Travelers

Evolutionary biology and biogeography are basically outside of my bailiwick here, not because I'm not interested in those fields (I am) but because, as is generally the case with all the sciences, a smattering of layperson's knowledge really doesn't qualify one to give an informed evaluation of advances in the discipline undertaken by people who have both years of scientific training and a thorough knowledge of the relevant literature. (Which doesn't stop any number of crackpots and pseudoscientists from jumping in with both feet, of course.) So I will only say of Alan de Queiroz's The Monkey's Voyage: How Improbable Journeys Shaped the History of Life that it is enjoyably written in a way that is comprehensible to a lay reader without being unnecessarily dumbed down, and that it puts forward what appears to be a plausible, if certain to be disputed, argument that may shed new light on how the world's flora and fauna came to be distributed where they are.

To sum up the book's argument very briefly, one of the early problems that Darwin and his contemporaries faced was how species that were clearly related to each other came to be distributed in places oceans apart; the example alluded to in de Queiroz's title is the monkey lineage, which is found in both Old and New worlds. The discovery of plate tectonics seemingly provided an answer: the ancestral homeland of these species had drifted apart, and the descendants went their separate evolutionary ways thereafter, in a process known as vicariance. The problem is that recent DNA studies suggest that the timing is all wrong.

If the opening of the South Atlantic caused the separation between platyrrhines [New World monkeys] and catarrhines [Old World monkeys], then that split in the evolutionary tree should have occurred on the order of 100 million years ago. To put this in some perspective, such an old date would imply that the New World and Old World monkey lineages, which we know are not early branches in the primate tree, are actually about 50 million years older than the earliest known primate fossils of any kind. In fact, these monkey lineages would have to be some 35 million years older than the first known fossils of any placental mammal.

The DNA evidence, which gets a bit complicated, suggests that the split in fact took place within the last 51 million years, and possibly as late as 33 million years ago. But if those numbers are right, then why are there monkeys in the Americas at all?

De Queiroz, a biologist at the University of Nevada, suggests that they got there the same way that a surprising number of seemingly out-of-place species came to be where they are: by accidental ocean crossings long after the continents had drifted apart. This may seem far-fetched, and in fact de Queiroz recognizes that the monkey example, of all the cases of potential oceanic dispersal that he examines, requires the greatest suspension of one's initial disbelief. But by building up careful evidence for other, less extreme dispersals, he makes a plausible case for even the ancestor of the platyrrhines as an accidental transoceanic migrant. The scenario — monkeys clinging to driftwood or to "islands" of vegetation swept out to sea by drainage from Africa's rivers (or, he might have added, by tsunamis) — would have extremely long odds against it, but one should bear in mind the vast quantity of time available; a one-in-10 million-years freak event might actually stand a fair chance of happening, given the tens of millions of years during which it could potentially have taken place. (The Atlantic would have been substantially narrower than it is now, and de Queiroz suggests that the monkeys — or maybe one pregnant monkey — might only have had to cling on for a week or so.)

There are many other well-documented case studies in the book — New Zealand, Hawaii, the Falkland Islands — and de Queiroz, whether or not he is ultimately proved to be right, appears to have done his homework. It seems likely that we'll be hearing more about his book in the coming years.