A kangaroo interrupts the Women's Premier League match between Belconnen United and Canberra FC in Canberra. File picture: Lawrence Atkin/Capital Football via AP

Canberra - Scientists have been able to establish a timeline for the evolution of the modern kangaroo, showing that ancestors of today's kangaroos appeared in Australia at least 25 million years ago.

According to researchers from Australia's Flinders University, modern kangaroos rapidly evolved and diversified to develop their current features around 3 million years ago.

It was largely due to environmental change - the spread of grasslands across the continent - according to the study published in the journal Science on Friday.

The marsupials are Australia's national animal, with an estimated population of around 45 million, almost double that of the humans on the continent. 

The scientists were able to establish the new timeline by analysing teeth, specifically their shape and wear, after studying about 1,600 specimen of modern kangaroos and more than 100 from fossilized related ancestors.

It helped them study the diet and the environment of the kangaroos. 

The scientists established that the kangaroos transitioned from having short and stumpy teeth, which are good for eating vegetation out of trees and shrubs, to high-crown teeth that extend further into the gums to help graze on tough grass after the spread of grasslands.

Previously it was thought that they evolved gradually as aridity spread across the continent between 12 million and 5 million years ago. 

Flinders University Professor Gavin Prideaux, one of the researchers of the study, said the findings help better understand how climate change has shaped the evolution of Australia's fauna.

"We are now exploring whether these evolutionary patterns hold up when we look at different parts of the skeleton, such as the feet, which might tell us about when and how hopping evolved..." he said.

Kangaroos and their close relatives represent the most diverse marsupial herbivore family ever to evolve.

Scientists say kangaroos started out on trees. Scientists say they came down to the land around 20 million years ago the trees.

In the past, the continent boasted giant kangaroos that weighed more than 250 kilogrammes, about six times the size of ones currently found. They most likely did not hop but walked, scientists say.

No one knows why they started hopping, but it is estimated they started doing so around 3 million years ago.