Some 470 million years ago, the first land plants emerged from prehistoric waters, put down roots in soil and ended up ruling the plant world. But scientists haven’t been certain about the family history of those pioneer plants.

By studying gene sequences of common fresh water algae, a team of University of Maryland researchers, funded by the National Science Foundation (NSF) has traced this family tree and identified a group of algae that are the closest living relatives of the first land plants. The scientists have moved a step closer to understanding how land plants evolved and came to dominate the terrestrial biosphere.

“What used to be a very short story – land plants evolved from aquatic algae – just became a much more interesting narrative,” said James Rodeman, program director in NSF’s division of environmental biology. “The new details in this part of the Tree of Life will guide research on how photosynthesizing organisms conquered the land.”

In a study published in the Dec. 14 issue of the journal Science, Maryland scientist Charles Delwiche and doctoral student Kenneth Karol confirm that the closest living relative of the first land plants is a group of green algae called the Charales, which survives today in fresh water around the world.

“Science has long believed that land plants are derived from primeval algae that became adapted to live on land, but we weren’t sure exactly how this happened, or which living algae were most closely related to land plants,” said Delwiche. “It’s an important part of the Tree of Life that has been unresolved.”

Although both the Charales and land plants can be traced back in the fossil record over a period of more than 400 million years, their common ancestor has been extinct for even longer and hasn’t been identified in the fossil record.

“Our data confirm that land plants and the Charales both evolved from a common ancestor that was a fairly complex organism,” said Delwiche. “We now can make specific inferences about what this organism looked like. It wasn’t just some sort of amorphous pond scum. It was made up of branching threads and reproduced with eggs and sperm.”

Scientists have thought that Charales and another group of algae called the Coleochaetales were almost equal cousins of the first land plants. Both groups share with land plants similar characteristics of growth, reproduction and cell division. But it wasn’t until Delwiche and Karol, along with collaborator Richard M. McCourt at the Academy of Natural Sciences in Philadelphia and University of Maryland student Matthew T. Cimino, studied the DNA sequences of four genes from 40 different plants and algae that the lineage could be traced with certainty.

“Plants didn’t write diaries or letters for us to study, but they do have genetic sequences that can reveal their evolutionary history,” said Karol. “The DNA of the Coleochaeteles shows many similar characteristics to land plants, but the Charales are even more closely related. The genes we examined show not only that the Charales are related to the first land plants, but that they are the closest living relatives.”

“These findings can help us understand what properties allowed land plants to dominate the biosphere,” said Delwiche. “It’s really exciting to know that we still have plants that look like the ancestors that were underfoot when the dinosaurs roamed the earth.”

The study was funded by the “Partnership for Enhancing Expertise in Taxonomy” program of the National Science Foundation.