In 1999, a Canadian primatologist named Alain Houle built up a how-possibly argument for this dispersal event by estimating how long it might take a natural raft to cross that narrower, Eocene Atlantic. Assuming wind and current speeds typical of the modern Atlantic, and with the wind adding greatly to the hypothetical raft’s speed by pushing against the crowns of trees growing on it, he came up with a surprisingly brief trip length of seven to eleven days. There are plenty of things to argue about in Houle’s calculations, particularly the very strong effect of the wind, as if the raft were a sailboat, and the assumption that a crossing would take the shortest possible route across the ocean. Still, even if his calculations underestimate the journey’s duration by a factor of two or three, they suggest that the crossing of a fairly wide Atlantic Ocean by monkeys is not out of the realm of possibility.
9.6 Possible island-hopping routes across the Atlantic based on a reconstruction of land configurations for 40 million years ago. Redrawn and modified from Bandoni de Oliveira et al. (2009).
Another thing to consider is that the Atlantic is getting deeper as it gets older, because ocean floor subsides as it cools down from its beginnings as hot magma. This raises the possibility that certain areas that are now submerged might have been islands at some point. A Brazilian biology graduate student named Felipe Bandoni de Oliveira and two of his colleagues recently used a model that takes subsidence into account, along with plate tectonic movements and sea-level fluctuations, to reconstruct the “landscape” of the Atlantic over the past 50 million years. Their model indicates that between 40 and 50 million years ago, there were many more islands in the Atlantic than there are today. As they pointed out, those islands could have served as stepping stones for monkeys and other organisms; instead of making one extremely long voyage, monkeys could have island-hopped shorter distances, building up populations on each successive island before another chance rafting event transported some of them to the next stepping stone (see Figure 9.6). The process would be analogous to the way Polynesians gradually colonized the islands of the South Pacific, although the monkeys’ voyages would have been far more sporadic and difficult (since they didn’t have boats), and probably would have taken millions rather than thousands of years.
The location of these ancient Atlantic islands actually would have required a much longer overall voyage from Africa to South America than would a nonstop crossing at the ocean’s narrowest point. Thus, it is far from clear that the stepping-stone journey would have been easier than the nonstop voyage.44 The importance of this exercise, then, is not so much to suggest what probably happened, but rather to add plausible routes, thus expanding the possibilities for what was undoubtedly an unlikely crossing no matter how it might have taken place. One can think of the stepping-stone idea as enhancing the how-possibly story through the accretion of small probabilities, just as buying more tickets to the lottery will increase one’s chances of winning (without actually making winning likely).
The general point is that the problem of distance is not as daunting as it might at first have seemed. Forty million years ago, the Atlantic was apparently dotted with islands and roughly half as wide as it is today. It was obviously a huge barrier to the dispersal of land organisms, but not as huge as it would later become.
PATTERNS WITHIN THE IMPROBABLE
My argument to this point for monkeys crossing a wide Atlantic can be summarized as follows: Timetrees and fossils rule out alternative hypotheses, indicating that the Atlantic crossing did happen, and how-possibly scenarios suggest how that crossing could have been accomplished. We can add here a third class of arguments, namely, observations of ocean journeys by other groups that suggest that the monkey case actually fits into some larger patterns, and is therefore not as inexplicably singular as it might at first appear.
The first observation is simply that land vertebrates have crossed the Atlantic far more often than is generally recognized. There is now strong evidence, mostly from molecular dating studies, for at least eleven such cases, including the monkeys. Admittedly, most of these ocean crossings have been by lizards or snakes, but one other crossing was by a mammal, the ancestor of the caviomorph rodents (guinea pigs, capybaras, and their relatives), an event that rivals the monkey’s journey in its apparent improbability. Furthermore, in all eleven cases, the crossing has gone from the Old World to the New World rather than the other way around. The reason may lie in the nature of the ocean currents flowing in the two directions; in particular, a raft from Africa can pick up a current heading west right from the coast, whereas the only major current running eastward (the North Equatorial Counter Current) originates well off the coast of South America. It is thought that the configuration of those currents has been in place since the opening of the Atlantic, and therefore they would have influenced ancient crossings such as that of monkeys. In short, the Atlantic crossing by monkeys is not a completely singular event, but is one of at least eleven such crossings, all explicable by the paths of ocean currents, and these eleven crossings include another seemingly implausible voyage by mammals, that of the caviomorph ancestors.
The monkey crossing also fits a taxonomic pattern. I argued above that primates without boats are not great ocean voyagers. That is certainly true if one compares them to really proficient long-distance dispersers, such as many plants, strongly flying insects, ballooning spiders, tiny snails, birds, bats, geckos, tortoises, and crocodiles, among others. However, it is not true in relation to most other mammals. In fact, among orders of mammals (other than bats and marine taxa), primates are probably second only to rodents in their ability to cross significant expanses of ocean. Thus, the Atlantic crossings by monkeys and rodents, although obviously improbable events, actually fit an overall pattern; if you had to pick two kinds of land mammals to make a transoceanic voyage, those would be the two you would pick.
Ironically, Darwin, in the course of arguing against special creation in The Origin of Species, also categorically ruled out ocean-crossing monkeys. “Why, it may be asked,” he wrote, “has the supposed creative force produced bats and no other mammals on remote islands? On my view this question can easily be answered; for no terrestrial mammal can be transported across a wide space of sea, but bats can fly across” (italics added). One wishes that Darwin could have seen the patterns just described and the evidence from the primate fossil record and timetrees, not to mention the evidence for improbable ocean journeys in general. In 150 years, much new evidence has come to light, and many things unknown to Darwin have been uncovered. As the Darwinian tree of life has been built up, using fossils and the anatomy and DNA of living species, strange and miraculous events have been revealed. Among many other things, the evidence shows that mammals have in fact been transported naturally across wide spaces of sea.
THE MONKEY WAS A SWAN
An obvious question arises from considering the monkey’s transatlantic voyage and other improbable ocean crossings: Do these rare occurrences really matter in the history of life, or are they just inconsequential curiosities, bits of biogeographic trivia? In his 2007 book The Black Swan, the writer, stock market analyst, and self-described flaneur45 Nassim Nicholas Taleb made the case that, at least in human affairs—whether social, political, economic, or personal—rare events are often extremely influential. In fact, Taleb argued that most important human events are unusual and cannot be predicted. Forecasting the fates of nations, the vicissitudes of the world economy, or your own personal destiny is next to impossible, because you cannot depend upon your experience of how things usually are or of what usually happens to anticipate what will happen. Things may carry on predictably for a while, but then something big and completely unforeseen takes place—Archduke Ferdinand is assassinated and the world tumbles into the Great War, the stock market rolls along happily on Monday and inexplicably crashes on Tuesday, two hijacked passenger jets explode into the World Trade Center. Taleb called such events “black swans” because Eu
ropeans thought such a creature was an impossibility until they ventured to Australia and found the native and mostly black Cygnus atratus. He defined “black swans” by three characteristics: they’re rare, they have great impact, and they’re not predictable except in retrospect.
DINOSAURS TOO?
Seventy million years ago, near the end of the Cretaceous, a dinosaur died for some unknown reason and ended up buried near the shores of an ocean, on ground that would eventually become northeastern Italy, near the city of Trieste. This creature was closely related to the well-known duck-billed dinosaurs and, like them, it walked on two legs, had a long snout, and ate plants that it ground up between thick batteries of tightly packed teeth. Compared to the duck-bills and other near relatives, it was conspicuously dwarfed, being a “mere” twelve feet long, and it was probably a fast runner, judging by the proportions of its hind-limb bones. Its hands were like narrow, bony pillars, useless for grasping but perhaps used for balance when moving over rough terrain, as a person might use a pair of walking sticks. After it died, this particular dinosaur was quickly buried by sediment, and, through the vagaries of geological history, it ended up as a beautiful, nearly complete fossilized skeleton. Today the specimen is prominently displayed in a geological museum in Bologna.
This species of dinosaur has been dubbed Tethyshadros insularis. As the name implies, it lived on an island that, in broad context, was part of a large European archipelago in the Tethys Seaway, which separated Eurasia from Africa and the island continent of India (still many millions of years away from its collision with Asia). More specifically, the island was a piece of the Adriatic-Dinaric Carbonate Platform (ADCP for short), at the time an arc of islands similar to the modern Bahamas. The ADCP had perhaps been connected to an Afro-Arabian continent early in the Cretaceous, which could explain the existence of T. insularis, just as Gondwanan dinosaurs persisted on the islands of India, Madagascar, and Zealandia long after those areas had separated from the rest of the supercontinent. However, that sort of vicariance scenario almost certainly does not hold for T. insularis, for a simple reason: the record of sediments indicates that, toward the middle of the Cretaceous, high sea levels submerged the ADCP, wiping out its entire land biota. From a biogeographic point of view, this “drowning” transformed the ADCP into oceanic islands, as was the case with some other continental fragments. Under this scenario, any Late Cretaceous land organisms on the ADCP must have colonized the area by crossing a significant expanse of ocean.
As gigantic land animals with little known propensity to swim in the ocean, dinosaurs are usually viewed as thoroughly landlocked, and consequently their distributions have almost always been explained through overland movements and vicariance. However, if the “drowning” scenario for the ADCP is correct, Tethyshadros must be an exception to the rule that dinosaurs only dispersed by land. The progenitors of T. insularis are thought to have arrived by island-hopping from Asia, with each “hop” presumably involving a voyage on a natural raft, or, somewhat less plausibly, dinosaurs swimming or floating in the ocean.
Like monkeys crossing the Atlantic, these ocean voyages by Tethyshadros may sound absurd. In addition to the geological evidence for the drowning of the ADCP, however, the collection of other fossils found with T. insularis hints that the fauna was that of an oceanic island. In particular, the only other vertebrates identified to date are pterosaurs, which could have flown to the island, and crocodylians, a group known to have crossed saltwater barriers many times. Furthermore, in Hungary, in Late Cretaceous strata representing another part of the European archipelago, paleontologists have unearthed a species of ceratopsian (horned) dinosaur called Ajkaceratops kozmai whose occurrence also seems best explained by an island-hopping journey from Asia. The examples of Tethyshadros and Ajkaceratops and a few other, more tentative cases indicate the need to revise our views on the dispersal abilities of dinosaurs, just as work on living species has forced a reappraisal of other groups, such as frogs, burrowing reptiles, and mammals.
Whether or not one buys Taleb’s argument for the overarching importance of black swans in human affairs, there is no denying their occurrence, if not their fundamental significance, in evolutionary history. The most notorious such event is the extraterrestrial impact that probably finished off the dinosaurs and many other taxa, thus providing opportunities for other groups, such as mammals. That event was in some sense a chance occurrence and wiped out more than half the species on Earth. We are likely a result of that black swan from space. Stephen Jay Gould and others have argued that evolutionary history has been punctuated by a series of such unpredictable events, determining the kinds of organisms that have come into being, which groups have survived and which have not, all the way back to the origin of life.
In this chapter and in Chapter Eight, we have dealt with high rungs on the ladder of increasingly improbable dispersal events, ending with the rarest of the rare, monkeys crossing an ocean. Can we think of these cases as black swans? These dispersal events certainly qualify as highly unexpected; nobody with a knowledge of the characteristics of worm lizards and monkeys, for instance, would have guessed that those animals would cross the Atlantic Ocean. But rare does not necessarily mean impactful. Are we dealing with the equivalent of Leif Ericsson’s voyage to North America, which seems to have had little influence, or Columbus’s, which deeply altered the course of world history?
Many rare dispersal events, such as skinks reaching Fernando de Noronha, probably fall into the Leif Ericsson category. However, others have had a much larger impact. In the case of worm lizards, for instance, one colonization event apparently generated a radiation of nearly a hundred modern species spread from southern South America to the West Indies. There’s a good chance that some of them will be burrowing through the soil of the New World long after we humans have disappeared from the Earth.
For monkeys, the single colonization of the Americas produced some 130 modern species in a startling array of forms. Monkeys are such a common sight in the forests of the New World tropics, and are such a key part of ecological communities—as foragers, prey, and seed dispersers, for instance—that it seems incongruous to think that their presence is a fluke, the result of a single epic journey in the face of almost unfathomable odds. 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 bald-headed uakaris. Or the raft founders, the monkeys drown, and we end up with, well, who knows exactly what, but something very different; maybe some of the monkey niches are eventually filled by rodents or marsupials, or maybe not. If you were around in the early Eocene and had to make a bet, you’d definitely take the second scenario—the monkeyless Americas—over the first. It just goes to show that in evolutionary history, as in human lives, odd and unpredictable things happen, and sometimes they change the world.
41Incidentally, the ocean-crossing worm lizards eventually gave rise to the species that made the much shorter but still substantial voyage from South America to Fernando de Noronha.
42This range of 31 to 51 million years is from the lowest confidence limit to the highest confidence limit from these studies.
43The fossil record by itself is somewhat equivocal about whether monkeys reached South America from Africa by crossing the Atlantic or from Asia by crossing the Pacific. However, the Atlantic route seems far more likely, both because it is much shorter and because of the moderate handful of cases of Atlantic crossings by other land vertebrates. The only plausible case for a land vertebrate crossing the Pacific is that of the ancestor of Brachylophus iguanas reaching Fiji and Tonga from the New World, but this example remains controversial (Noonan and Sites 2010).
44An intriguing factor in the stepping-stone explanation is that it suggests the possibility of a “smoking gun,” an unusual piece of evidence that could strikingly corr
oborate the reality of an ocean crossing (see Cleland 2002 for discussion of the “smoking gun” concept). Bandoni de Oliveira et al. (2009) noted that that deep-sea drilling in the South Atlantic has recovered fossils of shallow-water organisms as well as rocks that must have been formed either in shallow water or above the water in places that are now up to a mile below the ocean’s surface. These findings could indicate the location of ancient islands reconstructed in Bandoni de Oliveira et al.’s model. Imagine that a sample from one of those submerged islands contained the fossilized bits of a monkey, and that this fossil turned out to be the closest relative of New World monkeys. That would be the geographic equivalent of a missing link, the intermediate piece that corroborated the larger sequence, in this case the stepping-stone journey across the ocean.
45A flaneur is a person who strolls about aimlessly.
Cerithideopsis is a genus of small snails that live in intertidal mangrove and mudflat environments. The North American sister species Cerithideopsis californica and C. pliculosa from the Pacific and Caribbean coasts, respectively, are quite distinct genetically, having been isolated since the emergence of the Isthmus of Panama 3 million years ago. However, in certain areas on both coasts, one can find snails with mitochondrial DNA sequences that are geographically out of place, indicating that some of their recent ancestors were from the “wrong” ocean. This misplaced DNA suggests that Cerithideopsis snails jumped over the land barrier twice within the past million years, and that they did it both ways, once from the Pacific to the Caribbean and once the other way around.
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