The Monkey's Voyage: How Improbable Journeys Shaped the History of Lifeby Alan de QueirozBasic Books, 2014 There are some basic definitions to get out of the way before leaping into Alan de Queiroz' utterly fascinating new book The Monkey's Voyage. The first of these is our author's speciality, "biogeography," which is the study of how and why living things move around on Earth. Another such term is "vicariance," which describes types of living things - say species - that get split into separate non-communicating groups by insuperable external circumstances like the rising of a mountain range or the submersion of a land bridge. And at the back of these terms and many others would be "Gondwana," the name scientists have given to the southern half of the immense 'super-continent' that existed hundreds of millions of years ago and eventually fragmented due to continental drift. And if, as the nuns taught you back in school, we were to use those three words in a sentence, it might go something like this: "Biogeographers have long looked to Gondwana to explain Earth's vicariance."In other words, since oceans are uncrossable, all linked species of surface-dwellers on Earth are linked, somehow, by land: there were ur-populations, and they got split up and fragmented by a constantly-reshuffling planetary crust. If you want to study similarities between, say, African and South American monkeys, you study how Africa and South America parted ways.De Queiroz, an evolutionary biologist at the University of Nevada, begs to differ and argues passionately - and entirely convincingly - that the basic premise of that "since" is flawed, because oceans aren't uncrossable. Quite the opposite: he envisions thousands of years of the most extraordinary, unsung expeditions - not vicariance but rather voyaging. He acknowledges the staggering number of variables, but he paints an easily-imagined hypothetical:

For instance, consider a reasonable scenario for monkeys colonizing South America from Africa. Start with a group of African monkeys lounging in a tree by a major river, a river similar to the modern-day Congo or Niger. Heavy rains have been falling for days, and the resulting floodwaters undercut a large chunk of the bank, including the monkeys' tree, and the whole piece falls into the water and is swept away. This natural raft is carried many miles downriver, eventually reaching the ocean, where it gets caught up in the westward-flowing current. On the ocean, the monkeys eat anything edible they can find on their small, floating island. When it rains, they drink the water that briefly pools up. Weeks later, this raft, now waterlogged and barely buoyant, is grounded on the South American coast, and the few surviving monkeys, scrawny and dehydrated, splash onto the beach and disappear into the adjacent forest. They stay together, feeding on unfamiliar but edible fruits and insects, eventually mating, giving birth, and raising young. A thousand years later, their descendants form a substantial population that will ultimately give rise to the entire radiation of New World monkeys.

The essential question underpinning de Queiroz' book - "why is our world the way it is?" - is answered by this picture of random chance moving random animals over vast and inhospitable distances, of life managing to swarm everywhere, whether it be spiders using webbing-strands to 'balloon' into the lower stratosphere and circle the globe or small animals hitching rides in the thick leg-feathers of ocean-crossing birds, or those desperate raft-dwellers riding the currents that move the seas. "The entire biota of the Earth," our author asserts,

... specifically, the identities and locations of living species - has been profoundly influenced by natural ocean crossings. This influence has been so great, I will suggest, that if one could somehow go back in time and eliminate all of these overwater colonizations, and then jump back to the present, the living world as we know it would be transformed into an alien one.

In compelling support of that transformation, if not quite its means, de Queiroz asks us to reflect on the gigantic effects migrating species have had on the world. An obvious example would be the spread of the flea, traveling on migrating rats, that bore the bubonic plague (somewhat unaccountably, our author also cites the example of the potato, Solanum tuberosum, which he says was first cultivated 4000 years ago in what is now Peru and which was brought to Europe in the 1500s, a rather fanciful scenario severely at odds with my grandmother's solemn pronouncement that the potato was given to County Donegal by the Virgin Mary shortly after the expulsion from Eden, but then, nobody's perfect) - and the changes that can be wrought by such well-traveled interlopers, now rather unhelpfully called "invasive species," cause changes in their turn, until, as de Queiroz confidently asserts, those changes make the world anew. It's a fantastically well-done account (our author makes it all the more vivid with a steady side-commentary of personal anecdotes), the best possible popularization of a paradigm-shifting new line of thought about the history of life on Earth.At one point de Queiroz remarks with surprising honesty about the drudgery that can result from reading a succession of scientific papers. "You read fewer and fewer pages," he admits, "instead merely skim the summaries ... you find, above all, that even the "take-home message" of most papers gets lost somewhere in the dim neuronal passageway between short-term and long-term memory."Your dim neuronal passageways will have no problem here. The Monkey's Voyage will keep you thinking long after you've turned the last page.