We have land plants to thank for the oxygen we breathe. And now we have a better idea of when they took to land in the first place. While the oldest known fossils of land plants are 420 million years old, researchers have now determined that pond scum first made landfall almost 100 million years earlier.
“[This] study has important global implications, because we know early plants cooled the climate and increased the oxygen level in the Earth’s atmosphere,” conditions that supported the expansion of terrestrial animal life, says Tim Lenton, an earth system scientist at the University of Exeter in the United Kingdom who was not involved with the work.
For decades biologists have been trying to come up with a reliable birth date for land plants. Lacking backbones and hard shells, plants leave relatively little behind in the fossil record, so researchers suspect even the oldest plant fossils don't represent the first flora.
Some scientists have tried to use plant genetic data as "molecular clocks"—knowing a typical mutation rate, they can estimate how long ago various species split based upon their differences in DNA—to figure out their evolutionary history. But they have been unable to sort out the lowest, or earliest, branches of the plant family tree. At that base, vascular plants—which include the trees, crops, and flowers we are most familiar with—came along sometime after liverworts, hornworts, and mosses. Yet the order in which those three other groups appeared has been a mystery and has stymied molecular clock studies.
Philip Donoghue thought that if he brought enough computer power to bear on this problem, he could solve this mystery. Donoghue, a paleobiologist at the University of Bristol in the United Kingdom, and his colleagues started with previously collected genetic data on more than 100 plant and algal species. They tested every permutation of the relationships of the four groups of plants with several kinds of these analyses and factored in the ages of dozens of plant fossils as a reality check.
The exact configuration of the base of the plant family tree doesn't matter to dating the first land plants, Donoghue and his colleagues report today in the Proceedings of the National Academy of Sciences. All the analyses indicate that land plants first appeared about 500 million years ago, during the Cambrian period, when the development of multicellular animal species took off.
The new analysis “shows that the first land plants arose earlier than we thought, regardless of current uncertainties about which land plants evolved first," Lenton says.
Donoghue's team has also applied its computer power toward resolving these uncertainties. Plant scientists once considered liverwort the most primitive existing plant because it lacks roots and pores for gas and water exchange, but a few recent studies had suggested that liverwortlike plants were not the earliest land plants. Donoghue now agrees. Liverworts are most closely related to mosses and once had roots and pores but lost those traits over time, his team reports this month in Current Biology.
"The assumption has been that the ancestral plant is physiologically like a liverwort," Donoghue says. But his group’s analysis suggests that ancestor likely had rudimentary pores and roots, and thus might grow better, process more soil and more carbon dioxide, and therefore have been more influential in Earth's biogeochemistry than researchers have thought.
"It’s nice to see they came to the same conclusion that we did" about liverworts, says Jim Leebens-Mack, a plant evolutionary biologist at the University of Georgia in Athens who helped gather the data Donoghue used for the new analysis.
Leebens-Mack also praises the study that calculated when land plants appeared as the most comprehensive to date. If Donoghue's results are right, "This changes the entire timeline for the origin of terrestrial life and the subsequent pace of evolutionary change in plants and associated animal (and fungal) groups," says Pamela Soltis, a plant evolutionary biologist at the University of Florida in Gainesville. "Also, these earlier dates would mean that changes to the Earth happened at a slower pace than we might otherwise think."
Leebens-Mack is still cautious. Molecular clock analyses always come up with older dates than fossils, he notes, so "I always take these estimates with a pretty big grain of salt.”