BASIC BOOKS, JANUARY 2014Baobab trees crossed the Indian Ocean, perhaps as seeds drifting on marine currents. Iguanas from the New World travelled thousands of miles over the Pacific to reach Fiji and Tonga, probably by hitching rides on natural rafts of vegetation. A small carnivorous sundew plant somehow made it from Australia to the top of a sheer-sided mesa in northern South America. Frogs, traditionally considered hopeless ocean voyagers, traversed seas from mainland habitats to colonize the islands of São Tomé and Príncipe, the Seychelles, the Philippines, Sulawesi, and the West Indies. Crocodiles, burrowing lizards, rodents, and monkeys dispersed to the Americas from Africa via the Atlantic.
All of these improbable journeys, and many others, are now supported by genetic evidence as well as other data. As I describe in my book The Monkey’s Voyage, these cases collectively represent a major sea change in the field of biogeography, from a view in which distributions broken up by oceans were typically explained by the fragmentation of landmasses through continental drift to a more balanced perspective that recognizes the great importance of natural ocean crossings. Recent molecular-clock studies, in particular, have often refuted hypotheses that invoke continental drift in favor of relatively recent long-distance oceanic dispersal. For those of us who were raised thinking of continental breakup as the default explanation for such piecemeal distributions, the geographic history of living things has been turned on its head.
In some cases, these dispersal events represent oddities that likely have had little impact on the overall course of life on Earth. For instance, the iguanas that apparently rafted to Fiji and Tonga produced only a few descendant species, confined to a handful of South Pacific islands. Similarly, the sundew lineage that colonized South America from Australia is represented in the New World by a single species found only on the summit of one small massif on the border of Brazil and Venezuela.
But other ocean crossings have had far more widespread and profound effects. For example, the monkeys, rodents, and burrowing lizards that colonized the Americas from Africa (probably all between 30 million and 50 million years ago) gave rise to a total of some 450 living species. Those are 450 lineages that would not exist in the absence of chance dispersal, and their presence has undoubtedly redirected the evolutionary paths of many other groups as well—their parasites, prey, and competitors, for instance. These and other similar cases give strong support to the idea that unpredictable events that would have seemed minor and inconsequential when they occurred can have far-reaching effects through deep time—a few dehydrated, starving African monkeys made landfall on the South American coast, managed to survive and thrive, and the Neotropics were changed forever.
A recent study of the evolutionary relationships of fossil primates highlights the profound influence of chance dispersal in a particularly obvious way. The phylogenetic placement of various groups in this study can be debated, but, if true, the results imply that, some 40 million years ago or earlier, an ancient primate species made an unlikely, accidental journey from Asia across the Tethys Sea to Africa, which was then an island (PNAS, 109:10293-97, 2012). Over many millions of years, those seafaring ancestors’ descendants evolved into diverse lineages, including baboons, guenons, mangabeys, and the monkeys that would go on to raft from Africa to the New World. Those ancestors would also give rise to gorillas, chimpanzees, Australopithecus, Homo erectus, and us. Given the massive impact of our species on the planet, this case, if it holds up, will rank as an extreme illustration of the profound influence of improbable, unpredictable ocean crossings on the history of life.
Alan de Queiroz is an evolutionary biologist and adjunct faculty at the University of Nevada, Reno. Read an excerpt of The Monkey’s Voyage.