Plants Crawled onto Land Far Earlier Than Expected

Plants may have appeared on land far earlier than expected. Scientists have found that some green algae may have been hanging out on land millions of years before they evolved the same adaptation as land plants.

Land plants have developed an alternating life cycle. Presumably, this allowed their offspring to float inland and conquer Earth. Since 1980, botanists have suspected that algae may have been able to conquer land before this adaptation occurred. Until now, though, little evidence has existed to corroborate that theory. Now, scientists have genetic and morphological evidence that corroborates this.

"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 Jesper Harholt, one of the researchers, in a news release. "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 researchers also explored structures (or rather, the loss of structures) in algae that are hard to explain if algae lived only in water. For example, some green algae have lost their flagella, which allow single-celled organisms move around in water. All of the algae that are close relatives to land plants also no longer have an eyespot, which is used to swim toward light.

Cell wall traits combined with the recently sequenced genome of terrestrial green algae revealed that this green alga shares a number of genes with land plants related to light tolerance and drought tolerance. This, in particular, shows that the traits arose linearly rather than by convergent evolution.

"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 Peter Ulvskov, one of the researchers. "We have to turn this thinking on the head-we have the evidence now."

The findings are published in the journal Trends in Planet Science.

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