When did plants crawl from the sea? New study holds clues.

Solid ground's first settlers  – plants – may have arrived earlier than we thought.

Scientists agree that all plants evolved from green algae. According to previous assumptions, these ocean-dwelling algae developed alternating life-cycles – a fundamental characteristic of plant life – just before taking root on land. But new evidence, published Wednesday in the journal Trends in Plant Science, may suggest otherwise.

"Our original quest was to go backwards in evolution to find simpler model organisms for our research," says co-author Jesper Harholt, of Carlsberg Laboratory in Copenhagen. "We found that the species we looked at did not get simpler in precisely the way we anticipated."

In the 1980s, botanical researchers proposed a new model for primordial terrestrialization, arguing that land plants could have descended from algal species that had already adapted to life on land. But until now, nobody could provide proof in support of that model.

While studying freshwater algae, Dr. Harholt and colleagues noticed that some species had notably complex cell walls. The cell wall provides essential structural support to land plants, which might otherwise be weighed down by gravity. Aquatic plants tend to have simpler cell walls, since gravity is counteracted by buoyancy underwater.  

Harholt’s "secondarily aquatic" algae were among the most genetically similar to terrestrial plants, which suggests that their common ancestor could have lived on land for hundreds of million years before developing alternating life-cycles.

"'Secondarily aquatic' means that we deal with terrestrial organisms that adapt to an aquatic life style, rather than organisms that have been aquatic all the time,” says co-author Peter Ulvskov, a professor of plant science at the University of Copenhagen. "A bit like whales, which are aquatic animals that have retained land animal traits from their ancestors."

Other physical data seem to support the new theory. Over evolutionary time, some of these algae lost their flagellae, which are tiny whip-like structures that help single-celled organisms swim. And most have no eyespot – a light receptor organelle that algae use to orient themselves in water. But indisputable proof remains elusive.

"We will not find fossil evidence," Harholt says. "The difference between an aquatic and a terrestrial alga is not preserved in fossils."

Many questions remain. For example, what evolutionary trajectory did algae follow on the path to terrestrial life? With only structural and genetic data to go on, researchers expect some degree of skepticism from the field at-large.

"Aquatic green algae, as direct ancestors of land plants, have been in the textbooks for a long time," says co-author Øjvind Moestrup, an evolutionary biologist and algae expert at the University of Copenhagen. "Whether doubt will prevail depends on the corroborating observations from researchers with specialization complementary to ours."