Life emerged billions of years ago, but a lack of fossil evidence meant the exact timeline remained unclear -- especially around the time bacteria first moved onto land.

Fossils found in Western Australia's Pilbara region suggest that microbial mats of prokaryotes (organisms lacking a nucleus) started colonising shallow marine environments up to 3.5 billion years ago.

The structures were made up of tiny grains of sediment held together by sticky mats of cyanobacteria -- ancient photosynthetic bacteria.

Lead researcher Tara Djokic, from the Australian National University, said the microbial mat fossils -- known as stromatolites -- were previously identified in rocks but dated back to 2.7 billion years ago, until now.

"Our discovery of more ancient stromatolites provides additional evidence for the ancient emergence of life on Earth and suggests some of the earliest life may have been microbial communities that thrived near hot springs," she said.

Researchers collected 20 rock samples with preserved microbial mats from the Dresser Formation, which is the top sedimentary succession in the 3.5 billion-year-old Pilbara Craton in Western Australia.

They are among the oldest preserved sediments found on the planet, as the Earth formed some 4.5 billion years ago.