The Monkey's Voyage Page 3
As I wandered around New Zealand, from the subtropical forests in the north to the glacial valleys of the south, I was constantly running into signs of Gondwana. Not signs as in biological or geological evidence, but signs as in signage. Almost every nature preserve and national park had signs or pamphlets mentioning the Gondwanan origins of New Zealand’s flora and fauna. The country seemed to be part of both the British Commonwealth and an even larger league of nations, the fragments of the former southern supercontinent.
At Nelson Lakes National Park, in the northern part of the South Island, I walked in a mossy forest of the famous Gondwanan trees, southern beeches. Many of the tree trunks were blackened by a fungus that grows on the honeydew that drips out of the rear end of a scale insect, giving the forest a slightly diseased look (although the fungus apparently does no harm to the trees). Still, the trees were beautiful, their foliage delicate and layered, in places making them look like overgrown bonsai. Thumbing through a small field guide to the trees of New Zealand, I picked out, by the size and shape of their leaves, at least three species—red, silver, and mountain beech. Nothofagus fusca, Nothofagus menziesii, Nothofagus solandri. According to the landmasses-as-life-rafts story, they are all part of a lineage that has been in New Zealand since the breakup of Gondwana.
For a while the forest trail seemed to wander aimlessly, finding and then losing the course of a small creek, but eventually it gathered purpose on a long set of switchbacks up toward a ridgetop. Reaching the treeline (the bushline, to Kiwis) I was startled to find it unlike anything I had experienced in the mountains of North America. As I climbed, the beech trees got smaller and smaller, but the forest didn’t thin out gradually, as I had expected. Instead, within just a few steps, the dwarfed but still dense forest disappeared, and I entered a completely treeless alpine zone. It was like walking from woods into a farmer’s cleared field. This alpine area turned out to be as strange to me as the abrupt passage into it; in the Rockies or the Sierra Nevada, the vegetation above the treeline is sparse, or very short, or both, but in these New Zealand mountains, much of the treeless alpine area was thickly covered with tall tussock grass.
Just above the treeline I found a flat outcrop that made a good seat, where I caught my breath and admired the view across the deep blue of a large lake, Rotoiti, to the paler blues and greens of the mountains beyond. This seemed to be the place where most people turn around, or, at least, the place where whoever planned the trail thought people should turn around, because, above this point, the path narrowed, and the carefully laid switchbacks became a steep beeline to the crest of the ridge.
On this last, gasping scramble to the ridgetop, with my nose almost in the dirt, I gained a greater appreciation for the subtle beauty of the alpine plants. All around me the tussock grass whipped and undulated in the heavy wind. Between the tussocks and beneath my clutching hands were stringy, dark-green plants with scaly leaves like a juniper, shiny yellow buttercups, and mats made up of rosettes of narrow sage-colored leaves. Wandering along the ridgetop I noticed a pale gray mound a couple of feet wide affixed to the flat surface of a rock. Up close, the mound resolved itself into thousands of leaves, each one rolled up into a tiny cylinder, hard to the touch. It was a vegetable sheep, a silly name but an apt one—from a distance, a group of these plants looks like a shepherd’s flock. Vegetable sheep are in the sunflower family, but, remembering Jared Diamond’s words, I thought of them as sunflowers from another planet.
On the ridgetop, the wind was roaring in my ears and threatening to blow me off my feet. But, to my relief, just a few steps down on the lee side of the ridge it was perfectly calm and quiet, as if someone had flipped off the switch on the wind machine. I had passed a few people on the trail, but now I was alone in the abrupt silence. I sat down, drank some water, and took in the view—the rocky ridge, the washed-out earth tones of the alpine landscape, the dark green of the beech forest below. The place felt untouched and ancient.
If I had visited this spot a few years earlier, I would have thought of the southern beech trees, the vegetable sheep, and the other plants as descendants of the flora that drifted off with New Zealand as it broke away from other parts of Gondwana. No doubt I would have felt the mythic power of that story as I sat in the quiet solitude of the mountains—Here I am, on an actual piece of Gondwana, surrounded by its ancient flora! Instead, an entirely different scenario passed through my mind. I imagined a tangle of trees, perhaps blown down by a storm, floating on a wide ocean thousands of miles from land, with fruit still in the trees’ branches, and seeds in the dirt stuck to their roots. In the dark recesses of the tangle, I envisioned spiders and crickets and lizards clinging to the branches.
And I thought, “It’s time for a new story. It’s time to change those signs.”
1The biotas of the tropical parts of Eurasia and the Americas are much more distinct, at least in part because the recent incarnations of the Bering Land Bridge have been too cold for tropical organisms to pass over by that route.
2This description is a simplified view of the origin of the Sea of Cortés; the process probably occurred in several stages and involved not only the Pacific and North American Plates but also smaller tectonic plates in the region.
3I have capitalized common names of bird species, following the established convention among ornithologists, but I have not capitalized the common names of species in other groups.
At 6:00 in the morning on December 14, 2004, an Aldabra giant tortoise (Dipsochelys dussumieri), the Indian Ocean’s analogue to the oversized tortoises of the Galápagos, ambled out of the sea at Kimbiji, 22 miles south of Dar es Salaam in Tanzania. Inspection of the tortoise’s shell showed faint concentric growth rings, indicating that the animal came from the native population on Aldabra, where the high density of tortoises leads to slow growth, rather than from introduced populations elsewhere in the Seychelles or on Changuu Island near Zanzibar. Aldabra also made sense as the point of origin based on the direction of prevailing currents. A trip from Aldabra to Kimbiji would cross 460 miles of ocean waters as the crow flies, and presumably somewhat farther as the tortoise floats.
The Kimbiji tortoise was emaciated, as one might expect, but even more telling was the fact that its front legs and part of its lower shell were covered with thickets of goose barnacles, like the hull of a boat. Barnacles settle as tiny larvae and, once fixed, do not move. From the size of the largest ones, it was surmised that the tortoise had been in the ocean for at least six weeks.
I.4 The Kimbiji tortoise. Photo by Catharine Joynson-Hicks.
Section One
EARTH and LIFE
Chapter One
FROM NOAH’S ARK TO NEW YORK:THE ROOTS OF THE STORY
PRELUDE: CROIZAT’S VISION
Léon Croizat sat at his desk, writing . . . and seething. For Croizat, an Italian botanist living in Venezuela, writing seemed as natural as breathing, and nearly as constant—from 1952 to 1962, his especially prolific period, he published four technical biology books totaling close to 6,000 pages—and when he wrote, he was often thinking about Charles Darwin. And when he thought about Darwin, he seethed. It was not about religion—Croizat was as complete an evolutionist as Darwin had been. However, in Croizat’s eyes, Darwin had gotten almost everything about evolution wrong. To begin with, Croizat believed that natural selection was a trivial part of evolution, not its main driving force. More than anything, though, he hated Darwin’s views of historical biogeography, of the means by which living things had acquired their particular distributions on the Earth.
Croizat had a grand vision, a unified theory of the geography of life. It boiled down to this: the distributions of groups, from orchids to earthworms to armadillos, all reflected the dynamic climatic and geologic history of the planet itself. Sea levels rose to inundate land bridges; ocean basins opened, dividing continents; island arcs plowed into continental margins. These chang
es in the configurations of landmasses and oceans left an indelible imprint on life. In fact, that imprint was so unmistakable that one could use the distributions of living things to reveal the history of the Earth. Find out where the orchids and the worms and the armadillos live, and the arrangements of the continents through time also would be revealed.
Croizat gave his theory a name befitting its all-encompassing nature, its power to explain the distributions of living things over the entire planet. He called it panbiogeography. He also provided a memorable phrase, probably the most memorable one in the history of the discipline, five words that captured the essence of his worldview: “Earth and life evolve together.”
Croizat’s panbiogeography ran counter to an idea that had a long history among biologists and naturalists, namely, that the discontinuous distributions of species and higher taxa often were the result of chance dispersal, of unpredictable, long-distance jumps. To the extent that such dispersal was common, it meant that distributions did not reflect Earth history. Terrestrial organisms, for instance, could move even among landmasses that were widely separated. To Croizat, this was lunacy, mere storytelling founded on absurdly improbable events. Beyond that, it robbed biogeography of any kind of generality, because a different story might apply to every taxonomic group. Perhaps snails had reached the Hawaiian Islands attached to the feathers of a bird, spiders by using long silk strands to float on storm winds, and bean trees as seeds embedded in a raft of vegetation. And perhaps ants and termites and bumblebees had not reached those islands simply because, well, because they had not. This view of biogeographic history was pure chaos, the antithesis of unification. And where did this pabulum come from? It came from Charles Darwin. To most biologists, Darwin was like a secular saint, even a deity, but to Croizat he was a fool and worse—he was the unthinking dilettante who had come up with an unsupportable view of the geographic history of life and somehow convinced almost everyone that he was right. A hundred years after publication of The Origin of Species, in which Darwin had presented his ideas on chance dispersal, the field of biogeography was still laboring under the delusions of the “master.”
Croizat thought it was time for this long, anti-intellectual chapter to come to an end, and that he, of course, would be the one to end it.
BEGINNINGS
At Down House, his country home in Kent, Charles Darwin worried about the implausibility of long-distance dispersal, especially dispersal over water. He thought it was a problem for his theory of evolution. How could the same species, or two species that were closely related by descent, turn up in regions separated by seas or oceans? For that matter, how did many species find their way to oceanic islands, which were separated from everywhere by ocean barriers? Darwin had all kinds of reasons to believe that species were connected by descent, but he thought this problem—the problem of related groups living in areas divided by large bodies of water—could be a sticking point for skeptical readers. In these cases, it almost seemed as if creation were a better explanation than evolution. Wasn’t it easier to imagine that God had created the same or related species in these widely separated places than to envision all manner of animals and plants making absurdly long ocean voyages? Could iguanas really have rafted from South America to the Galápagos on their own? Could beech-tree seeds have floated from Australia to New Zealand?
Darwin was aware of the other natural (as opposed to divine) explanation for such distributions, that is, the existence of former land connections. However, over time he had come to view the easy use of such explanations as little better than invoking the supernatural. It seemed like cheating, pulling something out of thin air, or, more precisely, conjuring up land out of the deep, unfathomable ocean. He and his close friend, the botanist Joseph Hooker, had a running argument about the subject. Like Darwin, Hooker had taken a long ocean voyage, as a naturalist aboard the HMS Erebus and the HMS Terror, and, seeing obvious similarities among the floras of various southern lands, he had suggested that plants had moved across now-sunken land bridges. Darwin did believe that lands had risen and fallen—he had seen evidence of rising land in the Chilean Andes and of subsidence in the coral islands of the Pacific—but he didn’t like using land-bridge explanations in specific cases when there was no geological evidence to back them up. In an 1855 letter to Hooker, he wrote, “It shocks my philosophy to create land, without some other & independent evidence” (that is, other than distributions of organisms). Hooker, for his part, was equally skeptical about some of Darwin’s ideas on the dispersal of plants and animals across water. He especially didn’t like Darwin’s penchant for suggesting transport on icebergs. The Erebus and the Terror had journeyed to Antarctica, crashing their way through ice floes, and Hooker had seen his share of icebergs. He had the impression that not many living things caught rides on them.
The issue of oceanic dispersal was important enough to Darwin that, from 1854 to 1856—while he was still waffling over how to present his evolutionary ideas publicly—he conducted a whole series of experiments at Down House to figure out whether plant seeds and other propagules could possibly cross large water barriers. He put the seeds of eighty-seven kinds of plants in bottles filled with salt water for weeks and months, then planted the seeds to see if they were still viable. He dangled the disembodied feet of a duck in an aquarium to see if hatchling freshwater snails would cling to them. Knowing that some fish would eat plant seeds, he forced seeds into the stomachs of fish, fed the fish to eagles, storks, and pelicans, and then tried to germinate the seeds he retrieved from the birds’ droppings.
The experiments convinced him that long-distance oceanic dispersal was a lot more likely than one might think. Many kinds of seeds survived after being immersed in salt water for 28 days, and a few survived for 137 days. The young snails did climb up onto the duck’s feet, suggesting they could hitch a ride to wherever a duck might fly (although the distance would be limited to the time it takes a tiny snail to dry up). Some of the seeds from the eagle, stork, and pelican droppings germinated, indicating another possible means of transport by birds. Careful, as always—Darwin was nothing if not a careful thinker—he reasoned that seeds on their own wouldn’t make it very far because they would sink. So he also collected dry branches with fruits attached and dropped these into salt water to see how long they could remain afloat. Combining the results of these floating-branch experiments with the seed-viability numbers and estimates of the speed of ocean currents, he calculated that seeds of 14 percent of plant species could travel at least 924 miles and still germinate at the end of the trip.
Darwin wrote quite a few letters to Hooker describing these results and, like most experimentalists, he seemed to take pleasure in conveying the difficulties of the work. “It is quite surprising that the Radishes shd [should] have grown, for the salt-water was putrid to an extent, which I cd [could] not have thought credible had I not smelt it myself,” he wrote in one letter. He also enjoyed what sounds like a self-effacing, Victorian version of trash-talking at Hooker’s expense: “When I wrote last, I was going to triumph over you, for my experiment had in a slight degree succeeded, but this with infinite baseness I did not tell in hopes that you would say that you would eat all the plants, which I could raise after immersion.” Eventually, he changed Hooker’s mind on the subject. At one point, Hooker even conceded that “I am more reconciled to Iceberg transport than I was.” Darwin had won a round for dispersal explanations.
It was not as if Darwin were the first person to think about oceanic dispersal. In fact, some 250 years earlier, in the late 1500s, there had been a surge of interest in both overwater dispersal and land bridges. What brought on this early attention to the geography of living things was, oddly enough, a shift from allegorical to literal interpretations of the Bible. In particular, taking the story of Noah’s Ark at face value meant that all the animals in the world, two by two, must have ended up in a crowd on the top of Mount Ararat after the Flood. This meant that somehow an
imals had repopulated the world from that single spot, which in turn required them to cross oceans. How had they done it? One theory was that transoceanic journeys had been made in stepping-stone fashion, with the animals swimming from island to island. Another was that animals had traveled as cargo on boats (the same boats with which people had repopulated the world). A third had animals crossing from the Old World to the New World on the lost continent of Atlantis.
An English historian of science named Janet Browne has argued persuasively that these biblically motivated ideas about the colonization of the world by animals (plants weren’t part of the Ark story) mark the beginnings of scientific thinking about the distributions of living things on the Earth. They also may represent early inklings of the dispersal-vicariance debate: the stepping-stone and cargo ideas are obviously about long-distance dispersal, and the notion of Atlantis as a land bridge looks like a vicariance hypothesis, with the continuous ranges of animal “kinds” being split into Old and New World portions by the drowning of the lost continent.