Kangaroos and cockatoos are synonymous with Australia and tigers and orangutans with Asia. Both these continents boast rich biodiversity that is also very unique. A simple yet popular way to understand these ‘separate greatnesses’ has taken the shape of the Wallace line.
What is the Wallace line?
In the late 19th century, the English naturalist Alfred Russel Wallace noticed a dramatic shift in the composition of organisms as he moved from Asia to Australia, New Guinea, and other islands nearby. He posited an invisible barrier in the ocean, later called the Wallace line, running between the islands of Bali and Lombok, striking north between Borneo and Sulawesi before curving south of Mindanao. To him this line was like a fence between the different kinds of animals on the two sides.
Wallace and others conducted eight years of fieldwork to carefully plot the line across many kilometres, in the process laying the foundations of modern biogeography: the study of how species are distributed and how they got there.
Over the years, the line has attracted considerable research interest. “The Wallace line … ties partly into the theory of evolution. Nowhere else on the earth do you see such a dramatic shift over such a narrow distance. Organisms are not just scattered randomly,” Jason R. Ali, honorary associate researcher at the Senckenberg Society for Nature Research, Germany, said.
What did Wallace find on Sulawesi?
At their closest, the islands of Borneo and Sulawesi are just over 20 km apart yet they support very distinct plants, mammals, and birds. Wallace was more baffled by Sulawesi. It’s one of the largest islands in the archipelago and home to species found nowhere else on the planet, including tarsiers (family Tarsiidae), the lowland anoa (Bubalus depressicornis), and the mountain anoa (Bubalus quarlesi), which are both of Asian origin. Yet Sulawesi is also home to Australian marsupials like the dwarf cuscus (Strigocuscus celebensis).
The island frustrated Wallace, who repeatedly redrew his line because he was unsure whether it belonged to Asia or Australia. He wrote in 1876 that the animals here showed “affinities” to Africa, India, Java, the Maluku Islands, New Guinea, and the Philippines.
Why do Sulawesi have species from both sides of the line while most others didn’t? Wallace had deduced the essential answer all those years ago but it has accrued greater depth with more research over time.
What does the ancient past say?
The line is part of the Malay archipelago, a geologically complex region with more than 25,000 islands.
Wallace figured that Sulawesi’s animal distribution could be explained if some of these islands had been joined with the Asian mainland in the past. As the islands broke off and drifted apart, the ancestral species on each island would have become isolated and evolved independently, creating the distribution Wallace saw in the 19th century. Since then, researchers have expanded this understanding by going further back in time. Millions of years ago, Australia broke off and drifted away from Antarctica. An ocean emerged in the growing gap and the water currents in its depths cooled the planet.
Meanwhile, Australia drifted north into Asia, creating the volcanic islands of Indonesia. Various studies found that variations in monsoons, aridity, and sea levels between these islands spurred island species to adapt to their new conditions and diversify, until as recently as four million years ago.
The movement of continents was one part of the puzzle. A study published in 2023 revealed another when scientists took a closer look at how species across the Wallace line were related. They analysed data of 20,000 species of birds, mammals, reptiles, and amphibians. Despite global cooling, they found, Malay’s tropical islands stayed warmer and wetter than Australia. Thus, Asian fauna used these islands as stepping stones to Australia whereas Australian species, having evolved in cooler climes, struggled to make their way across the islands to Asia. “Species from Asia can migrate through the rainforest-rich northern route, as the ecosystems are similar to their origins,” Ali said. “Australian species can only move into Asia along the southern route, around Timor and nearby islands. This path emerged much later — only a few million years ago — making migration more challenging for Australian species.”
Does the line matter?
By combining insights from multiple disciplines, the aforementioned studies helped explain Wallace’s findings to a degree that revealed the line to be a mirage: it was visible but the real reasons why it exists are rooted in the deeper facts of nature.
Today, even newer tools have joined older ones to further clarify the region’s biogeography. “We are learning more about which adaptations allow species to move throughout the region by using advanced evolutionary modelling and computer simulations,” Alexander Skeels, a postdoctoral research fellow at Australian National University, Canberra, said.
The factors that influenced species dispersal and settlement in the past are still relevant today.
The Indo-Malayan archipelago faces one of the world’s highest rates of habitat destruction. Understanding its biography will be crucial for ecologists to predict how species will respond to the loss of their homes, compounded by the effects of climate change.
“New technologies are helping us understand that ‘lines’ that separate Asia and Australia may be simplifying the story,” Skeels said. Ali echoed him, saying redrawing the Wallace line or any other line like it is “futile”.
“Different datasets and methods will reveal different results. These boundaries will always be fuzzy. Instead of redrawing lines, it is more valuable to focus on how these species will be affected by habitat destruction in future,” Ali added.
Rupsy Khurana is Science Communication and Outreach Lead at the National Centre for Biological Sciences, Bengaluru.
Published – March 05, 2025 08:30 am IST
