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Mr. Wallace's Line

Through the ocean just east of Borneo runs an invisible line that separates the world of tigers from the world of kangaroos. Getting across that line may have seen what made our ancestors truly human.

By Jared Diamond
Aug 1, 1997 5:00 AMNov 12, 2019 4:39 AM


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Far below my Garuda Indonesian Airways jetliner stretched a blue ribbon of water, the narrow strait separating the Indonesian islands of Bali and Lombok. Through the window I gazed down on a gorgeous landscape of tropical mountains, white sand beaches, coral reefs, and warm seas. To the tourists sitting around me, the sunlit scene promised sunbathing, snorkeling, spicy foods, and Balinese temples. That prospect appealed to me too, but I was thrilled by a different promise. I had read and dreamed of this strait for decades, and I was now at last taking it in with my own eyes. It constitutes part of Wallace’s line, which for a biologist is the sharpest and most famous boundary in the world, and the line whose crossing may have transformed our ancestors from glorified apes into real humans.

Wallace’s line is one of many biogeographic borders in the world, zones separating geographic regions with dissimilar groups of plants and animals. Most familiar to us are the divides between the native distributions of human populations--the lines that seem to divide us into races: the Atlantic Ocean, which separated Europeans from Native Americans; the Sahara, which lay between black sub-Saharan Africans and white North Africans; and the Indian subcontinent, between East Asians and the whites of West Asia and Europe. Not only humans are distributed geographically, of course. Any fanatical bird-watcher eager to spot as many of the world’s myriad bird species as possible knows that you have to visit tropical Central and South America to find toucans and antbirds, sub-Saharan Africa for turacos and mousebirds, tropical Southeast Asia for leafbirds and minivets, and so on.

By means of such localized distributions, biologists divide the world into six major realms termed biogeographic regions, areas that do not coincide with the seven continents. Among neighboring pairs of those six regions, the greatest contrast is found between the Oriental region (consisting of tropical Southeast Asia and neighboring islands) and the Australian region (Australia, New Guinea, and neighboring islands). One is a world of tigers, the other a world of kangaroos. Each region harbors entire suites of species absent from the other. Tropical Southeast Asia, for example, is home to a rich array of placental mammals--primates, squirrels, big cats, and hoofed mammals among them--that are not found in the Australian region. Among placental mammals, only bats and rats are native Australian species. Conversely, Asia completely lacks Australia’s many groups of marsupials (like the kangaroos) and its monotremes (egg- laying mammals like the platypus). Other animal and plant species are similarly localized: Asia is devoid of Australia’s wealth of cockatoos and eucalyptus, while Australia has none of Asia’s numerous woodpeckers and pheasants.

Why does such a contrast exist? Between Australia/New Guinea and the Asian mainland lies an almost continuous chain of islands--constituting Indonesia--most of which are separated by gaps of ocean less than 100 miles wide. With such apparently modest barriers to their migration, how did such a great contrast between Asian and Australian plants and animals arise?

We now take it for granted that different types of species live in different places. But early European naturalists had no way of anticipating that truth, and they were astonished when sixteenth-century explorers of other continents began bringing home exotic hummingbirds and lemurs rather than animals from familiar European groups. Why don’t climatically and structurally similar habitats on different continents support similar species? This question was initially posed in the centuries before Darwin, when biologists, like everyone else, believed in the origin of species by divine creation. Hence early biogeographers explained the distinctiveness of biogeographic regions by postulating that God, for inscrutable reasons, had chosen to create plants and animals at six widely scattered sites and had exercised creative imagination differently at each site. With Darwin’s demonstration of evolution, we have come to realize that the world’s biogeographic realms instead represent regions that have been more or less isolated from each other by geographic barriers, forcing evolution to proceed independently in each region. That’s easy to understand for independent biological evolution in North America and Eurasia, sundered by the North Atlantic Ocean and the Bering Strait. It’s harder to understand, though, for regions separated by less obvious geographic barriers--such as the Oriental and Australian regions.

The boundary between those regions was first recognized by Alfred Russel Wallace, the great nineteenth-century British naturalist famous as the co-discoverer (with Darwin) of evolution and natural selection. As Wallace described it, the line runs between Bali and Lombok, then continues north between the larger islands of Borneo and Celebes. Wallace recognized that on the west side of his line, Borneo and Bali, although they are islands, are home to representatives of most major groups of Asian mainland species. Many or most of those same groups, however, are absent on Celebes and Lombok, the islands just to the east of his line. Why did the boundary arise there? What’s the big deal about the strait between Bali and Lombok, making it so much more important than the straits to the west between Bali and Java or between Java and Sumatra, or the straits to the east between Lombok and Sumbawa or between Sumbawa and Flores?

To reach Borneo from my home in Los Angeles, I first flew to mainland Asia, changing planes in Malaysia’s capital of Kuala Lumpur before flying on to Malaysian Borneo. Accustomed as I am to New Guinea, populated by dark-skinned, frizzy-haired people who speak languages unrelated to any spoken outside New Guinea and nearby islands, I found Kuala Lumpur another world. It’s a city of Southeast Asian people, related to the South Chinese, who speak Malay, a language belonging to the widespread Austronesian language family. The flight to Borneo had brought me to an island, but I was still in the human sphere of mainland Southeast Asia.

In New Guinea, where my research frequently takes me, I work out of tents pitched in the humid jungle, sleep on the ground in a damp sleeping bag, and subsist on a diet of rice, crackers, and warm boiled water. In Borneo, I stayed at a wonderful tourist facility called the Borneo Rainforest Lodge, slept on a bed in a house cooled by fans, feasted on shrimp and steak, and drank chilled Guinness. However, any thoughts that comfortable lodgings meant an escape from other discomforts of the New Guinea jungle were dispelled in the first minute of my first morning at the lodge. I got up before dawn, went downstairs, grabbed one boot in the dark, and immediately felt an intense pain in the ring finger of my left hand. I pulled a small black object out of my fingertip--apparently the stinger of an insect that had been resting on my boot. The pain gradually subsided, and I thought no more about it until the next morning, when I found my finger grossly swollen. My wedding ring was tight about my finger’s base and threatened to become a gold tourniquet. With help, I finally wrenched it off. However, the swelling continued to spread over my hand until I couldn’t bend any finger or grip anything, and it took a week to subside. Evidently, in the Oriental region new stinging insects replace the familiar ones of New Guinea.

Even from the verandah of the island lodge, I could see and hear an animal world composed of species belonging to mainland Asian groups completely absent in Australia and New Guinea. Among primates, of which the Australian region has none except humans, gibbons were hooting for much of the day; richly red-colored leaf monkeys clambered in plain view just a few dozen feet from where I sat on the porch; and orangutans moved through the area. As soon as I entered the jungle, I could see elephant dung on the trails, barking deer startled me with their menacing coughs, and clouded leopards lurked unseen. The birds ranged from the huge argus pheasant to dozens of species of babblers and almost as many bulbuls, all belonging to Asian groups of species. In the rivers swam a rich assortment of Asian freshwater fish. The most conspicuous mammals after primates were squirrels, present in a wide range of sizes and a great variety of habits, from five-inch-long pygmy squirrels scampering on the ground to giant flying squirrels in the trees.

My first encounter with those aerial mammals came when I glimpsed a big dark shape, which I took to be an eagle as it soared through the air in a straight line 30 feet above the ground. It never occurred to me that a creature gliding across the valley, often without any visible loss of height, could be anything but a bird. In fact, giant flying squirrels floated slowly overhead, seemingly immune to the pull of gravity, and silent except for a loud thwunck at the end of their glide, when at the last moment they rotated their bodies from horizontal to vertical to slam against a tree trunk with their stomachs, thereby avoiding a head-on crash.

Borneo differed from New Guinea not only in its individual species: I was now in habitats with different structures and different biological processes. For example, instead of the New Guinea rain forest’s highly branched trees, mostly less than 100 feet tall, Borneo’s rain forest is dominated by dipterocarps, trees 160 feet tall or more, with straight trunks unbranched for the first hundred feet, prized by loggers. Although many dipterocarps were flowering profusely, I looked in vain for the parrots, honeyeaters, and other nectar-feeding birds that fill and pollinate flowering trees in New Guinea. Dipterocarp flowers rely instead on inconspicuous small insects called thrips for pollination. As for fruit- eating birds, New Guinea’s birds of paradise were absent here. Instead, Borneo’s wild fruits fed many of those Asian mammals and birds that I had been watching, such as the primates and squirrels, the hornbills and bulbuls. Evidently, Asian trees and those of New Guinea and Australia have evolved entirely different solutions to the twin problems of getting their flowers pollinated and their seeds dispersed.

Thus, Borneo’s lowland rain forest was a world apart from the New Guinea world familiar to me. But I felt more at home in other Borneo habitats, such as the mangrove forest with its birds closely related to New Guinea pigeons and flycatchers, and the mountain forest with its whistlers and cuckoo doves. I was beginning to suspect that Asia’s and New Guinea’s floras and faunas are woven together in a complex fabric within the same island, and that Wallace’s line twists between adjacent habitats.

Crossing Wallace’s line eastward from Borneo to Celebes, I felt at first as if I were still in the human world of Asia. The people whom I encountered near the airport still looked like Malaysians and South Chinese, and they still spoke Austronesian languages. But as I drove inland into the mountains, I was startled to see two men walking along the road who closely resembled New Guineans. My guide explained that they came from a hill tribe considered primitive bush people by Celebes city dwellers. I would guess that they are remnants of the human migration wave that spread eastward across Wallace’s line from mainland Southeast Asia, out through Indonesia’s islands, to New Guinea and Australia around 60,000 years ago. Within the past 7,000 years, those migrants were replaced on the Asian mainland and in most of Indonesia, but not on New Guinea or Australia, by an expansion of farmers out of South China.

Another clue that I was moving into a more New Guinea-like realm soon thrust itself on my attention. When I entered the Celebes rain forest to bird-watch, I felt a nibbling at my ankles that gradually spread up my legs to my private parts; it became an uncontrollable, maddening itch that left me incapable of concentrating. It was a sensation all too familiar to me: the itching from bites of a tiny red pest called the scrub mite, one of the most feared discomforts of the New Guinea jungle. Each nibble swelled up and drove me to an itching despair for several days; then finally each bite developed into an inch-wide, serum-filled blister that left a scar.

When I could bring my attention back to other wildlife, I found that the strait between Borneo and Celebes had cut off much of the Asian fauna as if by knife. Celebes has no leopards or other native carnivores, no apes and few monkeys, few large native mammals of any species, no pheasants or native bulbuls, few woodpeckers or babblers, and few freshwater fish. But I was not stepping into Australia instead. The sole Australian-related mammals of Celebes were two possums; there were no Australian birds of paradise or bowerbirds; and Australia’s diverse honeyeater family had only a single representative here. Celebes is a land neither of kangaroos nor of tigers. While its animals are still largely Asian, they are impoverished in diversity of species: many Asian groups are absent and not replaced by Australian groups. Evidently, Wallace’s line is the biogeographic boundary not between Asia and Australia, but between a rich Asian world and an impoverished one.

Yet Celebes is a lush island, with a great variety of habitats rivaling those of Borneo. It has mountains over 11,000 feet high, originally covered with wet forests, but also with local areas of dry scrub. Why is there such a marked contrast, across Wallace’s line, between the faunas and floras of biologically impoverished Celebes and of rich Borneo? The same contrast and the same question arise for the strait that I had flown over to the south, between Bali, with its squirrels and now- extinct tigers, and Lombok, without either, despite Lombok’s higher mountains and greater variety of habitats. What caused the steplike loss of Asian biota across Wallace’s line?

It is a tribute to Wallace’s genius that he deduced much of the explanation still considered correct today, even though he knew far less about local biological distributions and geology than we do now. He did know that the seas separating Borneo, Java, and Bali from each other and from the Asian mainland are shallow, no more than 75 fathoms deep. The straits to the east separating Celebes and Lombok from each other and from Borneo and Bali are much deeper. Geologists of Wallace’s time had already recognized evidence of past ice ages, when ice and glaciers covered much more of Earth’s surface than they do today. That evidence persists in North America as glacial features far south of the current limits of permanent ice and snow in Alaska and northern Canada. For instance, New England’s Cape Cod is easy for geologists to identify as the terminal moraine of vanished glaciers, while the thousands of lakes dotted over the otherwise flat terrain of Wisconsin and Minnesota originated as glacial lakes.

In the Pleistocene Epoch (which lasted nearly 2 million years, until 10,000 years ago) dozens of ice ages alternated with warmer interglacial periods, like the period we’re now experiencing, causing glaciers to advance and retreat. Each buildup of glaciers sequestered so much water that sea level over all the oceans dropped about 75 fathoms below its present stand. Hence what are now shallow seas and straits less than 75 fathoms deep were then dry land. Those drained straits and seas included all of today’s shallow seas between Borneo, Bali, Java, and the Asian mainland. In those days the Asian mainland didn’t terminate, as it does now, on the coasts of China, the Malay Peninsula, and India but extended east and south to encompass what are now the islands of Taiwan, the Indonesian islands east to Borneo and Bali, and Sri Lanka. That is, Wallace’s line marks the former eastern margin of the Asian continent. Each time the glaciers melted (most recently around 13,000 years ago), sea level rapidly rose, those low-lying dry lands once again became shallow seas, and Taiwan, Sri Lanka, Borneo, and Bali turned back into islands.

Whenever Borneo and Bali were part of the Asian mainland, Wallace inferred, freshwater fish reached them by swimming in rivers; weakly flying birds like pheasants and babblers and woodpeckers fluttered there; and tigers and other flightless mammals simply walked. With the glacial melting and the rise in sea level, all of those flightless or weak-flying populations became stranded on their now-insularized homes, where some survived and others went extinct. For instance, the tigers of Borneo died out and are known only as fossils, but tigers survived on Bali until their extermination by humans in modern times.

Unlike Borneo and Bali, Celebes and Lombok were never joined to the Asian mainland and could never be reached by walking or fluttering. Hence the only species of plants and animals to arrive there were ones adapted to crossing straits: powerful fliers such as parrots and bats; animals likely to be blown by the wind (like many butterfly species) or rafted on floating uprooted trees (like lizards and rats); or strong swimmers, including the ancestors of the native wild pigs and buffalo of Celebes. But Celebes and Lombok weren’t connected to the continent of Australia/New Guinea either. Corresponding to Wallace’s line at the former eastern margin of ice age Asia is a similar line at the former western margin of ice age Australia/New Guinea, beyond which kangaroos and bowerbirds couldn’t spread westward. The area of Indonesian islands between the two margins is called Wallacea by biologists, in honor of Wallace.

All the islands of Wallacea lack species with weak water-crossing ability, among both the Asian and the Australian biotas: kangaroos as well as tigers, bowerbirds as well as woodpeckers. Wallacean islands are biologically impoverished compared with both the islands to the west formerly joined to Asia (such as Borneo) and the islands to the east formerly joined to Australia/New Guinea (such as the Aru Islands).

Traveling from west to east across Wallacea, one passes from islands (such as Celebes) with an excess of strong-flying Asian species to islands (such as Timor) with an excess of strong-flying Australian species. On islands in western Wallacea, the few Australian species able to establish and maintain themselves do so especially in species-poor habitats (such as mountain forest and mangrove forest), where they face less competition from invaders from species-rich Asia. A related phenomenon applies to the human populations of the islands between Asia and Australia: Austronesian invaders from the west have occupied all of the good farmland. The few human populations related to modern New Guineans and aboriginal Australians retreated to habitats under less pressure from Austronesian farmers. Those relict human populations include the New Guinean-like hill people whom I saw in Celebes, the Negrito pygmies in the mountains of the Philippines, and New Guinean-like populations in the interiors of Timor, Halmahera, and other islands of eastern Wallacea.

Thus the main message of Wallace’s line is that it’s impossible to understand modern distributions of plants and animals over the globe without some understanding of history. If there hadn’t been extensive glaciers and lowered sea levels in the past, Borneo’s biota would now be much more similar to that of Celebes and not nearly so similar to that of the Asian mainland. Borneo wouldn’t harbor orangutans and rhinoceroses today. Past as well as present barriers are what made the biologies of Asia and Australia so different, by isolating those continents from each other and permitting much separate evolution on each of them.

Similar effects of past and present barriers to movements of plants and animals play themselves out at boundaries between the world’s other biogeographic regions. The same is true even for minor regions, such as eastern and western North America. I grant that California and New York aren’t as different from each other biologically as are the lands of kangaroos and tigers (though you might think so to hear how my friends in Los Angeles and Manhattan talk of each other’s states). But the West is still a land of hummingbirds--13 species west of the Mississippi, compared to just one in the East--and the East is still a land of wood warblers.

Any bird-watcher knows that the eastern half of the United States has its own distinctive bird species and races, including the rose-breasted grosbeak and Baltimore oriole, contrasting with the West’s black-headed grosbeak and Bullock’s oriole. Just as the filter of Wallacea separates birds of Asia and Australia, the persistent filter of the Great Plains and the ancient intermittent filter of now-vanished ice sheets separate the land of hummingbirds from the land of wood warblers, and the culture of Hollywood and cowboys from the culture of Manhattan and Yankee fans.

Perhaps the effect of Wallace’s line most relevant to us is its possible role in a decisive step of human evolution. Paleontologists tend to stress Africa as the cradle of humanity, to view Cro-Magnon Europe as the site where late ice age human culture flowered, and to neglect Australia as a remote outpost occupied by supposedly primitive Aborigines. Human behavior took a Great Leap Forward sometime between 100,000 years ago, when there were still no signs of art or complex tools anywhere in the world, and the period around 40,000 to 30,000 years ago, when great art and complex tools began to abound in Europe. Paleontologists usually assume that this development began among humans in Africa or the Mideast, then spread to Europe and finally (in diluted form) to our poorer cousins in aboriginal Australia.

But anatomically modern humans appeared in Australia before they did in Europe--probably by 60,000 years ago and possibly even earlier. To reach Australia, the protohumans who had reached Asia from Africa around one million years ago (as attested by the famous Java Man fossils) had to cross a dozen straits separating Australia from Asia. Those were the straits among Wallacea’s islands, starting with the straits at Wallace’s line itself. Our occupation of Wallacea and Australia provides by far the earliest evidence in human history for an ability to use watercraft. It is not until tens of thousands of years later that evidence for watercraft appears anywhere else in the world (in the Mediterranean). If sea-crossing technology is any gauge, the leap began in Wallacea, not in Africa or the Near East.

Zoologists Tim Flannery and Jonathan Kingdon have come up with some plausible, if speculative, explanations. The island of Lombok is easily visible from Bali, but it remained tantalizingly inaccessible to our ancestors for a million years after they had walked out to Java (and presumably to Bali, formerly connected to Java as part of the Asian mainland). Finally, some people managed to get from Bali to Lombok--perhaps they were accidentally swept to sea on a crude raft they were using to fish along the coast. To humans who had grown up on long-occupied and overexploited Bali, Lombok and other islands of Wallacea must have been a virgin paradise. Wallacea lies in the center of the world’s richest area of coral reefs, mangroves, and shallow-water marine fish.

On reaching the eastern end of Lombok, according to Flannery and Kingdon’s scenario, the colonists would have seen the next Wallacean island to the east. On occupying that one, they would have seen another island, and then yet another. Each next strait would have been a stimulus to improve our nascent watercraft technology; each new island, a stimulus to adapt to a new environment and to invent new technologies; each island’s untapped rich resources, the basis for a new human population explosion.

By the time the colonists had finished traversing Wallacea and had reached Australia and New Guinea, there must have been nearly a dozen such cycles of discovery, exploration, adaptation, invention, and population growth, constituting the leap in human creativity. As a result, the first Australians and New Guineans of 60,000 years ago led the world in technology and art, and that progress trickled back to their poorer cousins in Eurasia and Africa. Ultimately, the much greater expanse, terrestrial resources, and potential human population of Eurasia and Africa let humans of those continents overtake native Australians in technology. But Wallacea may have been the crucible of human creativity, and our ancestors may have crossed from apehood to humanity as they crossed Wallace’s line.

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