The earliest plant communities on land were made up of cryptogams, lichen-like plants, and likely looked like the mosses covering the Icelandic lava field pictured here. Photo by Paul Kenrick/University of Bristol
Feb. 20 (UPI) -- Animals could never have colonized land without plants leading the way. Now, new research suggests the first land plants began taking root 500 million years ago, 100 million years earlier than scientists thought.
Until now, scientists used the oldest known land plant fossils, roughly 420 million years old, to date their arrival on Earth's continents. In the latest study, scientists used molecular clock analysis to more accurately pinpoint the origin of the earliest land plants.
Molecular clock methodology involves the measurement of genetic differences between different species.
"These relative genetic differences were then converted into ages by using the fossil ages as a loose framework," Mark Puttick, a researcher at the University of Bristol, said in a news release.
In other words, scientists converted the genetic differences into units of evolutionary time and plotted backwards on a massive timescale, filling in the gaps in the fossil record and more accurately identifying the temporal origin of the first land plants.
"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," Puttick said.
Plants do more than perform photosynthesis, turning the sun's rays into energy to be harvested by animals. Plants also physically and chemically weather rocks, altering the carbon cycle that dictates Earth's atmosphere and climate.
An improved understanding of when and how plants came to colonize the planet can help scientists more accurately model ancient climate change.
"[This] study has important global implications, because we know early plants cooled the climate and increased the oxygen level in the Earth's atmosphere," Tim Lenton, an earth system scientist at the University of Exeter, who was not involved in the research, told Science Magazine.
Much as changes in the bacterial communities in early Earth's oceans increased the levels of oxygen and set the stage for more diverse and complex marine life forms, the arrival of land plants made life outside the water possible for an explosion of new land species.
"The global spread of plants, and their adaptations to life on land, led to an increase in continental weathering rates that ultimately resulted in a dramatic decrease the levels of the 'greenhouse gas' carbon dioxide in the atmosphere and global cooling," said Bristol researcher Jennifer Morris. "Previous attempts to model these changes in the atmosphere have accepted the plant fossil record at face value -- our research shows that these fossil ages underestimate the origins of land plants, and so these models need to be revised."
Morris and her colleagues published their analysis this week in the journal PNAS.