This scenario is rife with points at which the successful colonization could have been derailed. For starters, the monkeys might easily have chosen a different tree in which to rest. On its journey downriver, their raft of earth and vegetation might have meandered along some other course in the current, and thus perhaps could have ended up caught in a tangle of flood debris. Reaching the delta, the raft could have run aground on a sandbar, as many such rafts do today at the mouths of great rivers. The exact path across the ocean might have been different, depending on exactly when and where the raft picked up the westward current, and some of those alternate routes might have ended with the monkeys dying of thirst or starvation before ever making landfall. Finally, even if the monkeys reached South America, their tiny population could easily have disappeared, as extremely small populations often do, from simple bad luck: the only female in the group, for example, might have succumbed to disease, or been picked off by a predator. In short, the success of the colonization likely depended on a whole series of occurrences, any one of which could easily have gone a different way. And similar fortuitous sequences probably apply to nearly all cases of long-distance, chance dispersal—that is what makes them “chance” events—from seeds hooked onto a bird’s feathers to moths caught up in the wind to crocodiles riding ocean currents. In essence, these colonizations and their subsequent impacts hinge on events akin to the child running into the street at exactly 2:13 p.m., not ten seconds earlier or ten seconds later.
If one is still skeptical of the chance nature of these events, consider, finally, that many ocean crossings probably depend to a large extent on that archetype of unpredictable phenomena, the weather. In particular, many transoceanic colonizations likely occurred through the agency of severe storms, which, as we all know, are impossible to accurately forecast even a few days before they happen. Furthermore, current thinking about how weather is generated suggests that long-range forecasting is not only impossible given our current means of measuring the important variables (the distributions over the landscape of atmospheric pressure, wind velocity, temperature, and so forth), but is likely impossible in principle. The key finding here, from quantitative weather models, is that the generation of storms and other weather phenomena shows “sensitive dependence on initial conditions.” In other words, tiny differences in starting values (of pressure in one particular spot, for instance) can produce huge changes at some later time. This is the famous “butterfly effect”: a butterfly flapping its wings in Brazil, it is said, might set off a tornado in Texas. For our purposes, what this finding suggests is that the development of any particular storm is the epitome of a chance event, the kind of event that could be erased by a tiny perturbation in a replay of history. In fact, a “jot or tittle” in the Jurassic atmosphere could change the details of all subsequent weather.65 Those changes, in turn, would mean massive alterations in the history of long-distance dispersal.
In short, the means by which ocean crossings and other long-distance colonization events take place suggest that they are fundamentally unpredictable. Forget about Pikaia and the other creatures of the Burgess Shale; the shaping of life’s history by oceanic dispersal is a much more compelling illustration of the chance nature of that history. Run the tape again (starting, let’s say, from 50 million years ago), and South America would have no monkeys, caviomorph rodents, xenodontine snakes, or Renealmia ginger plants, but it would contain many lineages that never made it there in the history that we know. Similar changes would apply over every other part of the globe.
None of this is to say that chance dispersals are the only or even the main unpredictable events that have shaped the history of life. In fact, chaos theory—a research area that focuses on all kinds of systems that show “sensitive dependence on initial conditions”—suggests that such sensitivity is a property of many biological phenomena. For example, fluctuations in population size and the rate of spread of an infectious disease should be affected by small perturbations. Such phenomena almost certainly influence the long-term history of life. For instance, a small, chance perturbation that ultimately caused a population to go extinct (a fluctuation to a population size of zero) might have major ramifications. Similarly, a particular mutation (that is, a particular base-pair change at a particular site in a particular gene at a particular time in a particular individual’s germ line), the quintessential sort of chance event in biology, no doubt sometimes influences the grand sweep of evolution. For example, a mutation that produced a new advantageous trait could dictate the future course of evolution for a species, which could, in turn, influence the fates of many other species.
Nonetheless, among all these possible small, unpredictable perturbations with large effects, ocean crossings and other chance colonizations stand out. What sets this class of events apart is that we can identify individual instances of them and at least the most obvious of their far-reaching consequences. Strange though it may seem, this is not true even for mutations. Most kinds of mutations are actually common enough that, in a replay of the tape in which a particular mutation (that is, in a particular individual organism) did not occur, the exact same kind of mutant likely would already be present in the population or would soon turn up. Thus, at best we usually can only point to a kind of mutation—one that changes an alanine to a threonine at amino acid site 269 in an opsin gene, for instance—that has had a major impact. In that sense, mutations are more akin to “normal” rather than “chance” dispersal. They are analogous to, say, a common land-bird species colonizing an island very close to the mainland; replay the tape and the colonization still occurs, even if the exact same individuals are not involved this time around.66 Cases like monkeys or caviomorph rodents crossing the Atlantic are immune to that sort of argument because of their extreme rarity and improbability; rerun history and, almost without a doubt, monkeys and caviomorphs do not colonize the New World at all. This means that we can point to these events as the particular ones that have changed history. Furthermore, we don’t need to strain to imagine the consequences of these ocean crossings. Traveling through Latin America, the results are plain to see (and hear) in the form of species that would not exist without these colonizations—a troop of squirrel monkeys, noisily chattering as they move through a rainforest in Amazonia; the bell-like calls rising from the burrows of tuco-tucos on the Argentine pampas; a herd of capybara splashing through a flooded field in the Brazilian Pantanal. As testimonies to life’s unpredictability on a grand scale, there can hardly be examples more telling. They are the embodiment of Gould’s notion of history’s sensitivity to tiny perturbations.
For some, the view of life described here may seem disheartening. We discovered the geographic distributions of all kinds of organisms, a rich record of past life in the rocks, the changing configurations of continents and oceans, the means to reconstruct evolutionary trees and to use molecular data to turn them into timetrees, and what did it all tell us? That the course of life on Earth has been frequently buffeted, deflected into new pathways, by the random and improbable. That many of the key events in this history are essentially inexplicable in the sense that they cannot be derived from any natural laws (such as “Earth and life evolve together”). That these events are impossible to fully reconstruct. We may infer, for instance, that monkeys crossed the Atlantic sometime between 26 and 51 million years ago, but we will never know exactly how they did it.
This is not to imply that there are no generalizations that can be derived from studying chance dispersal. In fact, we have run across several of these, such as the inference that vertebrates have crossed the Atlantic and the Mozambique Channel in the directions predicted by ocean currents, and the deduction that nonaerial animals have much more trouble colonizing Hawaii from the Americas than from the Indo-Pacific. In fact, one of the obvious consequences of the renewed interest in long-distance dispersal is that many scientists are trying to discover such tendencies, retrieving patterns from the seemingly random. However,
this doesn’t change the fact that individual instances of successful dispersal and establishment are fundamentally unpredictable. Even if, after the fact, it supports a general tendency, the monkey’s voyage was a fluke.
Nonetheless, there is wonder in this view when we consider the immensity of nature it implies. Ocean crossings and other chance colonizations are outcomes of a game of almost unimaginably large numbers. Seen in isolation, they may seem mysterious and miraculous, but, taken in proper context, that mystery disappears and is replaced by understanding. These occurrences are like the man struck twice by lightning, in his own eyes an act of God or the Devil, but an event with high probability when the whole human population is taken into account. Similarly, the many seemingly implausible colonizations remind us that we are living in a tiny slice of a deep history, a history acted out over many millions of years, with a vast array of living organisms as the players, moved by their own powers, by inexorable ocean currents, by storms beyond count. We may be surprised by the notion that monkeys once crossed an ocean; we should be in awe of a history in which events of that sort (although not that particular event) are inevitable. The large number of these colonizations tells us that, in the long history of this living world, the miraculous has become the expected.
61Nathan Nunn and Nancy Qian generated these figures in a 2011 study by comparing population growth for regions where potatoes became an important crop to those for regions where it did not.
62The procyonid that colonized South America before emergence of the isthmus is thought to have given rise to several species, including one that was the size of a bear, but all of these lineages apparently went extinct by the end of the Pliocene (Koepfli et al. 2007).
63In The Voyage of the Beagle, Darwin wrote, “Considerable tracts of country are so completely undermined by these animals [tuco tucos], that horses, in passing over, sink above their fetlocks” (Darwin 1839, 79).
64Some paleontologists also have argued against Gould’s view of the Burgess fauna as representing an exceptional breadth of distinct animal forms, most of which are unrelated to later lineages. Interestingly, one of these paleontologists is Simon Conway Morris (1998), who initially agreed with Gould’s view and whose work formed the basis for much of what Gould concluded about the early evolution of animals.
65This is not to imply that such changes would affect climate, the long-term pattern of weather for an area.
66The effects of single, rare mutations have been demonstrated in experiments in which bacteria that were initially genetically identical were propagated as separate populations, and only some of the populations evolved specific genetic adaptations for using a food source. Presumably such mutations have had major effects on the large-scale history of life, but it would be difficult to point to an actual case, that is, to identify a single mutational event that occurred millions of years ago and dramatically changed the course of evolution. See Blount et al. (2008).
EPILOGUE: THE DRIFTWOOD COAST
It is a misty July morning on the Oregon coast. There is no horizon, just a blending of dull sea and dull sky. Inland lies a dark forest of Sitka spruce, western redcedar, and Douglas fir. Tara and I and our kids, two-year-old Eiji and almost-five-year-old Hana, are in the middle of a long road trip from Reno across California and up the coast to northern Washington, a journey that has become a lazy exploration of beach after beach—long, serene days spent flying kites, building and destroying sand castles and fairy houses, chasing gulls, chasing the waves, tending to scraped knees and small disappointments.
Scattered on the beaches is a profusion of organic debris, a disorganized museum of the recently or not-quite deceased. On this trip we have seen or will see giant strands of orange kelp, a pelican skull, the mysterious mustard-colored and grub-like husk of some unknown crustacean, disembodied gull wings, mussel shells, clam shells, abalone shells, bleached sand-crab skeletons gathered into detrital ribbons by the waves, dried-out thistle stalks, the desiccated corpse of a fledgling murre.
By far the most obvious debris is driftwood. It’s everywhere, from finger-sized pieces that Hana uses in her fairy houses to the trunks of large trees. To the kids’ delight, people have built driftwood lean-tos, driftwood tepees, driftwood forts, whole driftwood villages. In places, the ocean has tossed up great chaotic piles of the stuff, arrayed along the beach like mangled pike fences, as if the continent were defending itself against invasion from the Pacific. I’m amazed by the sheer volume of driftwood, but I later learn that these massive accumulations only hint at the amounts that washed up on these beaches before Europeans came, before people began in earnest to log the forests and clear the rivers for navigation.
On the beach at Carl G. Washburne Memorial State Park, I’m following the line of high water, looking, not quite idly, under any debris light enough to lift. I take hold of one four-foot-long piece of driftwood, charred from a campfire, and flip it over. On the underside are half a dozen mottled, grayish brown insects, ensconced in crevices and hollows in the blackened wood. I recognize them immediately (I was looking for them): they’re jumping bristletails, and I’m fairly sure they’re Neomachilis halophila, the mainland relative of the Hawaiian bristletails that John, Cheryl, and I have been studying. (Weeks later, with a dissecting microscope, I find that these Oregon specimens have only a single pair of water-absorbing vesicles on the underside of each abdominal segment, confirming their identity.) Helmut Sturm’s notion that bristletails could have reached Hawaii as eggs attached to driftwood jumps into my head. Even though I haven’t found the eggs, Sturm’s idea is suddenly transformed from the abstraction of words on a page to tangible form: here are his bristletails actually clinging to driftwood. And there is certainly no shortage of this kind of debris. I keep looking, turning over other pieces, and find several more N. halophila.
Later, Eiji joins me, not quite helping to find insects, but adding his toddler’s enthusiasm to the search. He stomps. He dances. He squeals about earwigs and bristletails (he calls them “briss-oh-tayuhs”). Already, he seems to have developed a visceral appreciation for small, creeping things. Half of my mind is with him, excited to see whatever turns up in our informal biological survey of the beach at Washburne State Park. The other half, though, is thinking about those Neomachilis drifting on the ocean, about the wondrous means by which the history of life has unfolded.
A little ways south of us, rocky headlands rise out of the mist. To the north, the nearly deserted beach stretches on for miles, making the two children appear even smaller than they are. From the west, the endless ranks of gray waves curl and crash to foam then slide hurriedly in thin sheets over the sand.
Beyond the waves, the horizonless ocean seems infinite, an impossibly wide barrier, yet I know that, in the fullness of time, many living things have crossed it.
ACKNOWLEDGMENTS
It’s a great pleasure to thank the large crowd of people who have helped me complete this book, including friends, family, research collaborators and other colleagues, and quite a few scientists whom I know only through phone conversations and email correspondence.
For critiquing chapters—an enormously important part of the process—I’m grateful to Merrill Peterson, Carol Yoon, Peter Wimberger, Chris Feldman, Marjorie Matocq, Peter Murphy, Angela Hornsby, Brandi Coyner, Mitchell Gritts, John Measey, Mike Pole, Matt Lavin, Mott Greene, Susanne Renner, Patrick O’Grady, Karen de Queiroz, Sean de Queiroz, Jade Keehn, Sarah Hegg, Jason Malaney, Guanyang Zhang, Heather Heinz, Joseph Collette, Eric Gordon, David Haisten, Derek O’Meara, Eric Stiner, and Rebecca Swab. I owe a special debt to my wife, Tara de Queiroz, and my friend and research collaborator John Gatesy. Both gave constructive and supportive feedback on the entire manuscript—Tara in a relatively calm way, John in his usual biting, provocative style.
I interviewed or more informally chatted with many people, mostly scientists, to gain their insights
, to check facts, and to provide more of a human touch to the narrative. For extensive telephone or email interviews I’m grateful to Anne Yoder, Dennis McCarthy, John Briggs, Michael Donoghue, Michael Heads, Gary Nelson, Matt Lavin, Susanne Renner, the late Robert McDowall, John Measey, Miguel Vences, Bob Drewes, Mike Pole, Dallas Mildenhall, Isabel Sanmartín, Steve Trewick, and Jorge Crisci. In this context, I especially want to thank Michael Heads, Dennis McCarthy, and Gary Nelson, who all knew that I disagreed mightily with their scientific views yet were unfailingly cooperative in answering my questions. For shorter correspondence or conversations I thank Norm Platnick, Andreas Fleischmann, Hamish Campbell, Ellen Censky, David Krause, Rob Meredith, Sean de Queiroz, Kevin de Queiroz, Jerome Salador, Mike Crisp, Nicolas Vidal, Patrick O’Grady, Peter Wimberger, Chris Feldman, Ben Normark, Rudolf Scheffrahn, Greger Larson, Graham Wallis, Jimmy McGuire, Cheryl Hayashi, and Steve Montgomery.
Any book about science builds on an enormous amount of previous work and, in that sense, the reference list can be viewed as part of the acknowledgments. However, I also want to single out five books on which I leaned especially heavily for information and/or inspiration: Janet Browne’s The Secular Ark; David Quammen’s The Song of the Dodo; the late David Hull’s Science as a Process; Mark Lomolino, Dov Sax, and James Brown’s edited volume Foundations of Biogeography; and Blair Hedges and Sudhir Kumar’s edited volume The Timetree of Life. This book would have suffered if those five others had not existed. I’m also indebted to Andreas Fleischmann, John Gatesy, Patrick O’Grady, Mike Crisp, Michael Heads, Gary Nelson, Jason Ali, Susanne Renner, and the late Robert McDowall for providing important unpublished manuscripts or results.
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