The plants conceivably boosted oxygen levels in the atmosphere high enough for animals to develop skeletons, grow larger and diversify

Blair Hedges, Penn State University

This coincides with the time when the planet was thought to be entombed in ice, the so-called Snowball Earth, a dramatic climate reversal that preceded animal life on land.

Researchers from Pennsylvania State University say the presence of plants may have altered the Earth's climate, triggered the big freeze, and paved the way for the evolution of land animals.

If their data are correct, green plants would have been growing on land well before the sudden appearance of many new species of animals that occurred about 530 million years ago, an event called the Cambrian Explosion.

Barren landscape

The earliest evidence of land plants and fungi appears in the fossil record around 480 million years ago.

Before that, the Earth's landscape was believed to consist of barren rocks, home to bacteria and possibly some algae.

But some have argued that it is quite possible for land-dwelling plants and fungi to have been around much earlier, but their primitive bodies were too soft to be preserved as fossils in rock.

Fungi and lichen - a mixture of algae and fungi - helped green plants move on to land

The Penn State team carried out the largest genetic study so far of when land plants and fungi first appeared on Earth in an attempt to resolve this debate.

Team leader Blair Hedges said: "Our research shows that land plants and fungi evolved much earlier than previously thought - before the Snowball Earth and Cambrian Explosion events - suggesting their presence could have had a profound effect on the climate and the evolution of life on Earth.

"Both the lowering of the Earth's surface temperature and the evolution of many new types of animals could result from a decrease in atmospheric carbon dioxide and a rise in oxygen caused by the presence on land of lichen fungi and plants at this time, which our research suggests.

"An increase in land plant abundance may have occurred at the time just before the period known as the Cambrian Explosion, when the next Snowball Earth period failed to occur because temperatures did not get cold enough," he added.

"The plants conceivably boosted oxygen levels in the atmosphere high enough for animals to develop skeletons, grow larger and diversify."

The team studied the "molecular clocks" of living species of fungi, plants and animals. They looked at 119 genes that have accumulated mutations over the passage of time, allowing the relative times of origin of each species to be estimated.

"Because mutations start occurring at regular intervals in these genes as soon as a new species evolves - like the ticking of a clock - we can use them to trace the evolutionary history of a species back to its time of origin," said Dr Hedges.

Alien life

The work, published in the journal Science, could even help in the search for extraterrestrial life.

"Possibly, the early history of life on Earth can give us clues for predicting the kinds of lifeforms that are likely to exist on planets in other solar systems from the chemical content of their atmospheres," said Dr Hedges.

Linda Graham, an evolutionary biologist at the University of Wisconsin, said the findings were "surprising and very exciting".

"We don't really have any corroboration with the fossil record yet," she said. "I think it will stimulate more work by palaeontologists with micro-fossils looking in earlier (rock) deposits than people had thought to look in previously."