Australia Drifts

The wildlife of Australia includes a lot of endemic species — that is, species that are found only in Australia, and nowhere else in the world. 83% of the Australian mammals are endemic, as are 85% of the vascular flora and 93% of the amphibians. The animals and plants of Australia are very different from the animals and plants of any other part of the world.

This is because Australia has been isolated from the other continents for a large part of its history; much more isolated than any of the other continents. Life in Australia has had the time and space to evolve in its own unique way, without having to invade or be invaded by life from the rest of the world.

It all started about 750 million years ago, when all the landmasses in the world had come together to form the supercontinent of Rodinia. This was not the first supercontinent, and nor was it the last. But it was the one that was there when life was first beginning.

Rodinia had lasted about 300 million years. But the Earth is an active place, and the land is always changing. Finally, the western side of Rodinia broke away, leaving a landmass of what is now Australia, India and Antarctica. The gap of ocean in between later became the Pacific Ocean.

Over the next 250 million years, the three continents met with Africa and South America to form a huge landmass known as Gondwanaland — or nowadays, simply “Gondwana”. This was the time when huge tropical rainforests covered much of the land. Even today, most of the native Australian plant life can trace its ancestry back to Gondwana.

Today, there are a lot of “Gondwanan forests” in Australia and its neighbouring islands, that have been declared World Heritage sites partly because they strongly resemble the ancient forests of Gondwana.

The collisions didn’t stop there. After Gondwana was formed around 500 mya. (million years ago), it continued drifting as one mass, all through the Palaeozoic — the time when life took hold and began spreading all over the oceans and lands (though not yet the sky). Somewhere in the middle (350 mya.), Gondwana met with Laurasia, the other northern supercontinent that had formed. The continents were all joined up again, in the supercontinent Pangea.

This time, evolution was in full swing. Now that the land was all joined together, evolution began to take similar routes all over the world. When reptiles began to dominate in the Mesozoic, they took over the whole world at once. Of course, there were many different species, each suited to its own area, but their general design was usually the same. The fossil record also shows species living in areas that are now widely separated, but were once all connected together.

The Triassic reptile Lystrosaurus, for example, has left fossils in Africa, Madagascar, India and Antarctica, but nowhere in between! These places seem widely disconnected today, but we know the explanation. Lystrosaurus didn’t live in Africa and Madagascar and India and Antarctica. It lived in Gondwana.

Pangea didn’t last long, but the Gondwana continents did maintain a connection for long after that. It was 50 million years ago, much after the end of the dinosaurs, when Australia finally broke away from Antarctica and started an evolutionary path of its own.

Of course, if Antarctica hadn’t drifted so far south, we might have had similar wildlife there, too.

Marsupials are usually associated with Australia. But did you know that the oldest marsupial fossils are from North America? The fossil remains of the Kokopellia, found in Utah, are estimated to be about 100 million years old. Meanwhile, the oldest records of marsupials in Australia are only 55 million years old. So it seems that marsupials first evolved in the northern hemisphere, and only later spread to South America and then Australia. But while the other placental mammals (that is, the non-marsupials) went on to dominate the rest of the world, Australia was left undisturbed and the marsupials were allowed to thrive.

Marsupials are not just a different family, like primates or canids, but a whole different category. While the placental mammals, which are the ones we normally see around, give birth to live young, and suckle them till they grow older, the marsupials give birth even earlier. The tiny fetus stays inside the mother’s pouch until it’s old enough to come out.

Just like the placental mammals include anteaters, flying squirrels and wolves, so the marsupials have numbats, gliding possums, and the Tasmanian tiger. Except that the Tasmanian tiger is now extinct.

For years, the pace of evolution in Australia has been the pace of continental drift. Australia was not so impacted by the Ice Ages that hit the Northern Hemisphere and caused many species to die out, allowing new ones to take their place. A 2010 study found that frogs in the Southern Hemisphere have, on average, been around for 4.6 million years, while those in the Northern Hemisphere are only 2.9 million years old. Probably because they were wiped out and had to re-evolve many times.

Meanwhile, in Australia, new species came only very slowly as the land drifted, and not so much due to extinctions and re-evolving. But in slowing down, it seems, Australian species are also forgetting how to adapt. For a long time, rats and bats were the only outside, non-marsupial species in Australia, entering from outside as the continental drift created new connections.

But now, connections have found a new way to form that is much faster than continental drift: humans.

Starting with the dingo 400 years ago, humans have constantly been introducing new species to Australia. Many of these are fierce competitors to the old species, and many old ones have gone extinct. Is this a disaster for Australia? Maybe it’s just evolution.

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This article was originally published in Sirius #203 27 Sep — 10 Oct 2015 “Australia Drifts”.