Early plant expansion timeline revised

Early land plants may have begun reshaping Earth's surface much earlier than previously thought — pushing the timeline back by about 30 million years, according to a new study.

The findings, resulting from research by scientists from China, the United States and the United Kingdom, were published on Tuesday in the journal Nature Ecology & Evolution.

The expansion of early land plants across terrestrial environments has long been a focal point in research on planetary evolution because of its role in raising oxygen levels and transforming Earth's habitats, paving the way for animal evolution.

To determine when early land plants spread and began influencing the Earth's system, researchers analyzed marine sediment records. They identified a pronounced increase in carbon-to-phosphorus ratios beginning around 455 million years ago. After evaluating possible driving factors, the team concluded that the most likely explanation is a significant rise in terrestrial net primary productivity linked to the early expansion of land plants.

Land plants differ fundamentally from marine primary producers because the organic matter they generate contains higher carbon-to-phosphorus ratios.

When that material is transported into the oceans, it increases the carbon-to-phosphorus ratio of marine sediments. The findings suggest that early land plants expanded during the Late Ordovician, a period marked by both mass extinction and recovery.

Further analysis indicates that the proportion of terrestrial organic carbon relative to total organic carbon buried in marine sediments at the time was comparable to modern levels. The researchers suggest the expansion may have begun on the Laurentian continent, part of what is now North America.

"Greater organic carbon burial would have promoted atmospheric oxygen accumulation while drawing down carbon dioxide levels. These effects may have been further strengthened by intensified silicate and phosphorus weathering linked to rapid land plant diversification," said Zhao Mingyu, the study's corresponding author and a professor at the Chinese Academy of Sciences' Institute of Geology and Geophysics.

Zhao said the combined processes may have driven oxygenation at Earth's surface. Abrupt environmental changes during the initial expansion could have triggered climate shifts, contributed to Late Ordovician glaciation and indirectly influenced mass extinction events during the period.

After organisms adapted to the new conditions, ecosystems recovered, and the higher oxygen levels may have supported the evolution of primitive vertebrates such as fish, he said.