How an Interconnected Planet Is Fueling the Brewing Viral Storm

To prevent a pandemic: A new book by Nathan Wolfe follows the emergence of new viruses, the discovery of old ones, and the tricky business of trying to prevent future deadly outbreaks. Image: Times Books

Editor's Note: The following is an excerpt from a chapter in Nathan Wolfe's new book: The Viral Storm: The Dawn of a New Pandemic Age (published October 11 by Times Books, an imprint of Henry Holt and Company, LLC. Copyright ? 2011 by Nathan Wolfe. All rights reserved).

The chytrid fungus has resulted in global frog deaths and in some cases extinction of entire frog species, a tragic loss for wildlife on our planet. In a 2007 paper, Lee Berger, one of the researchers who first identified the chytrid fungus, used language uncommon in conservative scientific journal articles when he wrote, "The impact of [chytrid fungus] on frogs is the most spectacular loss of vertebrate biodiversity due to disease in recorded history."

What happened with the chytrid fungus also gives us important clues to a larger phenomenon that affects much more than just amphibians. Over the past few hundred years, humans have constructed a radically interconnected world?a world in which frogs living in one place are shipped to locations where they've never previously existed, and one where humans can literally have their boots in the mud of Australia one day and in the rivers of the Amazon the next. This radically mobile world gives infectious agents like chytrid a truly global stage on which to act. We no longer live on a planet where pockets of life persist for centuries without contact with others. We now live on a microbially unified planet. For better or worse, it's one world.

How did we get to this point? For the vast majority of our history as living organisms on this planet, we had incredibly limited capacity to move. Many organisms can move themselves over short distances. Single-celled organisms like bacteria have small whiplike tails, or flagella, that allow them to move, but despite their molecular-scale efficiency, flagella will never push their owners far. Plants and fungi have the potential to move passively by creating seeds or spores blown by the wind. They also have adopted methods that co-opt animals to help them move, which explains the existence of fruit and the spores of fungi like chytrid. Nevertheless, precious few forms of terrestrial life regularly travel more than a few miles in the course of their lives.

Among the wonderful exceptions to the largely static life on Earth is the coconut palm. The seeds of the coconut palm (i.e., coconuts), like a number of other drift seeds, evolved buoyancy and water resistance, permitting them to travel vast distances through ocean currents. Among animals, some species of bats and birds are masters of space. The best example might be the Arctic tern, perhaps the most mobile species on Earth outside of our own. The tern flies from its breeding grounds in the Arctic to the Antarctic and then back again each and every year of its life. A famous tern chick was tagged on the Farne Islands in the UK near the time it was born in the summer of 1982. When it was found in Melbourne, Australia, in October of the same year, it had managed a twelve-thousand-mile journey in the first few months of life! It's been estimated that these amazing birds, which can live over twenty years, will travel about one and a half million miles in their lifetimes. It would take a full-time commercial jet pilot, flying at the maximum FAA permitted effort, nearly five years to cover the same distance.

Yet despite their wings, most bird and bat species actually live their lives quite close to where they're born. Only a few, like the Arctic tern, have evolved to regularly move great distances. Highly mobile species, whether bird, bat, or human, particularly the ones that live in large colonies, are of particular interest for the maintenance and spread of microbes. Among primates, only humans have the potential to move themselves great distances during a single lifetime, let alone in a few days. That's not to say that other primates simply stay put. Almost all species of primates move every day in their search for food, and young adults routinely move from one area to another before mating. Yet whether primate or bird, nothing on the planet?certainly nothing outside of the sea?matches humans in our capacity to move long distances quickly. The human potential to move, which now includes traveling to the moon, is unique and unprecedented in the history of life on our planet. But it comes with consequences.

Humans started globetrotting in earnest millions of years ago using our own two feet. Bipedalism gave us an advantage over our ape cousins in terms of our capacity to wander. And, as discussed in chapter 3, it had consequences for how we interact with the microbes in our environment. Yet our capacity to negotiate the globe in the amazing way we do now started with our use of boats.

The earliest clear archaeological evidence of boats dates to around ten thousand years ago. Found in the Netherlands and France, these boats (which might be better called rafts since they were made by binding logs together) were probably used primarily in fresh water. The first evidence of sea-going boats comes from a group of British and Kuwaiti archaeologists, who in 2002 reported finding a seven-thousand-year-old vessel that undoubtedly was used at sea. The archaeologists made their discovery at the Neolithic site of Subiya in Kuwait. Stored in the remnants of a stone building, the boat consisted of reeds and tar. Most strikingly, the bits of boat had barnacles attached to the tar, indicating that it was definitely used in the sea.

Employing genetics and geography, we can get a much earlier estimate for the first use of seafaring boats. The indigenous people of Australia and Papua New Guinea provide perhaps the best case for this. By comparing the genes of the Australasian people with other humans throughout the world, we can conclude that people reached Australia at least fifty thousand years ago.

During this time, our planet was a relatively cold place?it was the peak of an ice age. Since more of the Earth's water was locked up in ice, the sea level was lower, revealing pieces of land that connected what are currently islands. Many of the islands in the Indonesian archipelago were joined by these so-called land bridges.

Despite the land bridges that ice ages expose, we know that no one walked all the way to Australia. In particular, the deep-water channel between Bali and Lombok in present-day Indonesia, a channel around thirty-five kilometers long, would have required boats to navigate. So we can infer that these early populations also used at least some form of sea transport.

We know very little about these early Australian settlers, although we know that they traveled at a time before animal domestication so certainly didn't move with animals in tow. Nevertheless, their movements impacted how they related with microbes. When they first crossed from Bali to Lombok, they encountered a completely novel set of animals.

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