Australian scientists have obtained clues about the origin of life on Earth by identifying fossil "biomarkers" in rocks more than two and a half billion years old.

The biomarkers are all that remains of a primitive form of cellular life that lived in the Earth's early oceans. They are a billion years older than the previous evidence for complex cells.

The molecules come from ancient single-celled organisms that scientists call eukaryotes, microbes made of a cell with internal structure.

The research demonstrates that even two and a half billion years ago, evolution had produced the basic divisions of life we see on our planet today.

It seems life arose almost immediately after the Earth had cooled and a solid crust had formed. The first three billion years of the Earth's history was the age of microbes.

Self-replicating molecules

First to appear were self-replicating molecules. Then these molecules became more complicated and evolved a membrane inside which they were protected from the changing external environment. These were the first cells that became primitive bacteria, termed Prokaryotes.

Cells; the most important stage in the evolution of life

What happened next is still highly controversial. One idea is that one type of cell went to live inside another forming the first cell with an internal membrane, the so-called eukaryotes.

All animals and plants are classified as eukaryotes. The prokaryote group contains two types of bacteria; the eubacteria and the mysterious archaebacteria which are as different from eubacteria as they are from eukaryotes.