How Did Monkeys Cross the Atlantic? A Near-Miraculous Answer

The question of how species came to live where they live is one of the enduring puzzles in biology: How did a big flightless bird like the now-extinct moa find its way to New Zealand?  How did monkeys cross the Atlantic from Africa?  Why does an insect-eating plant on a remote plateau in Venezuela have its nearest living relative at the other end of the Earth, in Australia?

Beginning in the 1960s, with the triumphant vindication of continental drift and plate tectonics, “vicariance” became the standard answer for evolutionary biologists. It’s what happens when a barrier—a mountain range, or an ocean, say—separates a species into two or more populations and sends them down diverging evolutionary paths. (The word comes from the Latin vicarius, meaning a thing that takes the place of something else, like a vicar standing in for a bishop.)

In this new view of geological history, all of the land masses of the modern world once huddled together in the ancient supercontinent Pangaea. As these land masses drifted apart to their present positions, they simply carried plants and animals with them—or so vicariance proponents argued.

To nonscientists, this may sound like a straightforward and not particularly controversial development in intellectual history.  But in his lively new book The Monkey’s Voyage: How Improbable Journeys Shaped The History of Life, biologist Alan de Queiroz reveals just how loaded it was with anti-establishment furor.

Vicariance, together with the ideas on which it was built, were “movements heavily fueled not just by intellectual considerations, but also by this radical belligerence that at times seemed like a stratagem,” Mr. de Queiroz writes. He enlivens his tale with vivid portraits of combatants, notably the French-born eccentric Léon Croizat, who began self-publishing his massive tomes on biogeography in the 1950s, and Gary Nelson, who started out as a young rebel in the late 1960s at the American Museum of Natural History. Vicariance proponents favored a “style of argument based on equal parts shouting and sarcasm,” with the heaviest sarcasm heaped on “dispersalism,” the old, supposedly untestable idea that plants and animals arrived at their present locations more or less on their own power, through a series of chance long-distance voyages.

“Damned benighted dispersalism,” as de Querioz calls it, with tongue in cheek, dates back to Charles Darwin, who proposed that drifting icebergs may have served as a means of transport, and to early Biblical scholars, who theorized that, after Noah landed on Mt. Ararat, his menagerie dispersed around the world as cargo on ships, or by traveling from island to island, stepping-stone fashion.

To Croizat, dispersalism was “a world of make-believe and pretense” and for Nelson it was “a science of the improbable, the rare, the mysterious, and the miraculous.” Vicariance relied, by contrast, on the hard science of plate tectonics and on cladistics, a new way of organizing species into more carefully distinguished evolutionary lineages, according to shared traits.

All this gave vicariance a kind of mythic power, with the paradoxical result, as de Queiroz recounts it, of blinding true believers to the evidence that species disperse over unlikely distances even now.  Among the many examples he cites, white-faced herons migrated at least 1300 miles from Australia to populate New Zealand in the 1940s, and green iguanas populated the Caribbean island of Anguilla in 1995, after a hurricane carried them 175 miles from Guadeloupe.

The new science of molecular clocks–timelines based on careful analysis of changes in DNA–argues even more powerfully that vicariance alone cannot explain why many species occur where they do, according to de Queiroz.  Monkeys, for instance, originally came to South America from Africa.  The vicariance argument suggests that this happened as the two continents pulled apart roughly 100 million years ago.  But numerous molecular clock studies say the split between Old and New World monkeys happened just 30 to 50 million years ago.  Moreover, monkeys only appear in the New World fossil record “as if out of thin air” 26 million years ago.

So how did they get there? De Queiroz makes a scientific case for the near-miraculous:  Land rafts periodically calve off the continents and drift long distances with the current, he writes.  When such a raft carried a few pioneering monkeys to South America 40 million years ago, the Atlantic may have been just 900 miles wide and powerful currents could have shortened the trip to as little as a week.

Molecular clocks also undercut the idea that the flightless moa, that “most iconic” example of vicariance, got to New Zealand via continental drift alone.  Instead, de Querioz writes, new genetic evidence places South America’s tinamou, a bird that can fly despite being heavy-bodied, deep in the same evolutionary line, implying that the moa could have flown to New Zealand and only later become flightless.  And if monkeys and moas could make such voyages, so could smaller animals and of course plants.

De Queiroz manages to keep this story both informative and highly entertaining.  He’s not a polemicist, but a raconteur, a guide down the boulevard of ideas, pointing out colorful characters on route.  He introduces Kary Mullis, for instance, as an LSD-sampling madman who once skiied down an icy street in Aspen with traffic speeding by on both sides, convinced that he would die only by hitting a redwood, “and there weren’t any redwoods in Aspen.”   Mullis turns out to matter to the story because he developed the fundamental technology of the DNA revolution—and won the Nobel Prize for it.

De Queiroz is also perfectly willing to admit when some piece of evidence doesn’t quite measure up, but is merely the best we have to go on at the moment.  “It is the business of evolutionary biologists and other historical scientists,” he writes, with disarming honesty, “to draw conclusions from obviously flawed and potential misleading information.  That is simply the nature of historical evidence …”

The resulting tale of how the world was populated willy-nilly—and of our own fumbling attempts to understand it–makes for a splendid intellectual history.

This is a book review I wrote for the Wall Street Journal