagascar, Africa, and Australia. There are crocodiles in most warm parts of the world, including all the major Southern Hemisphere landmasses. There are hystricognath rodents (a group that includes guinea pigs) in South America and Africa. These and many other similar examples collectively make up one of the great conundrums of biology, a riddle that has intrigued naturalists since Darwin’s time (and, in some sense, even before that). What can explain this profusion of far-flung, fragmented distributions? How on earth could a giant flightless bird or a southern beech, with seeds that cannot survive in seawater, cross a wide expanse of ocean?
For most of these cases, the answer, the one that we now find in textbooks, came from geologists more than biologists: the flightless birds and the baobabs, the crocodiles and the beech tree seeds didn’t have to cross oceans, because the oceans weren’t always there. At one time, all the major southern landmasses were part of the enormous supercontinent of Gondwana. However, about 160 million years ago, rifts began to form in the Gondwanan crust, like cracks in an eggshell. The supercontinent began to break up along these fissures, the pieces drifting apart at far less than glacial speed as magma welled up through the crust and spread out as new ocean floor. The Atlantic Ocean Basin formed, pushing Africa and South America apart. Zealandia, a continent including present-day New Zealand, New Caledonia, and other islands, drifted away from a combined Australia and Antarctica, the latter two continents also eventually going their separate ways. India, once attached to Australia, Antarctica, and Africa, famously wandered north and plowed into Asia, forming the Himalayas in the process. This is all part of the worldview of plate tectonics, a theory that, with a flurry of evidence, was swiftly transformed to fact in the 1960s: the Earth’s crust is made of giant plates that carry continents and get pushed around as magma spreads out from rifts in the crust. Continents drift.
The pieces of Gondwana carried with them not just soil and bedrock, but also the animals and plants of the supercontinent—the ratite birds, the crocodiles, the southern beech trees, and countless others. Where once there had been a single, continuous Gondwanan biota, now there were many descendant Gondwanan biotas wandering off to their separate fates. The reality of continental drift means that there is no need to invoke miraculous ocean crossings by flightless birds and southern beech seeds. The plants and animals of the Southern Hemisphere didn’t have to move; the continents moved for them.
The landmasses of the Southern Hemisphere have been called “Gondwanan life-rafts,” a set of giant Noah’s Arks that carry with them to this day the ancient supercontinent’s flora and fauna, albeit transformed by millions of years of evolution. This landmasses-as-life-rafts story is the iconic tale of historical biogeography, the study of how the distributions of living things change through time. It’s the textbook example of how the creation of physical barriers—in this case, seas and oceans—can fragment the distributions of groups of organisms. It’s a story simultaneously so obvious and so elegant that it’s barely worth arguing about.
Or is it?
It’s June 2000. My girlfriend (now wife), Tara, and I have flown to San José del Cabo, near the southern tip of Baja California, and, instead of heading down the coast to party in Cabo San Lucas (where we would have been in our element about like flounders on a freeway), we’ve rented a jeep and driven some thirty miles north into a different world altogether. We’re in a rocky arroyo that drains the eastern slope of a small mountain range called the Sierra de la Laguna, in the company of a few cows and burros, but no people. It’s hot and bright, the forested hillsides brown and bare of leaves in the dry season, the sun glaring off the white boulders and sand of the arroyo.
The two of us are crouching next to a nasty, spiny shrub that someone has sarcastically and misogynistically dubbed a buena mujer. Tara, maybe thinking about now that the nightclubs in Cabo don’t sound so bad after all, is reluctantly gripping the neck of a very large garter snake while I work my fingers down the snake’s body to where it disappears into a hole beneath the shrub. The snake has some kind of purchase underground and I’m pulling her out a fraction of an inch at a time, trying not to wrench her too hard in the process, trying also (and unsuccessfully) to avoid jabbing myself on the buena mujer. The process is exhausting, not because it’s physically difficult, but because we’re fighting against the will of another being; with each pull I feel the snake resisting and I sense her muscles straining and tearing. For all she knows, this is a life-or-death struggle, and she imparts that sense of urgency to our side of the encounter as well. Tara, who’s more afraid of snakes than I am but also feels more empathy for them, is not enjoying this episode.
After ten profanity-filled minutes, we get the snake out. I’ve been studying garter snakes for years and usually find them subtly beautiful, but even I have to admit that this is not a pretty snake. She’s messy looking, mostly black but with ragged, dark brown stripes along her sides, as if someone used the torn edge of a piece of cardboard to draw her pattern. The fact that she’s trying to sink her teeth into me as I drop her into a pillowcase doesn’t help. What this snake lacks in disposition and looks, though, she makes up for in other ways. For starters, she’s one of the biggest garter snakes I’ve ever seen. Back home, when we measure her, she turns out to be almost three-and-a-half feet long, huge for a garter snake and the largest specimen of her subspecies ever recorded, a bit of trivia worth a paragraph-long note in a herpetological journal. I end up using her, along with other snakes caught on this trip, in experiments showing that members of her species change the way they forage depending on the depth of the water, a shift that may mirror the way their feeding behavior has evolved. This snake also turns out to be pregnant and, two months later, she will give birth in the lab to a dozen tiny black garter snakes, all much prettier than their mother.
My real reason to remember this snake now is not her size or her offspring or her foraging behavior though—it’s her location, the fact that she came from southern Baja California. The distribution of her species, Thamnophis validus, is what got me thinking about organisms catching rides on drifting tectonic plates. It’s why I began thinking about the fracturing of Gondwana.
I.1 A garter snake, Thamnophis validus, from the Sierra de la Laguna, near the southern tip of Baja California. Photo by Gary Nafis.
Baja California is not one of the Gondwanan fragments, but its geologic history is reminiscent of the breakup of the southern supercontinent. At one time, the peninsula of Baja California was just another part of the mainland. No sea separated Baja California from the rest of Mexico, so many terrestrial species must have inhabited both what is now the southern part of the peninsula and the adjoining part of mainland Mexico; there was nothing to stop a mouse from walking (or a seed from being carried by a mouse) from the one place to the other. However, between 4 and 8 million years ago, a crack in the Earth’s crust began to form, a fissure between Baja California and the mainland. This rift is at the same border between tectonic plates as the San Andreas Fault, along which the Pacific Plate moves northwest and the North American Plate slides southeast, generating countless California earthquakes. In Mexico, instead of plates sliding past each other, that rift formed and grew wider and wider until, at some point, the fissure broke through to the Pacific Ocean, and seawater poured into the gap, creating the Sea of Cortés.2 In other words, Baja California is part of another “life-raft,” although the raft is still moored at its northern end to the continent. Biologists who study this region believe that when the Sea of Cortés formed, many kinds of animals and plants were isolated on the peninsula, creating odd cases in which populations in southern Baja California have their nearest relatives on the other side of the sea. In western Mexico, then, it’s as if we are catching the breakup of Gondwana in a very early stage, with Baja California playing the part of one of the smaller continenta
l fragments, like Madagascar or New Zealand.
Our dark garter snake, T. validus, is one of those species that occurs both in Baja California and across the Sea of Cortés on the Mexican mainland. These snakes are found in the slow rivers, irrigation canals, and mangrove swamps of the coastal plain along most of the western edge of the mainland, but in Baja California they occur only near the southern tip, mostly in the rocky arroyos of the Sierra de la Laguna. T. validus is one of the species that supposedly caught a ride on the peninsula as it drifted away from the continent (see Figure I.2).
This “incipient life-raft” story is a compelling hypothesis for the distribution of T. validus, but nobody had ever collected the critical genetic data to test it. Robin Lawson, a fellow herpetologist and evolutionary biologist, and I decided to do just that. Between us we took two more trips to Mexico, and, with the help of Tara, my graduate student Matthew Bealor, and an amateur snake enthusiast named Phil Frank, we collected T. validus specimens from sites spanning about eight hundred miles of Mexico’s west coast, from Sonora to Michoacán. Then we sequenced some of the genes of these garter snakes along with the ones Tara and I had collected in the Sierra de la Laguna.
The results were clear and striking: the Baja California snakes were genetically almost identical to some of their mainland counterparts. The genes we were looking at—genes in the mitochondria that code for proteins—evolve very quickly. Thus, if the peninsular snakes had been isolated from mainland snakes for several million years, as the landmass-as-life-raft hypothesis required, the genes of the two groups would have become quite different from each other. The fact that they were instead nearly identical had a clear implication: the life-raft hypothesis, based on the slow movement of tectonic plates, could not explain why T. validus is in Baja California.