The Tree (36 page)

The continents move slowly—only a few centimeters a year—but they have had plenty of time, and over the past few billions of years their peregrinations have been prodigious. Five hundred million years ago (in Cambrian times) the present landmasses were scattered islands. Places now in the tropics have at times been near the poles, and vice versa. Places that are now in the heart of continents have been islands, and present-day islands have been part of mighty continents. Most of what is now North America was an island that straddled the equator. Siberia was a subtropical island in the Southern Hemisphere. And so on.

All this was before any plants had come onto land, and long before there were trees. But by about 265 million years ago (the mid-Permian), when already there were plenty of trees, all the islands had massed together to form one great landmass known as Pangaea. By about 200 million years ago (in the early Jurassic), when the conifers and cycads were in their pomp and the flowering plants were yet to come on the scene, Pangaea began to split more or less in half to form two great “supercontinents”: Laurasia to the north, and Gondwana to the south.

Ever since—through all the time that the flowering plants have been evolving—those two great supercontinents have been breaking up. Laurasia has split to form present-day North America, Greenland, Europe, and most of Asia. Gondwana has fragmented to become present-day Antarctica (a huge continent), South America, Africa, Arabia, Madagascar, India, and Australia, plus a fairly long catalog of today’s islands, including New Zealand and New Caledonia.

The details of this redistribution are roughly outlined in the figure on pages 286–87. Two features in particular are outstanding. Note, first, that South America was an island for tens of millions of years, until it finally made contact with North America, via what is now called Panama, around three million years ago—which in geological time is very recent. India, too, was an island that drifted north for tens of millions of years, until it finally crunched into the south of Asia. The crunch took place about sixty million years ago (not long after the dinosaurs disappeared)—and the impact, slow and inexorable as it was, caused the rise of the Himalayas. (The Himalayas would still be rising were it not for the erosion that tends to keep them at the same height.) Australia is still an island, drifting north. Opinion is divided on whether it will miss Asia altogether, crunch into China, or obliterate Japan.

All the time the continents have been shifting, so the modern groups of plants and animals have been emerging. Modern land mammals, which could not easily cross the widening oceans, still show its effects very clearly. So it was that in the nineteenth century Alfred Russel Wallace pointed out that the mammals of Southeast Asia were quite distinct from those of Australia and New Guinea. In Asia there were cats, pigs, and deer; while Australia was dominated by marsupials and contained the unique egg-laying monotremes—the platypus and echidnas (though there is also an echidna in New Guinea). The boundary between the two faunas became known as Wallace’s line. Although Wallace didn’t know it, his line marks the border of Laurasia and Gondwana.

Continental drift must also enrich our attempts to explain why modern trees are where they are. To be sure, each kind must have arisen in one particular place, its center of origin. Each may then have spread to other places, and then spread again, and perhaps the earliest ones in the original center went extinct. But all the time this was happening the ground itself was shifting, as the continents wandered the globe, breaking apart and sometimes joining up with others.

Through such ideas we begin to see why different modern-day islands have such very different characters. Madagascar has been an island for a very long time, but it used to be part of Gondwana. When it broke from the mother continent it contained a random collection of plants and animals—the latter (perhaps) including the ancestors of lemurs, but not of modern carnivores or hoofed animals. Whatever plants it had on board were free to evolve in all kinds of strange directions, without competition from continental types. The traveler’s palm,
Ravenala,
the baobabs, and the Didiereaceae are among the results. New Caledonia too is a fair-sized, long-isolated fragment of the erstwhile Gondwana; but New Caledonia began its island existence with a different collection of Gondwanan creatures on board, which now are almost as spectacularly strange as those of Madagascar. In contrast, Hawaii, the Galápagos, the Azores, and the Canaries did not begin as bits of continents. They arose from the bottom of the ocean, in volcanic eruptions, and began life as bare rock. They, too, have their own unique collections of endemic creatures. But all their inhabitants are descended from ancestors that were brought in by wind or water, or flew or swam, or hitched a ride on the fliers or swimmers. Creatures with no such abilities stayed away. Conifers (apart from one bird-dispersed juniper on the Azores) are notably absent from midoceanic volcanic islands. New Caledonia is in the middle of nowhere but it is a fragment of continent, and its indigenous conifers, borne from Gondwana, are among the wonders of the world.

About 225 million years ago the world’s continents were joined to form a supercontinent called Pangaea.

Eventually Pangaea started to break into two separate continents.

Gondwana broke up to form Antarctica, Africa, Australia, and South America, plus India, Madagascar, New Zealand, and New Caledonia. Many (though by no means all) of the trees that now live in those southern lands were Gondwanan in origin.

The continents are still drifting. Perhaps in a few million years Australia will collide with Southern Asia, as India once did. Perhaps it will crunch into Japan. Or perhaps it will slide past Japan and into the North Pacific. Each of the possible scenarios will be dramatic, though none is urgent.

Britain was also part of an ancient continent, as Madagascar was, and has been linked to mainland Eurasia from time to time in the past few million years as the sea level fell during the ice ages. Yet it lacks much of the variety found in neighboring Eurasia and has no endemics to speak of, apart from a few fish, including various subspecies of char and trout. But here we encounter yet another complication. Britain endured the rigors of the ice ages. During the ice ages the British Isles were linked to mainland Europe, which should have enriched its flora and fauna. Instead, or at least equally, the ice wiped out what was there. Madagascar has long been in tropical latitudes and escaped any such purge. This story belongs to the next discussion, however: why the tropics are so much more various than temperate latitudes.

With these general principles in place (center of origin, secondary centers of diversity, continental drift), and aided by modern techniques (analysis of DNA and the steadily improving fossil record), biologists are now putting flesh on the theoretical bones. The Royal Society meeting of March 2004 showed how rich—and unexpected—the realities are now proving to be.

REALITY: A FEW CASE HISTORIES

Where Did the Amazon Forest Come From?

Take, for example, the American tropics (the neotropics), the most diverse ecosystem of all. South America is a fragment of Gondwana that broke away from Antarctica sometime in the Cretaceous, rafted north as an island for the best part of 100 million years, and finally docked with North America around three million years ago. (Although “finally” in this context simply means “most recently.” The travels of continents, like the Flying Dutchman’s, will never end.) The relationships of animals can be easier to see than those of plants, and it’s clear that South America has many strange animals—notably the sloths, anteaters, armadillos, and its own suite of marsupials—that are peculiar to itself and reflect the Gondwanan origins of South America and its long isolation as an island. But South America also contains northerners, including jaguars, pumas, and various deer, which came in from North America. So we might expect to see the same pattern among trees: basically a Gondwanan island flora with a few incursions, mainly from North America.

Yet this commonsense assumption holds up only to a very limited extent. Thus, the characteristic flora of northern South America at present is tropical rain forest. If the trees of the forest are indeed Gondwanan in origin, their ancestors ought to have been there tens of millions of years ago. But Dr. Robyn Burnham of the University of Michigan has studied fossil plants in Bolivia and found very little evidence of any tropical rain forest at all before about sixty million years ago. The oldest rain-forest fossils seem to date from around fifty million years ago. She has, however, found clear evidence of tropical rain forests in several sites in North America that are more than sixty million years old. This suggests (it does not prove, but it suggests) that the great rain forests of the neotropics, including South America, arose in the northern continent, Laurasia; and that the trees found their way to South America long before South America met up with North America around three million years ago, in the Pliocene. This fits in with other evidence of many kinds that suggests that South America had contact of a kind with northern continents long before the Pliocene. For example, the
small
South American cats—the ocelot and margay—seem to have been in place since well before the Pliocene. So, too, have South America’s monkeys, like the howlers, capucins, and tamarins. The ancestors of these animals also arose on other continents, but they got to South America before North and South America most recently collided. Presumably there were other land bridges in the past, now long gone—or if not literal bridges, then at least chains of islands, as are still to be seen through the Caribbean.

However, molecular evidence from Dr. Toby Pennington, at the Royal Botanic Garden Edinburgh, strongly suggests that the present-day trees of the South and Central American forest did not come from North America either. Their closest relatives, at least of the groups he has looked at, often seem to be in Africa. This seems to take us back to the original idea—that the plants of Africa and South America are both simply Gondwanan. But close comparison of DNA suggests that many of the present-day South American plants originated
after
South America broke from Gondwana—and that by some means or other they entered from Africa. The picture grows more and more mysterious.