The Invisible Barrier Keeping Two Worlds Apart (2023)


In between two of the islands of Indonesia, there’s an ancient line that is both real and…not real.

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In between two of the islands of Indonesia, there’s, an ancient line that is both real, and not real., You can’t see it, but it’s there.

All the same.

If you stood on the coast of Bali and looked east to the shores of Lombok, you’d, be staring right at the line’s narrowest point: a 32 km stretch of water that seems pretty unassuming., This invisible barrier weaves its way through the entire Malay Archipelago, the largest collection of islands on the planet.

See, on the western side.

The animal life is characteristic of Asia, featuring rhinos, elephants, tigers, and woodpeckers, to name a few.

But cross, the line, and things suddenly change.

You won't find those same species on the eastern side.


The islands have a totally different cast of ecological characters, including marsupials, Komodo, dragons, cockatoos, and honeyeaters.

This is what scientists call a biogeographic boundary.

The meeting point of two regions of biodiversity that are highly distinct.


This particular line, called the Wallace.

Line, is perhaps the sharpest and most iconic of all.


How did this invisible line come to be? Why? Does it shape the distribution of so many species? How? Did we figure out the path it takes? And? How can it be both real and imaginary?? The Wallace line was first sketched out in 1859 by a guy named, wait for it, Wallace.

Alfred Russel Wallace to be exact.

A British naturalist who you might have also heard of as the co-discoverer of natural selection.

That concept came to Wallace in a literal fever dream, as he lay bedridden with malaria during part of his eight-year trip around the Malay Archipelago.

And, while he’d end up being overshadowed by Darwin on that front.

The second-best idea he had on that trip was the existence of the Wallace line.

I mean.. know.

Two good ideas in one trip.

This idea helped to forever establish him as the father of biogeography.

The study of the distribution of living things.

He had spent his voyage observing and collecting as many species as he could, hopping from island to island across almost the entire archipelago.


It was, as he moved east from Bali to Lombok that he first noticed something intriguing.

Even, though the islands were separated only by a narrow strait, the change in animal life, wasn’t, gradual and subtle.

It was sudden and distinct.

Wallace saw the differences in animal life between the two as being even more striking than between England and Japan!.

It was birds that initially caught his attention.

Certain species that were plentiful on Java and Bali - like the yellow, headed weaver, coppersmith barbet, and the Javanese three-toed woodpecker - didn’t exist at all on Lombok.


This abrupt shift extended to mammals and even many insects, too.

Almost, as if an invisible barrier was separating two different worlds.

But why? And how? The biogeographic line that he drew, which others would tweak in later.

Years, didn't just reflect the proximity of the islands.

In fact.

Some islands on opposing sides of the line are closer to each other than many islands on the same sides are to one another.

So Wallace realized that other, more mysterious forces must be in play., Like, say, geology.

He, recognized that the geological past shapes the biological present.

And, because the distribution of living species today partly reflects ancient geological events.

He saw biogeography as a way to uncover epic chapters of the planet’s history that might otherwise have been unknowable.

These concepts are easy to take for granted today, -.

We talk about them all the time here on Eons - but they were still fairly new ideas in Wallace’s day.


By taking this perspective, he concluded that the western islands must have once all been connected to each other, and to the Asian mainland.


Today they are surrounded by shallow seas.

This is only the result of a geologically recent rise in sea levels.

How else.

Could the big animals of that side, like tigers and rhinos and tapirs, have ended up on the islands? Because they’re, now separated by expanses of water that are way too wide for those species to cross.

He had a similar thought about how the islands east of Java and Borneo had formed -.

At least some of them were the remnants of a former Australian continent.

Wallace had a hunch that, throughout all of that change, deeper waters with strong currents between the two regions must have prevented many species from crossing from continent to continent when sea levels were lower.


This is still preventing many species from crossing today when sea levels are higher and the continents are fragmented into neighboring groups of islands.

Even many flying bird and insect species obey the line, ones that aren't capable of crossing those stretches of open.


Wallace had pulled together many pieces of the puzzle, but he and other scientists at the time were missing.

One key idea to complete the picture… Plate tectonics.

The surface of the planet is not static, of course, it’s dynamic.

It’s made up of individual large sections or plates that move and collide over vast stretches of geologic time.


Yet another concept that’s easy for us to take for granted, but which is actually a relatively recent addition to our understanding of the world.

In fact, plate tectonics only became widely accepted in the late 1960s – more than half a century after Wallace’s death.

And also, around the time when I was born., Which means, I'm..

...I'm as old as plate.


We now know that plate tectonics shapes our planet in many ways, including forming and deforming continents, raising up island, chains, and building mountain ranges.

And studies have shown the Malay Archipelago to be one of the most complex tectonic regions in the world, a meeting point of multiple plates, all jostling for space.


This is responsible for not only the area’s many volcanoes and frequent seismic activity, but also the peculiar contrasts of its animal life.

Because by the 1980s, scientists were able to say with confidence that the Wallace line is, at its core, a result of plate.


Wallace, had correctly identified that two former continuous land masses had existed on either side of this line in the deep past.


We know them as the paleocontinents of Sunda in the west and Sahul in the east, both of which existed during the ice ages, when more water was locked up in ice and sea levels were lower., Wallace didn't know it, but while they’re pretty close now, the two partly-sunken continents used to be much, much further.


The Sahul continent of the eastern side of the line, encompassed Australia, Tasmania, New Guinea, and the Aru islands.


It only approached the Asian Sunda continental shelf in the west, around 20 to 25 million years ago in the late Oligocene or early Miocene, epoch.

This was a result of the Australian plate, slowly drifting north over tens of millions of years after breaking away from Antarctica in the south, bringing its distinctive community of birds, reptiles, and marsupials with it.


Even though the species of each side are neighbors, now, they’d been evolving separately for eons, their two worlds, only colliding fairly recently in evolutionary terms.

And in between them, immediately east of the line, a complex force of plate, tectonics created a chain of new islands in an area of the archipelago.

That’s now called Wallacea.

These oceanic islands differ from the continental islands that flank them in that they were never connected to either of the greater land.


They were ecological.

Blank slates waiting to be filled in with whatever creatures could make it there.


Those ended up being mostly species from the Australian side, seeing as the Wallace line acted as a barrier to Asian species.

Moving east.

Take, for instance, the Komodo dragon, a giant monitor lizard that today lives on a handful of islands in eastern Indonesia.

Their fossils first appear in mainland Australia more than 3 million years ago in the Pliocene epoch, only reaching their current Indonesian island homes in the Wallacea region, around 1 million years, ago.

And, even now, the deep waters with strong currents that weave between the two regions, including the strait between Lombok and Bali, still limit the dispersal of many species across the line, keeping the differences in their evolutionary history.

So strikingly visible.

This is what created the stark contrast: bisecting, the jungle of islands that Wallace first sketched out in 1859, and that still fascinates biogeographers today.

Wallace’s invisible line may not be real in a physical sense, but it shows just how loudly ancient geological events can echo through time, and how they shape the diversity and distribution of life in strange and contrasting ways.

And, while Darwin might get virtually all the credit.

As the guy who figured out how species came to be, Wallace is still recognized as a pioneer in figuring out how species came to be where they are.

So plate.

Tectonics also explains why Earth has supercontinents! You can celebrate this fact with our Saga of the Supercontinents poster that features four of these continental configurations.

Available now at


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If it makes me laugh, then it must be a joke.

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Subscribe at for more epic epochs., Alfred Russel Wallace, to be exact..., Alfed, Russel, Wallace..., Alfred, Wussel Wallace, to be exact...

It was birds that initially..

It was birds.

That- It was bir- eh- ...Oh, my god.

Ok mouth.

You don't like me and I, don't like you.

Let's just do this.


The Invisible Barrier Keeping Two Worlds Apart? ›

The Wallace Line is an invisible and imaginary line that stretches across the Indonesian islands to demarcate the evolutionary differences of the fauna in that region. It gives us an interesting insight on the biogeographical history of the area.

What is the invisible line of the Wallace? ›

The Wallace Line is an invisible and imaginary line that stretches across the Indonesian islands to demarcate the evolutionary differences of the fauna in that region. It gives us an interesting insight on the biogeographical history of the area.

What is the invisible line that animals don't cross? ›

The Wallace and Weber Lines. The Wallace and Weber lines are imaginary dividers used to mark the difference between species found in Australia and Papua New Guinea and Southeast Asia. This is especially apparent when considering the difference in mammals between the two regions.

What are the invisible lines of the Earth? ›

Lines of longitude, also called meridians, are imaginary lines that divide the Earth. They run north to south from pole to pole, but they measure the distance east or west. The prime meridian, which runs through Greenwich, England, has a longitude of 0 degrees.

What are the invisible lines on the Earth called? ›

Lines of latitude, also called parallels, are imaginary lines that divide the Earth. They run east to west, but measure your distance north or south. The equator is the most well known parallel. At 0 degrees latitude, it equally divides the Earth into the Northern and Southern hemispheres.

Which two animals can see behind itself without turning? ›

ONLY two animals can see behind themselves without turning their head and they are rabbits and parrots. Their eyes are on the sides of their head, instead on the front and this allows them to swivel 360 degrees.

What is the most invisible animal? ›

Cystisoma have mostly transparent bodies that reduce their visibility to predators. But they also rely on an antireflective coating to make them even more difficult to see.

What animal is almost invisible? ›

Top 10 Camouflaged Animals Summary
#1Long-Eared Owl
#2Arctic Fox
#3Arctic Hare
6 more rows
May 1, 2023

What is the blank around the Earth? ›

Note:As the Earth orbits around the Sun, the Moon orbits around the Earth.

What are the two imaginary lines on the Earth? ›

Two types of imaginary reference lines are used to locate positions or points and to make accurate globes and maps. These lines are called parallels of latitude and meridians of longitude.

What are the 5 imaginary lines on Earth? ›

Latitude, including the Equator, Longitude, the Prime Meridian, the Tropic of Capricorn and the Tropic of Cancer.

What is the invisible line that divides the globe into two hemispheres? ›

The Equator is the imaginary horizontal line at 0∘ latitude at the center of the earth. It divides the earth into the Northern and Southern Hemispheres.

Do humans have invisible lines? ›

Humans apparently have stripes on their body that are mostly invisible to the human eye. Called Blaschko's lines, they were discovered by dermatologist Alfred Blaschko in 1901. These stripes, which form V patterns on the back and S patterns on stomach, are due to the way embryonic cells proliferate.

Does the Earth have imaginary lines? ›

Parallels of latitude and meridians of longitude are the two imaginary lines. Note: The earth revolves on its axis daily. If the axis were a pole or a line, the north and south poles are the two imaginary points where it would enter and exit the earth.

Which animal Cannot turn? ›

Brown-throated sloths have fairly thin necks, and can rotate them through 270 degrees. That sounds handy for spotting predators or potential mates, assuming the sloths can work themselves up to care about such things. But other species of sloth have thickly muscled necks and can barely rotate them at all.

Which animal Cannot turn back? ›

However, because the kangaroo and emus cannot walk backward, the Australian authorities decided to include them on the coat of arms to symbolize the country's resolve to only move forward and never backward.

What animal can turn back time? ›

The 'immortal' jellyfish, Turritopsis dohrnii

To date, there's only one species that has been called 'biologically immortal': the jellyfish Turritopsis dohrnii. These small, transparent animals hang out in oceans around the world and can turn back time by reverting to an earlier stage of their life cycle.

What animal has no fear? ›

It is not for nothing that the honey badger has earned the reputation of being the most fearless animal in the world. Despite its small size, this carnivore is known for its ferocious defensive abilities and aggressive nature.

What animals don't feel fear? ›

For example insects, arachnids and crustaceans don't feel any type of emotion. They don't show any signs of fear or pain. This is just down to the fact that their brain is too simple to hold this information.

Which animal sees all? ›

One animal who's vision we have even more to learn about than the butterfly is the mantis shrimp – taking the crown for the animal with the most complex vision. The mantis shrimp has between 12 and 16 photoreceptors and can see both UV and polarized light (two things that humans cannot see).

What animal is not self aware? ›

Dogs. Dogs were previously listed as non-self-aware animals. Traditionally, self-consciousness was evaluated via the mirror test. But dogs, and many other animals, are not (as) visually oriented.

What animal is blind but can see? ›

Born Blind

The olm, the blind cave salamander that looks like a baby dragon, is another naturally blind animal, which can yet perceive light through its hidden eyes and skin.

Can animals see what humans can't see? ›

However, some animals see colors we cannot. Spiders and many insects can see a type of light called ultraviolet that most humans cannot see. Other animals, like snakes, are able to see infrared light. You can use the chart below to explore what colors certain animals see and how they compare to human color vision.

How did Wallace explain the Wallace Line? ›

To account for the phenomenon, Wallace proposed an imaginary line to divide the region into two parts. The animals found on each side of the line can be shown to have an evolutionary connection to either Asia (to the West of the Wallace line) or Australasia (to the East).

What is the Wallace Line and where is it? ›

The Wallace Line does not actually exist in reality. It is an imaginary line that intersects the Lombok Strait between the Indonesian islands of Bali and Lombok to the south, and extends north through the Makassar Strait between Kalimantan (Borneo) and Sulawesi.

What and where is the Wallace Line explain the significance of the Wallace Line for our understanding of biodiversity and plate tectonics? ›

The Wallace Line is an imaginary boundary that runs between Australia and the Asian islands and the mainland. This boundary marks the point where there is a difference in species on either side of the line.

What is Wallace's line of Huxley? ›

The Wallace line or Wallace's line is a faunal boundary line drawn in 1859 by the British naturalist Alfred Russel Wallace and named by the English biologist T.H. Huxley that separates the biogeographical realms of Asia and 'Wallacea', a transitional zone between Asia and Australia also called the Malay Archipelago and ...

What is the main importance of the Wallace Line discovery? ›

The significance of the line is that it identifies a major (though not entirely abrupt) faunal discontinuity: many major groups of animals (especially birds and mammals) found to the west of the line do not extend east of it, and vice versa. Wallace's Line divides the Australian and Southeast Asian fauna.

What is the difference between Wallace's line and Weber's line? ›

Wallace as the dividing line between Asian and Australian fauna in the Malay Archipelago. Weber's line is a line of supposed 'faunal balance' between the Oriental and the Australasian faunal regions within Wallacea.

Why don t animals cross the Wallace Line? ›

The primary reason is simply the ocean's depth and width. Animals have little incentive to try to cross any fairly large body of ocean. Animals sometimes migrate across raging rivers, but in that situation the other side is clearly visible.

How did humans cross the Wallace Line? ›

Scientists have proposed that the most recently discovered ancient human relatives – the Denisovans – somehow managed to cross one of the world's most prominent marine barriers in Indonesia, and later interbred with modern humans moving through the area on the way to Australia and New Guinea.

Who was the person that the Wallace's line or Wallace Line was named after? ›

Wallace Line, boundary between the Oriental and Australian faunal regions, proposed by the 19th-century British naturalist Alfred Russel Wallace.

What animals are in Wallace's line? ›

In the mid-19th century, Wallace discovered a mysterious line that separated two different faunal universes. This line separated marsupials from tigers, and honeyeaters and cockatoos from barbets and trogons.

What is the key concept of Wallace and Wegener on biogeography? ›

Wallace came to much the same conclusion that Darwin published in the Origin of Species: biogeography was simply a record of inheritance. As species colonized new habitats and their old ranges were divided by mountain ranges or other barriers, they took on the distributions they have today.

What did Wallace conclude about where new species arise? ›

From his observations of species in the Malay Archipelago and the Amazon, Wallace concluded that new species arise near similar species. When did Wallace and Darwin meet?

What is the missing link in Darwin theory? ›

Distinguished Professor Ross Large has uncovered the missing link in Darwin's theory of evolution. That link is the trace element selenium. His research has found that selenium correlates to nearly every major extinction and growth event in Earth's 4.5 billion year history.

What is the basic theory behind Darwin Wallace's work? ›

Darwin and a scientific contemporary of his, Alfred Russel Wallace, proposed that evolution occurs because of a phenomenon called natural selection. In the theory of natural selection, organisms produce more offspring than are able to survive in their environment.

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