DNA evidence shows the common ancestor of New World monkeys and Old World monkeys lived 40 million years ago, long after South America and Africa became separated. So how did the ancestors of New World monkeys make the trip? ‘The Monkey’s Voyage,’ by Alan de Queiroz answers this question.

This book begins with a small struggle in an arroyo near the southern tip of Baja California. It’s June 2000, and the author and his girlfriend, Tara, are trying to pull a snake from a hole. She’s gripping it by the neck. He’s reaching down the snake’s body trying to gain purchase. “The process is exhausting,” Alan de Queiroz writes, “not because it’s physically difficult, but because we’re fighting against the will of another being; with each pull I feel the snake resisting.”

Out comes a huge black garter snake, three and a half feet long. They pop her into a pillowcase, and de Queiroz takes her home to study. (He’s an evolutionary biologist who teaches at the University of Nevada.) De Queiroz knows that this particular species is found only at the southern tip of the peninsula and on mainland Mexico, which is 120 miles away across the Sea of Cortez. The question that intrigues him is this: How did that snake’s ancestors get across all that water?

De Queiroz and Tara are married now. Recently he hung a big map on their wall, “The World of Wild Animals.” Ostensibly, it’s for their children, he says, but he’s the only one who keeps looking at it. There’s a rhea in South America and an ostrich in Africa, staring at each other across the Atlantic. There’s also a mandrill in Central Africa and a South American capuchin doing the same. These birds are cousins; so are the monkeys, and so are tens of thousands of other species of animals and plants that are now separated by the world’s oceans. How did that happen? All of them have been forced apart at some point in the past by a process as implacable and incomprehensible as Alan and Tara must have seemed to that snake in the hole.

When I picked up this entertaining book, I thought I knew the answers to these questions. Charles Darwin did a beautiful if funky series of experiments at his country house in Kent, to explain how plants and animals might have crossed the oceans. In his day, the reigning explanation was supernatural: God put them there. Darwin’s thinking was more mundane. In the mid-1850s, he filled bottles with salt water, and added seeds and plants to find out how long they’d float, and whether they could soak in brine for months and still germinate. The seeds proved to be hardy and so did Darwin. “It is quite surprising that the Radishes shd have grown,” he wrote to a friend, “for the salt water was putrid to an extent, which I cd not have thought credible had I not smelt it myself.”

Darwin concluded that seeds could have floated across large bodies of water on the rafts and mats of vegetation that often drift out of the mouths of the world’s great rivers, and animals could have traveled with them on Darwin’s arks. In other smelly experiments he demonstrated that seeds could also have traveled across water in the gizzards of seabirds. Hatchling freshwater snails could have made the crossing clinging to the webs of ducks’ feet. And so on.

One generation after Darwin, a young explorer and meteorologist named Alfred Wegener reduced these mysteries still further. “Please look at a map of the world!” he wrote to his fiancée. “Does not the east coast of South America fit exactly with the west coast of Africa as if they had formerly been joined?” Other people had noticed that before, but Wegener made a study of it. He concluded, correctly, that our present continents must all once have been part of a single supercontinent, which has since come to be called Pangaea. When Pangaea broke up, innumerable species of plants and animals were slowly rafted apart by continental drift. In the 1960s, Wegener’s radical vision of the earth’s history was vindicated by the new science of plate tectonics.

By that time, what with Darwin’s arks and Wegener’s jigsaw puzzles, most scientists thought they understood the essential principles of biogeography, which is the study of the distributions of the world’s animals and plants.

That’s the way I thought things stood to this day, until I read “The Monkey’s Voyage.” But there was a peculiar episode in biogeography back in the 1950s and ’60s, a sort of mutiny on the Beagle. De Queiroz makes quite a tale of it. It seems a little gang of Young Turks in biogeography got so excited by plate tectonics that they concluded that continental drift explains everything. They threw Darwin overboard, and his stinking radishes with him. It was a ’60s thing. One of the revolutionaries, a passionate Italian botanist named Léon Croizat, wrote that Darwin was “a very unhappy thinker,” “congenitally not a thinker,” “essentially not a thinker,” “not born a thinker,” “anything but a thinker” (italics Croizat’s). Another hothead, a Swedish entomologist named Lars Brundin, an expert on midges, railed against the “negative, sterile and superficial” thinking of Darwin’s followers, “troubled biogeographers.”

Biogeography was in turmoil.

Fortunately, in the 1980s, DNA evidence resolved the controversy. Consider that black garter snake from the tip of Baja, for instance. Its DNA reveals that it parted from its family on the coast of the Mexican mainland a few hundred thousand years ago. But geologists know that the Sea of Cortez began forming millions of years ago. So the snakes couldn’t have been pulled apart because of plate tectonics. They must have drifted across the sea on one of Darwin’s arks.

It seems Darwin and Wegener were both right after all. DNA evidence proves that an amazing number of species got where they are today on precarious rides across bodies of water. The book’s centerpiece is the incredible journey of the New World monkeys. According to the best DNA estimates, monkeys crossed the Atlantic from Africa to South America roughly 40 million years ago. At that time, the continents were only about half as far apart as they are today. The shortest distance between them was probably only 900 miles. It’s likely that there were also a number of islands, now submerged, that might have offered a chance rest stop or two along the way. Given favorable winds and currents, a big raft of vegetation with trees growing out of it could have crossed the Atlantic in just a couple of weeks. “A raft from Africa reaches the South American shore, some exhausted monkeys amble off and, over millions of years, give rise to squirrel monkeys, howlers and capuchins, owl-eyed night monkeys, and baldheaded uakaris,” de Queiroz writes. It’s a wonderful story to contemplate, very high up on the ladder of unlikelihood, but it probably happened.

De Queiroz writes in a pleasant, relaxed style. If anything, his book’s organization is a little too relaxed for my taste, with charts, maps, photographs and sidebars defining technical terms scattered throughout, and big blocks of anecdotes in italics tacked on at the end of each chapter. It reads like an eclectic scrapbook, full of interesting bits from hither and yon. But then, that’s life.

THE MONKEY’S VOYAGE

How Improbable Journeys Shaped the History of Life

By Alan de Queiroz

Illustrated. 360 pp. Basic Books. $27.99.

Darwin’s Arks ‘The Monkey’s Voyage,’ by Alan de Queiroz