The channel between Bali and Lombok lies squarely on Wallace's Line, the famous geographic divide named after the nineteenth-century British biologist Alfred Russel Wallace who, along with Charles Darwin, codiscovered natural selection. While the distance between Bali and Lombok was no greater than that between many of the waterways separating the hundreds of islands along the Indonesian archipelago, Wallace noted that animal populations on either side of the channel differed extensively. And while he didn't have the precise models for ice age water levels that we have today, he surmised that this biological divide existed because Bali and Lombok were never connected by a land bridge, something we now know to be true.


Like humans, other animals take advantage of land bridges, but unlike these earlier settlers who had boats, the animal populations that couldn't fly long distances were largely stuck on one or another side of this deep-water barrier. When the first explorers left Asia for the Australasian continent, making the thirty-five-kilometer hop from Bali to Lombok, they took a fairly short trip by boat but a huge leap for primates. When they crossed this divide, these early explorers entered a world that had never seen monkeys or apes before. They also encountered completely new microbes.

These early settlers would have been hit with novel diseases from Australasian animals and their microbes, infectious agents that had never seen a primate before. Yet the impact of these agents for the human populations as a whole was likely limited, since the small population sizes of the settlers wouldn't have been able to sustain many kinds of agents.

It's hard to know exactly what the first trips across Wallace's Line were like. They may have been colonization events with small groups that were then completely cut off. Perhaps more likely they were short initial forays into new lands, followed by the establishment of temporary outposts, much as we consider colonizing the moon. The actual way in which the new lands were colonized would have played an important role in determining the flow of microbes in either direction. And while these first Australasian humans almost certainly had some connections to the "mainlanders" they left behind on Bali, that contact may have been very infrequent. Yet some new Australasian infections that had the potential for long-lasting human infection could very well have made their first forays into human populations on the Asian side of the divide.


The use of boats to visit new lands would continue with increasing frequency over the forty or so thousand years following this first colonization of Australasia. We have much better knowledge of what later trips were like and how they connected microbially distant lands. Perhaps the peak of boating-based colonization before modern times occurred among the Polynesian populations of the South Pacific.

Among these Polynesian journeys, probably the most incredible was the first discovery of Hawaii, over two thousand years ago. For the first lucky settlers, finding this island would have been truly like finding a needle in a haystack. To give a sense of scale, the largest island of the Hawaiian archipelago, also named Hawaii, has a diameter of around a hundred miles. And the Southern Marquesas, whose inhabitants were the most likely first colonizers of Hawaii, are some five thousand miles away. To imagine what it would have been like to hit the mark, imagine we blindfolded an Olympic archer, then spun him around and asked him to hit his target—the ratios are about the same. One can only imagine how many boats (and their inhabitants) were lost before the fortunate finally made it.

On their long trips, the Polynesians probably lived largely on caught fish and rainwater. Yet they traveled with a veritable biological menagerie. They brought along sweet potatoes, breadfruit, bananas, sugarcane, and yams. They also traveled with pigs, dogs, chickens, and probably (unintentionally) rats. Having all of these domesticated species meant that the flotillas carried not only life support for the Polynesian explorers, but also minirepositories of microbes, which would spread and mix with local microbes in the places that they discovered.


The boat journeys of the Polynesians, as remarkable as they were for their time, pale in comparison to the global shipping that emerged in the fifteenth and sixteenth centuries. By the time Europeans reached the New World, in the late fifteenth century, thousands of massive sailing ships were plying the waters of the Atlantic and Indian Oceans and the Mediterranean Sea, moving people, animals, and goods back and forth between the countries of the Old World.

The impact of smallpox on New World populations is the most dramatic known example of the way that the connections formed by shipping can influence the spread of microbes. Some estimates suggest that as many as 90 percent of the people living in the Aztec, Maya, and Inca civilizations were killed by smallpox brought by boats during European colonization, a massive and devastating carnage. And smallpox was only one of many microbes that spread along the shipping routes of this time.

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