Stand on the shores of Bali and look east. Across the water channel, the island of Lombok sits on the horizon.
This narrow 21-mile-wide stretch of ocean, known as the Lombok Strait, separates these two islands. A motorboat can make the crossing in about an hour, so animals—birds, at the very least—should have no trouble making the journey.
And yet, something stops them. Not a storm. Not a predator. Just an invisible line, holding them back.
In 1858, British naturalist Alfred Russel Wallace wrote Charles Darwin a letter while burning with fever in a remote hut in the Malay Archipelago. He had spent years meticulously documenting species across the region, but it was in that delirious, malarial haze that the realization hit him: The natural world was not a seamless blend of species, but a patchwork divided by invisible barriers.
One of those barriers—the one running through the Lombok Strait—was the most striking of all: Wallace’s Line.
Few Animals Have Ever Crossed Wallace’s Line
Before Wallace’s fateful journey through the Malay Archipelago in the 1850s, the prevailing belief was that species were static and distributed based on environmental conditions, with little consideration for Earth’s historical processes.
Wallace’s meticulous documentation of species distributions shattered this assumption. He observed that despite their proximity and environmental similarity, the islands of Bali and Lombok had vastly different fauna.
To the west of the Wallace Line—in Borneo, Sumatra and Java—you’d encounter creatures with deep Asian roots: tigers, rhinoceroses, elephants and primates.
Cross eastward into the Lesser Sunda Islands, Sulawesi and New Guinea, and the faunal landscape shifts to Australian species—marsupials, such as tree possums climb through forests, cockatoos and lorikeets speckle the skies and egg-laying mammals like echidnas make an appearance.
What Caused This Stark Division?
The answer lies beneath the ocean’s surface. This is ironically fitting, considering that fish are one of the only animals that freely cross the imaginary line without consequence.
During the recent Ice Ages, when sea levels fell, land bridges connected many Southeast Asian islands to the mainland, allowing species to migrate freely.
But the deep waters of the Lombok Strait never dried up. It remained a formidable barrier, preventing species from crossing, leaving two distinct evolutionary worlds to develop independently.
Wallace’s discovery reshaped how leading minds of the time understood evolution—and even pushed Darwin to publish his work faster.
For Some Species, Wallace’s Line Changed The Course Of Evolution
The crab-eating macaque, or Macaca fascicularis, a highly adaptable primate found west of Wallace’s Line, developed a long tail that aids in its arboreal lifestyle. Thriving in the forests from Thailand to Borneo, it has also successfully integrated into urban environments.
Meanwhile, the booted macaque, or Macaca ochreata, found east of Wallace’s Line in Sulawesi, evolved a more robust body and a notably shorter tail, adaptations suited to the island’s dense forests and differing food sources.
The separation imposed by Wallace’s Line has shaped their behaviors, as well; while crab-eating macaques are opportunistic and often raid crops and human settlements, booted macaques remain more reclusive, relying on the secluded resources of Sulawesi’s forests.
The Sunda frogmouth (Batrachostomus cornutus), a nocturnal bird found in Southeast Asia, belongs to an ancient lineage largely confined to the Sunda region. Its adaptations make it well-suited to life in dense tropical forests, where it relies on exceptional camouflage to remain undetected.
Meanwhile, on the Australian side of Wallace’s Line, the tawny frogmouth (Podargus strigoides) and its relatives have evolved separately, adapting to open woodlands and drier habitats.
The divergence between these frogmouth lineages dates back to the Oligocene, approximately 30-40 million years ago, making it one of the oldest known vertebrate divergences across Wallace’s Line.
This long-standing separation suggests that island arcs and shifting land masses may have once provided a pathway for dispersal, only to later become isolated through geological changes.
What Happens When A Species Crosses This Invisible Boundary?
Many animals that evolved on one side of Wallace’s Line are finely tuned to their specific environments. If a tiger somehow made it into the Lesser Sundas, for example, it might struggle in an ecosystem that lacks larger prey items it relies on for food.
Some species, however, might thrive if they crossed Wallace’s Line. Freed from the selection pressures of their original environment—such as competitors, predators, or disease—successful colonizers could exploit new ecological opportunities, much like many invasive species that flourish in novel habitats.
Bats are one of the terrestrial lineages that provides some exceptions to this iconic biogeographic boundary. Unlike most terrestrial species, they can traverse water barriers with relative ease through flight. Certain bat species, such as flying foxes, are found on both sides of the Wallace Line, as they can travel long distances in search of food and roosting sites.
Phenomena like Wallace’s Line highlight the incredible diversity of life within the natural world. How do you feel about the boundless diversity that nature has to offer? Take a science-backed test to see where you stand on the Connectedness to Nature Scale.
