Land plants may have evolved much earlier than we thought

A new study pushes back the date of emergence of land plants around 80 million years to 500 million years ago.

Article by Morgan Erickson-Davis.

A new study drastically upends conventional wisdom about when plants colonized land, pushing back the date about 80 million years to around half a billion years ago. The new date more closely aligns with when land animals emerged, and could help advance our understanding of how and when Earth’s physical and biological systems formed.

While previous estimates relied on limited fossil evidence to gauge when plants made the jump to land, researchers from the University of Bristol used “molecular clock” methods to analyze the genetic differences between living plant lineages. They then translated these differences to ages by comparing them to dated fossils to establish an evolutionary timeline for land plants as a group.

Their results were published this week in Proceedings of the National Academy of Sciences. Instead of emerging 420 million years ago – the age of the oldest known fossil land plants – the study indicates land plants first appeared around 500 million years ago.

Rhynia gwynne-vaughanii — 400 million-year-old fossil plant stem from Aberdeenshire, Scotland. Image courtesy of The Natural History Museum, London.

This pushes the emergence of land plants back into the Cambrian, a time period associated with a boom in the development and proliferation of multicellular life descriptively called the “Cambrian explosion.” Scientists believe land-dwelling arthropods first arrived on the scene mid-way through the period, which correlates to the study’s new date for land plant emergence.

“Our results show the ancestor of land plants was alive in the middle Cambrian Period, which was similar to the age for the first known terrestrial animals,” said co-lead author Mark Puttick.

The study also refutes another long-held assumption – that the earliest colonizers were similar to modern liverwort. Liverwort, lacking roots and pores for water and gas exchange, has long been regarded by scientists as the most primitive plant group. But the study’s results indicate the earliest land plants had rudimentary roots and pores, meaning that liverwort may have lost these features over time.

Scientists long thought that the earliest land plant probably looked like liverwort, which today lacks roots and pores. Photo by Eric Guinther via Wikimedia Commons (CC BY-SA 3.0)

Plant roots are a powerful erosive force, contributing significantly to the planet’s biogeochemical cycles – such as carbon and water cycles – by which matter moves between living and nonliving forms and locations. If land plants first appeared almost 100 million years earlier than previously thought and those plants had roots, the researchers believe this could have had a big impact on the Earth’s climate – and could change our understanding of the development of our planet’s physical systems.

“The global spread of plants and their adaptations to life on land, led to an increase in continental weathering rates that ultimately resulted in a dramatic decrease the levels of the ‘greenhouse gas’ carbon dioxide in the atmosphere and global cooling,” study co-lead author Jennifer Morris said in a statement.

“Previous attempts to model these changes in the atmosphere have accepted the plant fossil record at face value – our research shows that these fossil ages underestimate the origins of land plants, and so these models need to be revised,” Morris said.

Banner image: Early life on land resembled cryptogamic ground covers like this lava field in Iceland. Co-author Sylvia Pressel appears in the right of the picture. Image courtesy of Paul Kenrick

Citation: ‘Timescale of early land plant evolution’ by JL Morris, MN Puttick, J Clark, D Edwards, P Kenrick, S Pressel, CH Wellman, Z Yang, H Schneider and PCJ Donoghue in Proceedings of the National Academy of Sciences USA.

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