The expansion of early land plants across terrestrial environments has been a key focus in planetary evolution research due to its crucial role in raising oxygen levels and transforming the living environments of Earth's organisms, thereby driving the evolution of animals.

Now, new insights into this long-studied topic have been provided by scientists from China, the United States, and the United Kingdom. They have discovered groundbreaking evidence suggesting that early land plants may have started reshaping Earth's surface environment much earlier than previously recognized, pushing back the timeline by around 30 million years. Their findings were published on Tuesday in the journal Nature Ecology & Evolution.

To trace when early land plants spread and began influencing the Earth system, the research team analyzed marine sediment records and identified a pronounced increase in carbon-to-phosphorus ratios starting around 455 million years ago. An evaluation of potential controlling factors indicates that the most plausible explanation for this shift is a marked increase in terrestrial net primary productivity associated with the early expansion of land plants.

Land plants fundamentally differ from marine primary producers in that the organic matter they generate has higher carbon-to-phosphorus ratios. When this organic matter is transported into the oceans, it raises the carbon-to-phosphorus ratio of marine sediments. This finding suggests that early land plants may have expanded around that time in the Late Ordovician — a period experiencing both mass extinction and recovery.

Further research indicates that the proportion of terrestrial organic carbon to total organic carbon buried in marine sediments was comparable to modern values, and the expansion of land plants may have occurred first on the Laurentian continent, which is part of current 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, corresponding author of the study and a professor at the Institute of Geology and Geophysics of the Chinese Academy of Sciences.

Zhao added that these processes combined may have driven Earth's surface oxygenation. Sudden changes in the environment during the initial stages of the expansion may have led to climate changes, contributed to Late Ordovician glaciation, and indirectly influenced the mass extinction events during the period. After organisms adapted to these environmental changes, the ecosystem recovered, and the increased oxygen level might have been capable of supporting the evolution of primitive vertebrates like fish, he added.