In June 2000, Alan de Queiroz became curious about an enormous, ragged-looking garter snake that lived on the tip of Baja California. Like many other biologists of his generation, de Quieroz had been taught that species traveled the Earth to new habitats on slowly drifting continents. This snake had relatives on the other side of the Sea of Cortéz on Mexico’s mainland, and de Queiroz assumed that this population ended up on Baja 4 to 8 million years ago, when the peninsula split from the mainland.
But using a new method based on genetic sequencing to estimate when the two populations split, he found that it had occurred in the past few hundred thousand years. In other words, one or more pioneering garter snakes had probably floated across 120 miles of open ocean.
As de Queiroz prepared to write up the surprising results of his snake study, he discovered that the reptile was not an outlier. Biologists were finding that even after continents drifted apart, plants and animals somehow hopped between them. “Obviously, the continents had moved — nobody was claiming that the theory of plate tectonics was wrong — and obviously, they had carried species with them,” he writes, “but somehow, these facts did not explain nearly as much about the modern living world as we had thought.” Chance ocean crossings did.
In his engaging new book, “The Monkey’s Voyage,” de Queiroz makes the case that the vibrant and distinctive biological communities we see today were created by organisms rafting across oceans and soaring through the atmosphere. “The large number of these colonizations tells us that, in the long history of this living world, the miraculous has become the expected,” he writes.
To understand how contentious this notion is, de Queiroz takes us back to the 1950s and ’60s, when a wealth of new information emerged about continental drift. Geologists had long recognized that the coasts of South America and Africa fit together like puzzle pieces and had theorized that they were once a single landmass. But now measurements from the ocean floor revealed several ridges, including one in the middle of the Atlantic Ocean, where the sea floor was spreading before the scientists’ eyes. These discoveries provided a clear mechanism for how the continents creep along. Geologists determined that, approximately 180 million years ago, there was an ancient uber-continent called Gondwana, which sat on the equator and was composed of what are now South America, Africa, Antarctica, India and Australia.
Gondwana was also a revelation for evolutionary biologists. Its break-up, they surmised, was probably etched in the history of life. For instance, ostriches, emus and rheas, closely related birds found in Africa, Australia and South America, became a textbook example of this continental drift theory. Another famous example were southern beech trees, which are found in South America, Australia and other smaller pieces of Gondwana.
This theory was attractive because it was elegant and sensible, but, as de Queiroz colorfully describes, its proponents became a little too dogmatic about it. Léon Croizat, a self-trained botanist of French heritage who lived in Venezuela, coined the phrase “Earth and life evolve together” and believed that continental drift explained everything about plant and animal distributions. To him, the idea that plants or animals crossed oceans on their own was outrageous and unscientific. He characterized Darwin as “congenitally not a thinker,” in part because of Darwin’s suggestion that wolves may have reached the Falkland Islands on icebergs. Croizat came in for criticism himself. An eminent American paleontologist called him “a member of the lunatic fringe.”
Indeed, there had always been evidence that, over the long history of life on Earth, plants and animals made remarkable journeys. Consider, for example, that young spiders are carried on the wind by their silky threads and land on the decks of ships far from the coastline. Freshwater snails cling to the feet of migrating birds. And fishermen on the Caribbean island of Anguilla once watched a natural raft of logs get washed onto shore with 15 green iguanas on it, a species that had not previously existed there.
Proof of how important these journeys are in evolutionary history finally arrived in the late 1990s with genetic-dating studies, such as the one de Quieroz conducted on his garter snakes. We now know that the evolutionary history of ostriches, emus and rheas does not match the break-up of the continents. Some scientists believe that their common ancestor could fly and that they became flightless only after settling on their respective continents. Among the other creatures de Queiroz considers are New World monkeys and two other groups of mammals, which apparently rafted to South America on a clump of earth. Today, these three groups represent 73 percent of the land mammals living there.
De Queiroz, whose tone is self-effacing and reflective, admits that some may find this view of life unsettling. Unlike the laws of physics or the seeming order of the periodic table, the distribution of life on Earth has come about through a chaotic chain reaction, like the output of a Rube Goldberg machine. Of course, there’s another, more romantic way to look at it, which is that life charts its own course.
THE MONKEY’S VOYAGE
How Improbable Journeys Shaped
the History of Life
By Alan de Queiroz
Basic. 360 pp. 27.99