In the great tropical harbor of Manila Bay, two groups of men warily approach each other, their hands poised above their weapons. Cold-eyed, globe-trotting traders, they are from opposite ends of the earth: Spain and China.

The Spaniards have a big cache of silver, mined in the Americas by Indian and African slaves; the Chinese bring a selection of fine silk and porcelain, materials created by advanced processes unknown in Europe. It is the summer of 1571, and this swap of silk for silver—the beginning of an exchange in Manila that would last for almost 250 years—marks the opening salvo in what we now call globalization. It was the first time that Europe, Asia and the Americas were bound together in a single economic network.

The silk would cause a sensation in Spain, as the silver would in China. But the crowds that greeted the returning ships had no idea what they were truly carrying. We usually describe globalization in purely economic terms, but it is also a biological phenomenon. Researchers increasingly think that the most important cargo on these early transoceanic voyages was not silk and silver but an unruly menagerie of plants and animals, many of them accidental stowaways. In the sweep of history, it is this biological side of globalization that may well have the greater impact on the fate of the world's people and nations.

Some 250 million years ago, the Earth contained a single landmass known as Pangaea. Geological forces broke up this vast expanse, forever splitting Eurasia and the Americas. Over time the two halves of Pangaea developed wildly different suites of plants and animals.

Before Columbus sailed the Atlantic, only a few venturesome land creatures, mostly insects and birds, had crossed the oceans and established themselves. Otherwise, the world was sliced into separate ecological domains. Columbus's signal accomplishment was, in the phrase of the historian Alfred W. Crosby, to reknit the seams of Pangaea.