PERTH, 8 June 2006 - Enigmatic clusters of layered rocks in Australia, dated at more than 3.4 billion years old, provide the earliest evidence of life on Earth, according to a study published on Thursday.

The so-called Strelley Pool Chert, located in Western Australia's Pilbara region, has long been debated by palaeontologists as to whether it holds fossilised mats of early bacteria or - far less interestingly - ancient mineral formations caused by hydrothermal vents.

In a paper published in the British science journal Nature, Australian and Canadian researchers led by Abigail Allwood of Macquarie University in Sydney, describe the first full documentation of these rock structures, known as stromatolites.

The study describes seven different types of stromatolite, including cones, waves and "egg carton" domes, and concludes that they are microbial in origin.

And, it says, these formations appear in different environments, which suggests that they grew in reefs, rather like corals.

"The diversity, complexity and environmental associations of the stromatolites describe patterns that - in similar settings throughout Earth's history - reflect the presence of organisms," the paper argues.

The dating of early life on Earth is considered crucial to understanding whether extinct life could be found on other planets or moons in the Solar System.

Billions of years ago, Mars also had an atmosphere and was covered with water, which could have provided a toehold for bacterial life, according to some theories.