The Monkey's Voyage

by Alan de Queiroz

  In 1846 the blight reached its peak in Ireland, destroying some three-quarters of the potato crop, and with it the main source of nutrition for nearly all poor Irish. The toll on the population was devastating: it is estimated that, in three years, a million people died of starvation or, in a weakened state due to malnutrition, succumbed to typhus, cholera, and other diseases. At the same time, thousands of the living, deprived of essential nutrients, went blind or insane. Within a decade, the Great Famine had pushed some 2 million people out of Ireland, including more than half a million to the United States, radically changing the history of that country as well; if you’re an American with Irish blood, you probably owe your existence in a very direct way to the introduction of the potato and the potato blight oomycete to the Old World. In Ireland itself, the famine was such a cultural and political watershed that historians often divide the country’s history into prefamine and postfamine periods.

  Similar history-changing stories could be told about the ocean-­crossing potato in other countries where it eventually became a staple crop—Russia, Germany, China, and India, for example. One can make a strong case, for instance, that potatoes fueled the ascendance of Russia and Germany as world powers in the nineteenth century. More generally, it has been estimated that about a quarter of the rise in population that took place in the Old World from 1700 to 1900, from roughly 600 million to about 1.5 billion people, can be attributed to the adoption of potato farming.61 In other words, by 1900, the introduction of this one species had added some 200 million people to the Old World. Potatoes, in short, propelled world history into a radically different course from what would otherwise have been. Population explosions, famine, mass immigration, and the rise of world powers were all brought about because S. tuberosum came across the ocean in Spanish ships.

  The potato is obviously somewhat extreme in its influence, but countless other introduced species that humans have purposefully or inadvertently transported across oceans have had massive effects as well, from tomatoes, rubber trees, and cheatgrass to rats, pigs, and gypsy moths to the microorganisms that cause malaria, smallpox, and yellow fever. These human introductions have been so numerous, and their consequences so world-altering, that the era beginning with Columbus’s discovery of the New World has been called the “Homogenocene,” referring specifically to the fact that movements of species blend formerly distinct biotas, tending to homogenize them into a single, uniform global biota. Whatever we want to call this era, it seems abundantly clear that one can hardly understand human history and the nature of modern environments without recognizing the influence of the flood of species that we have moved across oceans.

  A strong message of the biogeographic studies presented in this book is that a parallel statement holds for the deeper past: 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. Quantifying the full effects of these chance ocean crossings with any accuracy isn’t possible, any more than it’s possible to know, for instance, all the historical ramifications of the introduction of potatoes to the Old World. History is simply too complex for such an undertaking. Nonetheless, just as we can infer, at a minimum, that the introduction of potatoes had a huge effect, we can also infer that the influence of natural ocean crossings has been enormous. This influence has been so great, in fact, that I can only hint at its scope.

  CHANGES IN SHALLOW TIME

  Imagine an apparently unremarkable day in the life. In the morning, your wife says she’s stopping at the grocery store on the way home from work and asks if there’s anything she should pick up. “Can you get some nutmeg?” you ask. “I need some for a cake.” It’s summer and you dress for the heat, which means more or less entirely in cotton—cotton T-shirt, cotton shorts, cotton underwear, cotton socks, and a cotton baseball cap when you go outside. After breakfast, you putter around with your daughter and son in the vegetable garden to see what’s ripe. The kids are picky eaters but good pickers, and they twist off a couple of eggplants and some oversized zucchinis. A watermelon is getting big, but it isn’t quite ready yet. The kids take the vegetables inside and then head off to school, and you stay home and write all morning and then drive to a Mexican fast-food place for lunch, where you order a burrito and are asked if you want it with black or pinto beans. “Pinto,” you say, “and I’d like the corn salsa.” There’s a large TV in the restaurant and, as you eat, you watch the news, a report of a storage compartment, washed out to sea by the Japanese tsunami, drifting all the way across the Pacific to a beach in British Columbia. The compartment is open, but, amazingly, some of its contents still remain inside, including a Harley Davidson motorcycle.

  After lunch you head home and try to keep writing, but the ideas are elusive, flitting around like butterflies, so you take a break and think about an upcoming family trip to the Yucatán. In a big book on Mayan ruins, two photos of blocky statues hewn out of the pale, volcanic rock jump out, one of a crocodile and one of a monkey god. The book also mentions a tame spider monkey that lives among the ruins of Copán. (You remember that there was a tame spider monkey at the Smithsonian field station on Barro Colorado Island in Panama years ago and wonder if this species has some special affinity for humans.) After that, more writing, or at least an attempt at writing, and then, almost as soon as the kids are home, pandemonium breaks out: your daughter sets the pet guinea pig down on the living room floor and seems shocked when, as usual, it scurries off and lodges itself under the sofa. The kids’ running and screaming has the pair of lovebirds chattering, too, but the kaleidoscopically colored panther chameleon seems nonplussed, only scanning with one turreted eye as it perches motionless on a branch in its terrarium.

  How would this ordinary day be different if living things had never made chance ocean crossings? Pretty much in every way: the evidence to date, mostly from molecular timetrees and the fossil record, suggests that all of the organisms mentioned had ancestors that dispersed naturally across ocean barriers. The nutmeg tree’s ancestors colonized the volcanic Banda Islands, near New Guinea, presumably from other islands in the area; the eggplant’s ancestors went from South America to India, probably via Africa; the zucchini’s from Africa to South America; the watermelon’s from Asia to an island Africa; the common bean’s from Africa to the New World; the corn’s from Africa to North America (although the evidence in this case is a bit iffy); the crocodile’s, spider monkey’s, and guinea pig’s from Africa to the New World; the lovebird’s from Australasia to Africa via Madagascar; and the chameleon’s from Africa to Madagascar. The oddest story belongs to cotton. Most of the cotton we wear comes from Gossypium hirsutum, part of a group of New World species that originated when one cotton lineage hybridized with another. The strange thing is that these two lineages appear to have independently colonized the New World from Africa by crossing the Atlantic. In other words, G. hirsutum is simultaneously the product of two separate chance, long-distance dispersal events, like a miracle squared.

  The point is that none of these lineages would even exist if their ancestors had not crossed oceans. Keeping our focus on human history for the moment, we can easily see that without these and other ocean-crossing taxa, many small details of the world as it exists today would necessarily be altered. For instance, all the zucchini breads and eggplant parmigianas would disappear. No more guinea pigs for genetic experiments or for Andean people to roast on sticks. No more nutmeg in the eggnog. However, the changes would go far deeper than that. Consider, as just one of a multitude of possible examples, that of the great triumvirate of pre-Columbian crops in the New World, beans and squash almost certainly evolved from ancestors that crossed oceans naturally, and the third, maize, probably did as well. Make all three of those plants disappear, and history would lurch into some unknown but fundamentally different space. The nature of prehistoric agriculture and its associated cultures would ha
ve been radically shifted almost everywhere from tropical Mesoamerica through the desert American Southwest to the eastern seaboard of temperate North America. The Toltecs and Mayans and Aztecs, the cliff-dwelling Anasazi, the mound builders of the Mississippi drainage and Great Lakes, the eastern tribes that encountered the Pilgrims—in this alternate history, none of these cultures would have existed in anything like their known form. In post-Columbian times, billions of tons of beans, squash, and corn would be erased from agricultural ledgers all over the world. No corn-fed hogs, no squash in the cellar or sacks of dried beans to get pioneers through hard winters, no Mexican food as we know it, no high-fructose corn syrup.

  Finally, there are indications that all of these details might be moot. In particular, some primatologists have argued from the fossil record that an ancestor of anthropoids, the group that includes monkeys and apes, must have crossed the Tethys Sea to colonize Africa from Asia. This conclusion is based on placing certain Asian and African fossils in specific positions within the primate evolutionary tree, and those placements remain controversial (as they often are with fossils). Let us assume for the moment, however, that this crossing of the Tethys really happened, and then imagine the consequences of erasing that unlikely journey from deep history. Without that ancestor, the whole African anthropoid tree would disappear. Colobus monkeys, baboons, and macaques would all wink out of history, along with the monkeys that crossed the Atlantic to populate the New World. There would be no gibbons, orangutans, gorillas, chimpanzees, Australopithecus, or Homo erectus. There would be no us. Darkness would descend, literally—the night would be lit once again only by the stars and moon.

  Contemplating the elimination of that ancient anthropoid ancestor and, with it, a large segment of the primate tree, leads into our next subject. In considering cases such as the civilizations founded on beans, corn, and squash, we’ve been taking a “shallow-time” view, pushing only slightly further into the past than the history of potatoes in the Old World. However, such examples represent just the thin surface of what would have to be different in a world without chance, oceanic dispersal. To more fully appreciate the ramifications of natural ocean crossings, one has to recognize that most of these events are far more ancient than any human introduction, which means that their immediate effects have had much more time to cascade into others, to expand their spheres of influence through deep time.

  As an illustration of that influence, we will turn, appropriately, to South America, the continent that, more than any other, put the seeds of evolutionary thought into Darwin’s mind, the same continent where Alfred Russel Wallace, noticing that the boundaries of species or varieties often coincide with the courses of great rivers, began thinking deeply about barriers to dispersal.

  THE BRANCHES THAT BECAME TREES

  Not so long ago, just a thin section on the geologic timescale, South America was an island continent. From roughly 30 million years ago, when a fairly tenuous link to Antarctica was broken, until the emergence of the Isthmus of Panama about 3 million years ago, South America was disconnected from all other large landmasses (see Figure 12.1). One has to go much further back, to 50 million years ago or so, to find the next most recent possible connection to North America (“possible” because some geologists doubt this connection existed), and then much further still, to roughly 100 million years ago, to see the final attachment between South America and Africa prior to the opening of the Atlantic.

  12.1 South America as an island continent. The reconstruction pictured is for the Middle Miocene, roughly 10 million years ago. Redrawn and modified from Schaefer et al. (2009).

  That geologic history is the setting for one of the great stories in biogeography, focusing on the origins and fates of South American land mammals. And that story begins with the continent’s long period of isolation. During and because of that isolation, a diverse and distinctive mammal fauna developed, including anteaters, sloths, armadillos, opossums, and a great assortment of entirely extinct groups, such as hyena-like and sabertooth-­cat-like forms that were closely related to marsupials, as well as a variety of hoofed mammals with unfamiliar names like Pyrotheria, Astrapotheria, Litopterna, and Xenungulata. In that same period, mammals from other continents generally did not enter South America, kept out by ocean barriers. Then the Panamanian Isthmus emerged, providing a corridor for the passage of land mammals (and other organisms); whereas before, a natural raft was needed for mammals to pass between the continents, now they could make their way by walking, hopping, or running. The result was a rapid and dramatic mixing of faunas: during a relatively short period—the past 3 million years—deer, cats, dogs, squirrels, peccaries, camels, and ten other North American mammal families colonized South America, while opossums, armadillos, anteaters, marmosets, porcupines, and thirteen other South American families moved in the other direction, colonizing North America. This mixing of faunas, dubbed the “Great American Interchange,” was a classic case of “geodispersal,” that is, the movement of many species into a new area after a barrier has been eliminated, in this case, the ocean barrier that had separated North and South America. As a prominent example in historical biogeography, it rivals Gondwanan vicariance.

  The colonizations of the Great American Interchange presumably happened by normal dispersal over the newly emerged land bridge, that is, by the expected “garden-variety” movement of organisms into suitable habitat. However, somewhat lost in this story is the fact that two North American groups showed up in South America shortly before the emergence of the isthmus. Specifically, sigmodontine rodent fossils have been found in Argentina in strata 4 to 6 million years old, and procyonid carnivores (in the same family as raccoons and coatimundis) in strata dated to about 7 million years old, also in Argentina. For the sigmodontines, molecular dating corroborates the notion that the dispersal event happened before the existence of the land bridge. Members of those two groups apparently arrived in South America by way of an archipelago in what is now Central America, probably using a series of islands as stepping stones.

  These island-hopping journeys by sigmodontines and procyonids don’t rank among the truly startling long-distance colonizations. However, given that terrestrial mammals are not adept at crossing substantial water barriers—even dispersalists like Charles Darwin and George Gaylord Simpson admitted that fact—these events certainly qualify as unexpected colonizations. Thus, although these two southward dispersals are sometimes treated as “heralds” of the Great American Interchange, they can also be placed in a different category: they are examples of land mammals that arrived in South America by chance, oceanic dispersal.

  We have already run across two much better known examples that fall into this category, the monkeys and caviomorph rodents (guinea pigs and relatives), both of which likely crossed the Atlantic from Africa (Chapter Nine) no later than about 26 million and 41 million years ago, respectively. Beyond these, there are a few less convincing cases. Thus, there are exactly four well-established examples of overwater colonization by land mammals in a period of some 50 million years, dating from the disappearance of the previous North America–South America land connection. Four may not sound like many, and certainly doesn’t compare with the tide of mammal groups that has passed over the Isthmus of Panama in the past 3 million years. However, that small number is misleading when it comes to thinking about the importance of overwater colonists in the history of the continent. It’s misleading for the simple reason that, during the course of deep time, evolutionary lineages can ramify. Single branches can turn into large evolutionary trees.

  That, as it turns out, is exactly what has happened for three of the four groups of oceanic dispersers—the monkeys, caviomorphs, and sigmodontines.62 In all likelihood, the ancestors of each of those groups arrived in the form of just a few individuals of a single species carried on a natural raft. From that tenuous beginning, however, each of those ancestral species evolved into a great range of forms. The diversit
y of New World monkeys, from tiny, acrobatic marmosets and tamarins to lanky spider monkeys and robust, deep-voiced howlers is well known, but the radiations of caviomorphs and sigmodontines are arguably even more impressive. The caviomorphs include, among many others, guinea pigs and chinchillas; a dozen species of prehensile-tailed porcupines; some sixty kinds of burrowing, gopher-like tuco-tucos; many species of arboreal spiny rats; the semiaquatic nutria (a common introduced pest outside of South America); tail-less, long-legged agoutis; mountain viscachas, all with big ears like a rabbit’s and a bushy tail like a squirrel’s; and the plains viscacha, which lives in communal burrows, looks like a giant guinea pig, and can both run rapidly and make prodigious leaps (see Figure 12.2). The world’s largest living rodent, the capybara, is a caviomorph, as was the world’s largest known extinct rodent, Josephoartigasia monesi, which had a skull twenty inches long and may have weighed as much as a rhino. Sigmodontine rodents are anatomically less strikingly varied—most of them look like mice or rats—but they occupy almost every kind of environment in South America, from deserts to alpine tundra to rainforests. Like the caviomorphs, the sigmodontines include terrestrial, arboreal, semiaquatic, and burrowing species. Most sigmodontines are herbivores of one sort or another, but some semiaquatic species eat arthropods and, occasionally, fish.

  Beyond their diversity of form and habits, the success of the immigrant rodents and monkeys shows up in a more subtle form also: these groups became so ubiquitous, forming such a large pool of potential dispersers, that they managed to make quite a few successful long-distance voyages from South America. In particular, all three lineages reached the West Indies, and the sigmodontines also traveled west in the Pacific to colonize the Galápagos at least three separate times, and east in the Atlantic to Fernando de Noronha (where, before going extinct, the immigrant rodent lived alongside the Mabuya skink from Africa).