History of plants pushed back by 100 million years
Plants colonised the continents 100 million years earlier than previously thought, new research shows.
Scientists now believe the first plants adapted to terrestrial life appeared around 500 million years ago.
Until then, for the first four billion years of Earth's history, nothing would have lived on the land except microbes.
The greening of the continents transformed the planet and paved the way for the first primitive land animals to crawl out of the sea.
Previously, scientists working out the timetable of life on Earth relied on the most ancient plant fossils, which are about 420 million years old.
The new study, published in the journal 'Proceedings of the National Academy of Sciences', used "molecular clock" techniques to provide a more accurate record.
"The fossil record is too sparse and incomplete to be a reliable guide to date the origin of land plants," said Mark Puttick, from the University of Bristol.
"Instead of relying on the fossil record alone, we used a 'molecular clock' approach to compare differences in the make-up of genes of living species. These relative genetic differences were then converted into ages by using the fossil ages as a loose framework.
"Our results show the ancestor of land plants was alive in the middle Cambrian period, which was similar to the age for the first known terrestrial animals."
The "molecular clock" method is based on the premise that certain genetic mutations accumulate at a predictable rate over time. By extrapolating back, they can be used to work out when life-forms diverged into different species.
The new findings have implications for climate modelling. Plants play a major role in the chemical weathering of continental rocks, a key process that helps to regulate the Earth's atmosphere and climate over millions of years.