Scientists believe life on Earth began evolving around 3.8 billion years ago.

But while they have been able to put a date on when life appeared, they are still far from knowing how it appeared.

Now researchers in the US and Italy say they have evidence that DNA-like fragments may have come with 'instructions' that guided their growth into complex life forms 4 billion years ago.

The researchers believe these fragments used their innate self-evolving abilities to grow into repeated chemical chains long enough to evolve into primitive life.

The study, by the University of Milan and University of Colorado Boulder, is based on a discovery in the 1980s that RNA can chemically alter its own structure.

RNA is similar to DNA, and carries out a number of jobs in our cells, including acting as an on-and-off switch for some genes.

Scientists believe that when life was in its early stages, RNA played a leading role in creating complex organisms before DNA and proteins were developed.

Many origin-of-life researchers say that RNA chains are too specialised to have been created as a product of random chemical reactions.

The new study, however, claims to provide an alternative theory by arguing primordial DNA-like fragments evolved in this way instead.

The researchers found the self-assembly of DNA fragments just a few nanometres in length have the ability to drive the formation of chemical bonds.

These connect together short DNA chains to form long ones, without the need for a separate biological process.

'Our observations are suggestive of what may have happened on the early Earth when the first DNA-like molecular fragments appeared,' said CU-Boulder physics Professor Noel Clark, a study co-author.

The study suggests that the way in which DNA emerged in the early Earth lies in its structural properties and its ability to self-organise.

In the pre-RNA world, the spontaneous self-assembly of fragments of nucleic acids – the building block of life - may have acted as a template for their chemical self-assembly.

'The new findings show that in the presence of appropriate chemical conditions, the spontaneous self-assembly of small DNA fragments into stacks of short duplexes greatly favours their binding into longer polymers, thereby providing a pre-RNA route to the RNA world,' added Professor Clark.