The origins of land-based life have been discovered on the Scottish Hebridean Island of Kerrera, in the form of a primitive fungus.

Tortotubus fungus, also found in Gotland, Sweden, is said to be one of the first organisms to make its way from the sea on to land.

It is the oldest example of a land-dwelling species ever discovered in the fossil record. Only sea creatures have been found which are older.

The fungus, which is no shorter than the width of a human hair, is likely to have made the transition from the oceans over 440 million years ago.

The early pioneer - known as Tortotubus - played an important role in laying the groundwork for complex plants and then animals to exist out-of-the-sea by beginning the process of rot and soil formation.

Dr Martin Smith, who carried out a study of the fungus while working at Cambridge University, said: "During the period when this organism existed, life was almost entirely restricted to the oceans: nothing more complex than simple mossy and lichen-like plants had yet evolved on the land.

"But before there could be flowering plants or trees, or the animals that depend on them, the processes of rot and soil formation needed to be established."

Dr Smith, now at the University of Durham, analysed a number of tiny microfossils from Sweden and Scotland, each shorter than the width of a human hair.

He was able to show that what were once thought to represent parts two different organisms actually belonged to one, at different stages of growth.

Fungi play a vital role in the nitrogen cycle, the process by which nitrates in the soil are taken up by plant roots and passed along the food chain into animals.

Tortotubus had a cord-like structure, similar to that of some modern fungi, allowing it to spread out and colonise surfaces.

In modern fungi such a structure is associated with decomposition.

"What we see in this fossil is complex fungal 'behaviour' in some of the earliest terrestrial ecosystems, contributing to soil formation and kick-starting the process of rotting on land," said Dr Smith.

What is less clear is what existed 440 million years ago for Tortotubus to decompose. It is likely there were bacteria or algae on land during this period, according to the scientists whose findings are reported in the Botanical Journal of the Linnean Society.

The pattern of growth of Tortutubus resembled that of mushroom-forming fungi, although there is no other evidence of mushrooms so long ago.

"This fossil provides a hint that mushroom-forming fungi may have colonised the land before the first animals left the oceans," Dr Smith added.