Evidence from deep beneath the desert in Oman suggests that plant life began its takeover of the Earth's land surfaces about 475 million years ago.
By Ivan Noble
BBC News Online science staff
The timing is much earlier than previously believed.
First came the moss, then the ferns...
Microscopic fossils recovered from boreholes and analysed by Charles Wellman, of the University of Sheffield, UK, and his collaborators Peter Osterloff and Uzma Mohiuddin, indicate that these first land-loving plants were similar to today's liverwort.
"They would have looked a bit like the moss you find on a badly kept lawn," said Paul Kenrick, a palaeobotanist at the Natural History Museum in London, UK.
"The plants themselves were miniscule. If you'd have been a botanist alive at the time, you'd have been crawling around on your hands and knees with a magnifying glass looking for them," he told BBC News Online.
Charles Wellman and his colleagues painstakingly recovered tiny fossilised spores from Omani rock known to date from the Ordovician Period, 443 - 489m years ago.
At that time, today's Oman was part of the supercontinent scientists refer to as Gondwanaland.
But it was clear from the team's analysis that the spores could not have been produced by land-dwelling plants already known to fossil hunters.
And crucially, the researchers also found fragments of the plants which generated the spores.
These fragments show that the spores were indeed released by land-dwelling plants and not marine-dwelling algae.
The findings tie in with recent analyses of the DNA of modern plants carried out in an attempt to calculate how far back in time they shared a common ancestor.
The emergence of plant life from the oceans on to land seems then to have been well underway by the time these spores were laid down.
The research work is described in the journal Nature.