Today’s how-possibly dispersal stories are usually just the last step in an argument that begins with evidence from fossils, the anatomy of living species, or DNA showing that long-distance dispersal is required to explain the distribution of some group. Many of these how-possibly stories probably couldn’t stand on their own. For instance, if there was no genetic or anatomical evidence to suggest that amphibians had dispersed from East Africa to the Gulf of Guinea islands, nobody would care about Measey and his colleagues’ arguments about floods in the Congo Basin, and camalotes, and the “freshening” of the surface waters of the ocean. (For that matter, Measey et al. would never have bothered to make those arguments in the first place.) A bizarre dispersal scenario like that only becomes really interesting when there’s at least a reasonable chance that it could be true.
However, this doesn’t mean that the how-possibly story is just the speculative fluff tossed in after the real work has been done, like a meringue after a meal of steak and potatoes. By overcoming incredulity that a thing could even possibly happen, a compelling how-possibly story can strengthen the case that the event actually did happen. That is obviously the intent of a prosecutor using a how-possibly argument: it’s a step toward a conviction, not just an intriguing story. In the case of the Gulf of Guinea amphibians, the how-possibly argument helps convince us that the small holes in the evidence for dispersal—for instance, the possibility that the island species are actually present but undiscovered on the mainland—are just that, small holes. If it were impossible to imagine how a frog or a caecilian could get from East Africa to São Tomé or Príncipe, those small holes would start to look bigger.
THE LADDER OF IMPROBABILITY
The study of the Gulf of Guinea amphibians isn’t finished yet, at least not in John Measey’s eyes. Measey has been contemplating a trip to the Democratic Republic of the Congo to look for caecilians, to get a better idea of exactly where the ancestors of the cobra bobo came from, but the realities of the location have dampened his enthusiasm. Based on secondhand reports of bright yellow caecilians, the place to look is Ituri Province, in the northeastern corner of the country. Biologically, Ituri Province might have a connection to São Tomé and Príncipe, but in human terms these areas are like night and day. Bob Drewes says he loves São Tomé and Príncipe partly because, unlike most of the rest of Africa, there is no “we vs they” there. Ituri Province, in contrast, is a region of violent tribal conflict, a place few foreigners visit unless they are missionaries or United Nations peacekeepers.
Whether or not Measey ever makes it to Ituri Province and finds the relatives of the cobra bobo, he, Miguel Vences, Bob Drewes, and their colleagues have already made a big contribution to the study of oceanic dispersal. They’ve shown that Darwin underestimated the ability of “batrachians” to cross saltwater barriers. At the same time, they’ve put a dent in the armor of vicariance biogeography, although just how big of a dent might depend on who’s looking. To my mind, the amphibian cases, taken together, are pretty significant. Amphibians are supposed to be the worst oceanic voyagers among all the major groups of land vertebrates, and yet we now have strong evidence from the work of Measey, Vences, Drewes, and their collaborators that these animals have reached Madagascar, the Comoros, the Seychelles, and São Tomé and Príncipe without the aid of people. Other phylogenetic studies extend that list of oceanic dispersal by amphibians—toads probably crossed from North America to South America when those two continents were still separated; fanged frogs reached Sulawesi, the Philippines, the Malukus (Moluccas), and the Lesser Sundas; slender salamanders colonized the California Channel Islands; and several frog lineages dispersed to and among the islands of the Caribbean. These studies show that one can no longer assume that related amphibian species separated by salt water must have been introduced by humans, or, as vicariance scientists would usually have us believe, reflect a former land connection. Oceanic dispersal is in play for amphibians, where before it was not.
Still, vicariance scientists can point to the fact that none of these cases involves a really long ocean voyage. If the Congo raft story is right, the oceanic part of the journey to Príncipe would have been about 630 miles, and that would be the longest known oceanic dispersal event for any amphibian. One might even say that the larger pattern fits the vicariance view pretty well. Frogs haven’t crossed the Atlantic. Salamanders haven’t rafted from North America to Australia. Caecilians, for their part, are found in East Africa, yet they haven’t even made the three-hundred-mile journey across the Mozambique Channel to Madagascar. Amphibians are certainly better at dispersing across saltwater barriers than people thought, but they don’t seem to be great at it. Furthermore, at least some of the deep branching points in the amphibian evolutionary tree occurred at times that indicate distributions related to continental movements. For instance, the caecilians and some of the frogs on the Seychelles are most closely related to groups in India, the expected connection for a continental drift explanation, and these Seychelles and Indian lineages separated before those areas split from each other, which is also consistent with vicariance.
So where does this leave us? It leaves us with some other cases of oceanic dispersal to contemplate, ones that are more extreme than the amphibian ones. Think of the Gulf of Guinea amphibian examples as one rung on a ladder of increasing improbability, with each rung representing a kind of dispersal event. The rungs near the bottom of the ladder are relatively easy events, such as birds reaching Japan from the Asian mainland, or gourds drifting to many islands. The middle rungs are less likely events, maybe anole lizards rafting among Caribbean islands, or garter snakes crossing the Sea of Cortés. With the amphibian cases, we’ve reached a high rung on the ladder, but not the top. Call that amphibian rung the “improbable and rare.” The next rung up might be the “mysterious,” and the final rung, the “miraculous.”
We’re about to keep climbing up the ladder.
38This species is now known as Gephyromantis granulatus.
39If new Malagasy frog species are being discovered at an incredible rate, how could Vences be confident that the Mayotte frogs were not members of some as yet unknown species on Madagascar? He was sure in large part because the frogs from Mayotte are lowland species found in second-growth forests, while nearly all of the new species on Madagascar are from undisturbed mountain forests. Lowland, second-growth environments on Madagascar have been extensively surveyed, and frogs that could be the same as the two Mayotte species have not been found.
40I would argue, uncontroversially, that The Origin of Species is not just a set of how-possibly arguments. For instance, a critical aspect of the book is that it shows that many biological phenomena make sense under evolution but not under creationism. In other words, Darwin shows not only that evolution is possible, but that the alternative explanation is untenable.
“That insects can be transported into the upper regions of the atmosphere by ascending air-currents was long ago remarked by Humboldt, and the subject has been discussed with his usual acumen by Whymper (Travels amongst the Great Andes of the Equator). Carried along in the higher air-currents these insects might finally be deposited at places far distant from their home. One reads occasionally extraordinary accounts of a rain of insects. A very circumstantial account was given to me when I was on Keeling Atoll of a shower of dragon-flies that fell on the islands, their remains being found in quantities in the lagoon. Dragon-flies, it is known, are often found at sea far from land, and one species has been observed nearly all over the world, including the Pacific islands.”
—H. B. Guppy, Observations of a Naturalist in the Pacific, vol. 2
Chapter Nine
THE MONKEY’S VOYAGE
THE LONG ROUTE TO FERNANDO DE NORONHA
Off of Brazil’s eastward-jutting shoulder, a few degrees south of the Equator, lies the small island of Fernando de Noronha along with about twenty even smaller islands and isl
ets, the emergent heights of a mostly submarine range of volcanic mountains. Like many islands, Fernando de Noronha once served as a penal colony; people have recognized for a long time that water can be an effective barrier to dispersal, at least for other people. In fact, especially unruly inmates at the prison on Fernando de Noronha were sometimes banished without guards or assistance to a small island called Rapta, northeast of the main island, to scrabble for survival on their own. No one was worried that they would escape; there were no trees on Ilha Rapta to fashion a raft from, and the prisoners weren’t going to swim a couple of hundred miles to the mainland.
Today Fernando de Noronha is a World Heritage Site and something of an ecotourist destination, a haven for seabirds, marine mammals, and fishes, all taking advantage of the nutrient-rich waters. It’s also a place that demonstrates the rule that oceanic islands tend to have few species of native land vertebrates; Fernando de Noronha has had only three that anyone knows about—a skink, a legless amphisbaenian (or worm lizard), and a rat that’s now extinct. The presence of just two lizards and a rodent jibes with the view that land vertebrates are lousy oceanic voyagers; Fernando de Noronha is a mere 220 miles from the Brazilian coast, yet only these three species have managed to colonize the island.
For the worm lizards and the rats, there isn’t much more to tell. Both species are (or were) closely related to South American taxa and apparently came from that nearby continent. However, starting more than sixty years ago, biologists have recognized something odd about Fernando de Noronha’s skinks. (Something other than the fact that they’re extremely common and extremely tame: One English visitor in the nineteenth century reported that, while he was cautiously climbing a steep bluff, using his hands to keep from falling, one of these lizards ended up crawling around in his trousers for nearly an hour. He described the lizard as having become “offensively familiar.”) Specifically, these skinks (Mabuya atlantica, sometimes called Trachylepis atlantica) lay eggs and have keels, or ridges, on their scales, which are unremarkable features except for the fact that South American species in the same genus give birth to live young and have smooth scales. However, some African species of Mabuya do lay eggs, have keeled scales, and have several other traits suggesting they are close cousins to M. atlantica. In other words, it looks like M. atlantica is part of an African branch of the genus, not the South American branch.
9.1 Mabuya atlantica, the skink from Fernando de Noronha. Photo by Jim Skea.
It’s possible that the obvious similarities of M. atlantica to African species are misleading, and that the Fernando de Noronha skinks don’t really belong with the African ones. For instance, egg-laying is probably ancestral for the whole genus (and therefore doesn’t indicate an especially close relationship within the genus), and perhaps keeled scales evolved independently in M. atlantica and African Mabuya, like wings in birds and bats. Maybe M. atlantica, despite appearances, is actually most closely related to South American Mabuya. It wouldn’t be the first time that obvious traits in common have led biologists astray. However, M. atlantica’s African roots have been verified by recent work: several studies using DNA sequence data have strongly supported the close relationship of the species to African rather than South American members of the genus. Thus, the ancestors of M. atlantica must have reached Fernando de Noronha by crossing almost the whole width of the Atlantic, presumably by rafting on the South Equatorial Current, which heads more or less due west from the central African coast toward the island. It’s a case that shows that identifying the distance required for colonization based on the nearest continent can be misleading—a relatively short journey for the rat and the worm lizard was a much longer one for the skink, coming from the opposite direction (see Figure 9.2).
9.2 Fernando de Noronha is only 220 miles from the Brazilian coast, but the skink came from Africa.
The ancestors of M. atlantica apparently made their transatlantic voyage within the past 3.3 million years (the maximum estimate for the age of Fernando de Noronha), crossing the ocean in roughly its modern configuration. They must have traveled at least 1,800 miles over water, and in fact, based on likely routes, using westward-flowing currents, the distance traveled was probably closer to 3,000 miles. A final irony is that, even if M. atlantica had been derived from South American species of Mabuya, it would have had a transoceanic journey in its recent evolutionary past: the same DNA studies that show that M. atlantica came from Africa also indicate that the ancestor of all other New World Mabuya arrived from the Old World within the past 9 million years or so, probably also dispersing from Africa across the Atlantic.
Molecular dating analyses indicate quite a few other ocean crossings by lizards and snakes, including two skink lineages and two gecko lineages traversing most of the breadth of the Indian Ocean, four different gecko lineages dispersing from Africa to the West Indies or South America, worm lizards making the journey from Africa to South America, and both blindsnakes and threadsnakes crossing from Africa to some part of the New World. The cases of the worm lizards, blindsnakes, and threadsnakes are especially surprising because these animals spend almost their entire lives underground, and such creatures are thought to be especially unlikely to cross oceans.41 These burrowing reptiles can’t fly or float on the air or stick to the feet of a bird, and, unlike geckos (and to a lesser extent, skinks), they don’t often cling to trees or other vegetation that might get blown out to sea. About the only way a burrowing reptile can get across an ocean without human assistance is on a raft containing chunks of earth, and the chunks might have to be large enough to remain unsaturated during the long voyage. For these reasons, such subterranean vertebrates have often been placed in the category of animals that simply do not cross oceans. A burrowing lizard or snake dispersing over the Atlantic is, in this view, as imaginary as a unicorn. But that view is apparently wrong.
These examples of lizards and snakes crossing entire oceans can stand for a high rung on our ladder of improbability, a step up from the amphibian cases. (However, the number of crossings by geckos—at least six and counting—suggests that this particular group may represent a lower rung.) Such events seem sufficiently implausible that many biologists have been very reluctant to invoke them, sometimes banishing them to the realm of the impossible. These ocean crossings seemed implausible or impossible, that is, until the evidence made them unavoidable.
Nonetheless, in hindsight one could argue that it isn’t so surprising that lizards and snakes have crossed the Atlantic and other oceans. After all, they’re small enough that a modest-sized floating island might contain many of their prey, provisions for the long journey. Also, they have relatively low metabolic rates, meaning that, even when they run out of food, they can last a long time before they starve or die of thirst. That’s especially true if they’re simply lying around, with their body temperatures equilibrated to the cool maritime air, their metabolic motors running at a very low idle. In such a cold state, many reptiles can go for months without food or water, certainly long enough to raft across the Atlantic or Indian Oceans.
9.3 Amphisbaena alba, a member of a large group of South American worm lizards descended from another transatlantic voyager. Photo by Diogo B. Provete.
Our next example, however, has none of these things going for it. At last, we are about to ascend to the top of the ladder, a case of oceanic dispersal apparently so absurd that it has, at times, provoked ridicule not only from biologists but also from creation scientists. People who have no vested interest in the topic have laughed at me when I’ve brought up this case.
We have reached the centerpiece example of the book.
THE MYSTERIOUS, MIRACULOUS VOYAGE
It’s a hot summer day and my wife Tara, our one-year-old daughter Hana, and I are having a picnic at Pyramid Lake, in the dry, sagebrush-and-greasewood country north of Reno. The white, pebbly beach is deserted, one reason we sometimes come here instead of crowded Tahoe. Gulls cruise alo
ng the shoreline. The occasional flock of white pelicans wheels in the bright sky. On the dun-colored, treeless mountain slopes surrounding the lake, one can make out faint bands, the remnants of higher, ice-age shorelines, looking like poorly spaced contours on a topographic map.
The water is cool, calm, and inviting. Some large black-and-white Western Grebes are drifting about a hundred yards offshore, and Tara and I take turns swimming out toward them while Hana, squatting on the shore, happily flips pebbles into the lake. The wind changes direction, now blowing out from the shore, and Tara comes back from her turn in the lake, saying it was hard getting back in. I glance up at the sky, feel the wind on my face, and figure I’ll take one more quick swim, trying to get as close as I can to the grebes.
Heading out from the shore is easy, and I end up so close to a grebe that I can see its devilish red eyes fixed on me, as if it’s wondering what to make of my disembodied head and shoulders appearing above the water. Swimming back in is a different story. The wind has picked up and the water is getting choppy and, although I put my head down and take what I’m sure are powerful strokes, when I look up the shore seems just as far away as before. It crosses my mind that many people have drowned in Pyramid Lake, often because they were out on the water when the wind turned. It also crosses my mind that I’m a dumbass for not thinking about that earlier. I’m not in a full panic, but I’m pushing hard now, trying to swim the way you’re supposed to swim, with deep, full strokes and regular breathing. Almost imperceptibly, the shore starts edging closer. Finally, after what feels like a long time (but is probably about five minutes), my toes touch sand. I wade up onto the beach, not quite exhausted, but definitely winded, and relieved to feel the earth under my feet.
The Monkey's Voyage Page 24
