Scientific Method —

How the vastly improbable has determined what animals got where

The Monkey’s Voyage argues that life on Earth is shaped by animals floating on rafts.

The new book The Monkey's Voyage asks a deceptively simple question: how did we get here? My grandparents took a boat to New York in 1948 after spending three years in a displaced persons camp in Germany after the Nazis had just murdered most of their families. But how did kiwis—flightless birds—get to New Zealand? And how did frogs—whose skin will quickly desiccate if exposed to either air or salt water—get to volcanic islands like Príncipe and São Tomé, off the east coast of Africa?

Taxonomic groups like species can get split into two or more populations by the development of a "barrier to dispersal;" in biogeography, this is called vicariance. These barriers can be formed by the shifting tectonic plates that move continents, widen oceans, and create mountain ranges, or they can be formed by climate changes that submerge or create land bridges or form vast, impassable deserts.

But geology doesn't only separate. All of the continental land masses used to be connected in one primordial supercontinent called Gondwana (you must admit, it sounds more scholarly than Pangaea). So southern beech trees—with seeds that cannot survive in seawater—are found on Australia and New Zealand, while crocodiles are found in South America. When Gondwana began to break apart about 160 million years ago, the different pieces acted like rafts for these species, keeping them—or their ancestors—afloat.

It is an intuitive, simple, and elegant theory to explain how similar species end up in wildly different places and has ruled biogeography since the development of plate tectonics in the 1960s. However, as is the case with many intuitive, simple, and elegant theories—like the one claiming that eating nuts will make you fat—there are reams of pesky data demonstrating that despite its appeal, it happens to be wrong.

The biologist Alan de Queiroz has collected much of this data, and he uses it to shoot down his vicariance-leaning biogeographer colleagues in his new book, The Monkey’s Voyage.

Darwin thought hard about how all of the different species got where they were. He had to fight people with a Biblical perspective, though he realized that it was absurd for all of the Earth's species to have dispersed from Noah's Ark after it alighted on Mount Ararat. These people thought God created each species and placed it into the appropriate locale.

Darwin wondered, then, why continental rainforests were fit for frogs, but island rainforests were not? He concluded that species moved between land masses not by vicariance, but by chance dispersal across oceans. He performed multiple experiments to demonstrate that different types of seeds and eggs could survive for days on end in seawater and could get caught in branches or even stick to birds' legs to get transported across long distances. Amphibians, however, could not do these things—that's why (he thought) none live on islands.

Vicariance supporters view such long-distance dispersal of species across vast stretches of ocean as something of a deux ex machina. Sure, Darwin showed that some seeds can survive in seawater. But for most terrestrial animals—like primates and other mammals—this method of colonizing distant lands would rely on a raft carrying a male and a female (or a pregnant female), along with enough food and water to get across the ocean without succumbing to weather before making landfall. It does seem somewhat unlikely. Spiders attached to a ship's rigging or floating on winds, a scientist could buy. Birds, insects, even plants getting across the ocean, ok. But monkeys? Come on.

According to de Queiroz, two factors make the idea of long-distance dispersal a more likely explanation for the data than vicariance. One is the idea of "deep time." The Earth has been around for four and a half billion years; the Cenozoic era, which began with the extinction of non-avian dinosaurs, has lasted for 65 million years. Over such an extraordinary length of time, extraordinarily unlikely things do happen.

The other factor has been the spectacular advances in DNA sequencing technology, which have enabled more accurate molecular clocks, giving us a better idea of when species diverged from one another. As it turns out, most species that are alleged Gondwanan relics didn't even exist until tens of millions of years after Gondwana broke up. That's not in the same ball park, even by vast geological standards of time. It's kind of hard to claim that monkeys rode on South America as it floated to its current location when it detached from Africa 50 million years before any primates even appear in the fossil record.

Diehard proponents of vicariance don't really trust molecular dating, so they pretty much ignore it. It is definitely a limited technique; it relies on the rate at which genes mutate, which is not even constant within a species, let alone between species. But the flood of genetic data that has become available of late is refining it. And it is probably more accurate than dating based on the fossil record, which is incomplete by its nature.

Long distance dispersal clearly does happen; people have witnessed it happen. de Queiroz ends each chapter of his book with a vignette, written all in italics, of people watching different types of species—including mammals—traveling through the sea: attached to the rigging of ships, on rafts made of tree branches, or just floating. Monkeys got to South America about 40 million years ago, when it was at least 900 miles away from Africa. de Queiroz does not claim to tell us how that trans-Atlantic journey actually did happen; that is something we will probably never be able to know. Rather, his point is that there is more than one way in which it could have happened.

The Monkey's Voyage: How Improbable Journeys Shaped the History of Life, by Alan de Queiroz. From Basic Books.

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