Early plants probably didn't invade and colonise the land on their own, they joined forces with fungi to conquer Earth. Newly-discovered fossils show that fungi and green plants moved from water on to land at about the same time, between 455 million and 460 million years ago.
The fossils of tiny fungal spores were recovered from a road cut in Wisconsin and their analysis is described in the journal, Science. While they were not directly linked to green plants, they push back the date of the earliest known land based spore fossils by 55 million to 60 million years, putting them into the same era when plants started to appear on land.
Plant and fungus interactions are common today, with up to 90 per cent of all green plants forming associations with what are known as "mycorrhizal fungi". These fungi grow around and into plant roots and help them absorb minerals and water. They give plants a substantial competitive advantage according to co-author, Prof Linda K. Graham of the University of Wisconsin, Madison. "Plants without mycorrhizal fungi are competitively inferior," she said. "Though they won't die, in a highly competitive situation or places with a dearth of resources, mycorrhizal fungi function as an auxiliary root system to provide additional nutrients."
Although they weren't recovered with plant fossils, the research team argues that there is strong circumstantial evidence for plant/fungal associations back to 460 million years ago. The fungi have been found in 400 million year old fossils in company with more evolved vascular plants, and similar modern fungi are known to form a simple association with today's liverworts and hornworts, relatives of the only group of land plants around 460 million years ago.
"The presence of both organisms at the same time suggests that there could have been such an interaction, and that organisms even then were interdependent," according to Dr Dirk Redecker, an expert on the evolution of fungi from the University of California, Berkeley.
The first land plants are thought to have taken hold as much as 505 million years ago but no pieces of land plants large enough to see without a microscope have yet been found from that epoch.
This may be due to the fact that these early colonisers mainly consisted of soft, easily decayed tissue, like modern liverworts. Only spores and microscopic parts of resistant protective tissue have survived to tell their fossil story. Even so, the work pushes back the oldest known land-based fungal spores by up to 60 million years.