Discovery identifies Australia as birthplace of all modern mammals

By Karen McGhee, Science and environment editor December 18, 2022
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Research has uncovered evidence that the evolution of mammals began in the Southern Hemisphere, not the Northern Hemisphere, challenging hundreds of years of scientific dogma.

A staggering international breakthrough has led to a remarkable new understanding that all modern mammals, including humans, had their earliest beginnings in the Southern Hemisphere, with some important developments occurring in Australia, turning conventional wisdom on its head.

The major force behind this astounding development is renowned Aussie scientist Professor Tim Flannery, an Honorary Associate at the Australian Museum in Sydney.

Tim, who’s spent extensive time as a field biologist in remote corners of the planet, discovering and naming scores of new species, and has been instrumental in bringing the world’s attention to the perils of climate change, has no doubt about the significance of this new breakthrough.

“I think it’s the most important piece of science, from a global perspective, that I’ve done in my lifetime,” he told Australian Geographic.

Tim is supported by a group of international scientific luminaries, including Professor Kris Helgen, the Australian Museum’s Chief Scientist; Dr Tom Rich, Senior Curator of Vertebrate Palaeontology with Museums Victoria; Dr Patricia Vickers-Rich, a Professor of Paleontology at Monash University; and Dr Grace Veatch from the Smithsonian, in the US.

Together this weighty band of experts has just published a paper in the scientific literature that’s likely to be seen as landmark in our understanding of life on Earth. 

Appearing in Alcheringa: An Australian Journal of Palaeontology and entitled The Gondwanan Origin of Tribosphenida (Mammalia), it’s not likely to be fodder for conversation around office water bubblers.

But this paper will undoubtedly set off a flurry of emails, texts and tweets across the globe among not only scientists interested in evolution but well beyond.

Ultimately, it’s likely to see the rewriting of high school and university textbooks the world over.

Origins of a theory

To appreciate all this, it’s important to understand there are three lines of mammals surviving on Earth today: the marsupials – the pouched mammals such as kangaroos and possums; the placental mammals, which includes most of Earth’s well-known modern mammals including dogs, cats, mice and of course, humans; and the monotremes – the weird egg-laying platypus and echidnas.

The scientific consensus has been for more than 200 years that the placentals and marsupials arose in the Northern Hemisphere and spread southwards. It’s called the Sherwin-Williams Effect. (Forget the monotremes for now: we’ve long-known they’re an odd ancient evolutionary offshoot that survives only in Australia and New Guinea, and nothing has changed there.)

“But the question has always been what was the common ancestor that gave rise to all the placentals and marsupials? And where was that evolution happening?” Kris Helgen says.

Some sort of small shrew-like creature in Victoria, it now seems, played a crucial role.

Realising the truth

The extraordinary new scientific understanding about when and where modern mammals evolved came about initially when Tim and Kris, two of the nation’s great scientific minds, were forced during pandemic lockdowns to revisit treasure-troves of evidence that have been lying around in museums and the scientific literature for about two decades.

“I was re-analysing these fossils that turned up in Victoria from the age of dinosaurs,” Tim explains. “And then I started looking more widely for similar sorts of fossils found elsewhere and it turned out all of them were in the southern hemisphere and all are Jurassic or Cretaceous in age [from 199–66 million years ago].”

“And we realised the thing that unites all these Southern Hemisphere fossils is they have these very strange, complicated molars that let the animals puncture shear and crush, all at the same time, what they were eating,” Tim says, explaining these distinctive teeth probably evolved to deal with the tough chitinous shells of insect prey.

These odd teeth are described as tribosphenic and the mammals they occurred in are known as tribosphenidans.

The first Southern Hemisphere fossils of these teeth were found in an ancient jaw in 1997 by Tom Rich, at a dig site now known as Flat Rocks, near the Victorian seaside town of Inverloch, which is also where Australia’s first dinosaur bone was found.

During the following 10 years, similar tribosphenidan fossils were found in Madagascar and South America and then much later in India – all land masses that were once connected in the great southern supercontinent Gondwana. It’s these records Tim started to revisit and have deep discussions about with Kris Helgen.

People had looked at these scattered bits and pieces of information previously but never in a comprehensive way to see if they revealed any sort of pattern.

A new truth

There are plenty of fossils of early mammals across the Northern Hemisphere. But it’s only in these fossils of mammals, which are older and only from the Southern Hemisphere, that you first see these unique tribosphenic teeth.

“So, we’ve been able to show that the relevant fossils that look like they are anatomically likely to be close to the common ancestor of marsupials and placentals are found exclusively in the southern continents and are from an older time period than the oldest mammal similar fossils seen in the north,” Kris explains. “And that indicates these groups of mammals had their ancestry in the southern continents at an earlier time period and then later colonised the northern continents. It absolutely turns our previous understanding on its head.”

Tom Rich elaborates further: “The hypothesis we are presenting is that [marsupials and placentals] originated in Gondwana and then dispersed to the northern hemisphere about 120 million years ago.” That’s about the same age as the oldest Victorian fossils with these weird teeth and slightly older than the oldest mammal fossils found in the Northern Hemisphere.

“And if you do plate tectonic reconstructions one of the best possible routes north is from what’s now Victoria through to east Asia via a series of islands that existed at that time.” 

So it’s thought these early mammal ancestors would have island hopped their way from land that’s now located on Victoria’s south coast,  northwards though Asia and across the rest of the world.

“Yes, it looks to be incredible,” agrees Tim. “I resisted the conclusion as long as I could, but the evidence is compelling. These shrew-like animals from Australian are actually the ancestors of both the earliest placentals and the earliest marsupials.”

tim flannery Related: Why Tim Flannery looks back on his 20 years of climate activism as a colossal failure