Documentaries about the history of life highlight the conquest of the land as our ancestors crawled from the seas. Far less attention, however, goes to the process by which plants made the same journey, giving the first animals their food supply. This might be just as well, because it seems we have had the process all wrong.
Dr. Jesper Harholt of the Carlsberg Laboratory, Denmark provides evidence for the theory that algae species established themselves on land, and it is from these early adopters that the terrestrial plants we know today evolved, as published in Trends in Plant Science. "Significant evolutionary steps in chariphytes [green algae such as stonewarts] occurred on land and not in water," Harholt and his coauthors write.
The theory dates back to 1980, but until now had little support. However, Harholt has found evidence in the walls of algal cells.
"We realized that algae have a cell wall that's similarly complex to terrestrial plant cell walls, which seemed peculiar because ancient algae were supposedly growing in water," said Harholt in a statement. Plants on land need strong walls to hold themselves up, but even large algae like kelp can rely on buoyancy. "We then started looking for other traits that would support the idea that algae were actually on land before they turned into land plants."
The algae most closely related to plants no longer have a light detector, which other species use to navigate towards light. Some have even lost their flagella, an essential feature for getting around in water for single-celled algae. Harholt and his collaborators even found that the genome of the algae species Klebsormidium flaccidum, sequenced in 2014, contains genes land plants use for drought tolerance, an odd feature for something that dwells entirely in water.
Putting these pieces together the authors conclude algal species existed on land before turning into plants, rather than plants emerging from algae in the seas before making the transition. "With all of the genomic and morphological data we have, it is very hard to explain, evolutionarily-wise, how algae lived in water all the way up to land plants," said coauthor Professor Peter Ulvskov of Copenhagen University.
The authors propose that algae transitioned via sandy surfaces, presumably first in inter-tidal zones and then in areas with heavy rainfall.
Suggested path by which plants evolved on land from algae that had made their way out of the water. Panny Kondor
The theory is hard to prove since such ancestral algae would not have fossilized well, but they hope further genomic studies will reveal how the 250 or so genes that go into building complex cell walls came together.
Professor Øjvind Moestrup, also of Cophenagen University, contributed to the paper but admitted: "The strange thing for me is that if these green algae were terrestrial for a long time, how come so few of these species are still around? It could be because they were all outcompeted, but maybe one day we will find more green algae of this lineage."
Fungi are thought to have been even earlier land inhabitants, an estimated 1,300 million years ago.
2