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| Alan de Queiroz, The Monkey's Voyage: How Improbable Journeys Shaped the History of Life. New York: Basic Books, 2014. Pp. 360. Bibliography and Index. $27.99 (hardcover). |
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Supremely informative and entertaining, this book will appeal to anyone who does history on the grand scale. It is a personal account of the author's growing understanding of the field of historical biogeography, a branch of evolutionary biology that has made dramatic advances in the last half century. Two great breakthroughs, plate tectonics in the 1960s and molecular biology beginning in the 1980s, have effectively created verifiable knowledge of life on Earth set in deep time. Historians already familiar with Alfred Crosby's blockbuster, Ecological Imperialism (1986), recognize plate tectonics from his "crossing the seams of Pangaea," which was the massive transfer of plants, animals, insects and microorganisms between the New World and the Old beginning c. 1492. This era historical biogeographers call the homogenocene, when humans intervened and accelerated the dispersal of life-forms, which began with the development of agriculture some 10,000 years ago. But before agriculture or even before humans, how far back in time did biological transfers reach? What is or was the natural biological condition of a particular world area before human intervention? This book takes the reader to the epochs before human existence. We learn that life-forms have crossed the oceans, thus the seams of Pangaea, many times and in many directions since the continents drifted to their present position beginning some 100 million years ago. His introduction sets a personal tone, beginning with a description of a children's map, "The World of Wild Animals" that hangs in the de Queiroz home. This frames the prime question: how and when did life-forms come to be distributed around the world, and why do two or more distant places have the same or similar species? Why for example are there baobab trees in Madagascar, Africa, and Australia, or crocodiles on all the major land masses of the southern hemisphere? One possible answer is that they were all of a kind on the ancient megacontinent of Pangaea; then plate tectonics split the continents which then drifted over millions of years to their present position, carrying living things with them. Or did some life forms somehow improbably fly, swim, or float on rafts of driftwood or clumps of vegetation across vast oceanic expanses? The first explanation is vicariance, and it prevailed in historical biogeography from the 1960s, when plate tectonics became established theory. The second is dispersal, which since the 1980s has gained credence from the new science of molecular biology. The author begins by telling of his early career experiences with garter snakes in the Baja California. The Baja peninsula separated from what is now the Mexican mainland between eight and four million years ago. Garter snakes on the mainland are distributed in rivers, canals and mangrove swamps along the coast. A small population of them became established on the southernmost tip of the peninsula, probably within the past few hundred thousand years. DNA chronology proves when and therefore how they got there. There are no garter snakes on the Baja north of the tip, proving they did not slither all that distance. The Earth did not move. They did, somehow rafting or swimming across the narrow Sea of Cortés between the peninsula and mainland. The rest of the book describes a clash between biologists supporting one or the other explanatory mode. It is a classic piece of dialectics, which he believes resolves as a "recent sea change in biogeography, from a view dominated by vicariance to a more balanced outlook recognizing that the natural dispersal of organisms across oceans and other barriers is also hugely important" (15). The first of four sections begins by tracing the origins of evolutionary biology to the works of Charles Darwin and Alfred Russel Wallace in the 1850s. The geographic variation they observed in the distribution of plants and animals became a critical catalyst to development of the theory of natural selection as they wondered how life forms found their way across seas, mountains, deserts, or great rivers. About 1910 Alfred Wegener realized that continents and their continental shelves fit together like pieces of a puzzle, with due allowance for island fragments. Islands turn out to be important because many are of recent origin, some having popped up from the ocean floor recently enough to be seen and recorded by humans. Some observers imagined land bridges to islands where there is no geological evidence for them, but dispersals across oceans became generally accepted through the 1940s and 1950s. De Queiroz explains how vicariance became the dominant paradigm by the late 1960s, apparently with considerable animosity between the conflicting groups of biologists. First, the theory of plate tectonics became definitively established in geology through mapping of the ocean floors in the 1950s and 1960s. Then some biologists became enthusiastic about a technique for constructing cladograms, which are branching diagrams that demonstrate and measure the ancestry of related species. Although plate tectonics is not necessarily related to cladistics, the supporters of cladistics intent on rejecting the murky and impressionistic case for dispersals seized on drifting landmasses to explain how the same or related species became distributed around the world. Section two digresses to explain how molecular biology developed and how it measures time by mutations in DNA, allowing one to determine not only when but also where new species evolved. Molecular clocks, as they are called, produce only a rough chronology because rates of mutation vary by taxa and are not constant over time, despite occasional assertions as to their constancy. Species with short generations, for example, mutate more rapidly than long-living species, and changes ultra-violet radiation over time affect mutations. Nevertheless it is accurate enough when utilizing technical correctives, which de Queiroz explains, to show ages far too young by tens of millions of years for most plant and animal distributions to be attributable to breakup of Gondwanaland (the southern part of Pangaea that most interests biogeographers). Section three consists of a pile of examples, starting with the obvious and less distant dispersals, and moving to the less and less probable. A chapter, "The Green Web," shows how plants have populated oceanic islands more readily than animals. The cobra bobo is another. It is a legless burrowing amphibian originally from the eastern Congo basin that somehow washed to sea in a flood of fresh river water and found its way to Sao Tomé, an island less than thirteen million years old, never part of Africa, and 130 miles from the mainland. Miraculous and the book's centerpiece, was the transfer of monkeys from Africa to South America. Imagine a monkey, a pregnant female perhaps, or a small family of monkeys clinging to a clump of trees rooted in a chunk of earth torn by flood waters from the bank of a great African river, maybe the Congo, swept out to sea and propelled by the prevailing winds to the coast of South America. Floating islands like this have been observed on the open Atlantic Ocean (112), or in Southeast Asian archipelagoes (198), hundreds of them in fact reported in recent centuries. One or more of them almost certainly brought ancient monkeys sometime between fifty and thirty million years ago, and from them descended all the monkeys of the New World. Almost impossible to believe, yet de Queiroz and the experts he cites are convincing. Section four comprises the author's reflections on the development of knowledge in the biological sciences. He believes that something like a Kuhnian revolution has taken place and that a new paradigm has arrived. He also suggests that appreciation of time in its measureable depths contributes enormously to our understanding of the Earthly web of life: "one can hardly understand the history of living things on Earth and the nature of the modern world without recognizing the influence of species that have crossed oceans naturally, without the aid of humans" (285). Intriguing in itself, the book works also as a translator, a kind of Rosetta Stone for reading historical biogeography, otherwise shielded against the general reader by the bewildering terminology of cladistics. More importantly, de Queiroz gives the deep background, a perspective from which to measure the significance of human transoceanic voyages as a biological force. The historian accustomed to a tiny slice of time, since civilization or since Columbus, is amazed by the millions of years that biogeography requires as its scope, and cannot help but wonder if plants and animals crossed vast oceans to colonize islands and continents, why should human voyages seem so remarkable. If monkeys could cross the Atlantic by accident, why not pre-Columbian Africans, or Polynesians cross the Pacific to South America, and what's so impressive about Columbus or Magellan? Accessible and fascinating, this book makes a great eye-opener for high school or college students, possibly when studying oceanic voyages, or in a Big History course. It certainly would stimulate discussions. Paul V. Adams is professor emeritus of history, Shippensburg University, and past president of the regional Mid-Atlantic World History Association (1998). He is currently completing a book on the Philippines in world history set in deep time. He can be reached at hist.totale@gmail.com.
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