The first land plants were humble mosses rather than towering trees. Yet they may explain a spike in global oxygen levels that changed the planet forever.
Around 2.4 billion years ago free oxygen first appeared in the Earth's atmosphere, eventually infiltrating the oceans. This is known as the Great Oxidation Event. It made possible the appearance of complex organisms. Details of the timing and process are much debated. Yet even afterward, the Earth's atmosphere lacked the oxygen to support advanced animal life, or fire.
It was only somewhere around 400 million years ago that oxygen came to make up more than 15 percent of the atmosphere, let alone the 21 percent it is today. Professor Tim Lenton of the University of Exeter has attributed this second oxygen increase to the spread of bryophytes, most notably mosses, across most of the land surface.
"It's exciting to think that without the evolution of the humble moss, none of us would be here today. Our research suggests that the earliest land plants were surprisingly productive and caused a major rise in the oxygen content of the Earth's atmosphere," Lenton said in a statement.
The first land plants appeared around 470 million years ago, but oxygen levels remained low for tens of millions of years thereafter. Nevertheless, in the Proceedings of the National Academy of Sciences Lenton presents a model that these small plants changed the chemistry of the planet's rocks, and over millions of years triggered the release of oxygen into the atmosphere.
Plants release oxygen into the atmosphere through photosynthesis, but “The major long-term source of oxygen to the atmosphere is the burial of organic carbon in sedimentary rocks,” the paper observes. This prevents the carbon from binding to oxygen to form CO2, leaving it free to form molecular oxygen in the atmosphere. The burial rate, in turn, is determined by the amount of phosphorus leached from rocks, and the ratio of carbon-to-phosphorus in the buried material.
Marine plants have a 100:1 carbon to phosphorus ratio, but for mosses this is typically twenty times higher. Moreover, plants and fungi in their roots can increase rock weathering, releasing more phosphorus.
Combining these two factors, Lenton concludes the conquest of the land would have dramatically increased the carbon deposited, leading eventually to more atmospheric oxygen. He estimates that, despite their diminutive height, these early bryophytes captured 30 percent as much energy through photosynthesis worldwide as plants today.
Ancient oxygen levels are poorly recorded in the geological record, and some of the data we have is contradictory. However, rare examples of charcoal appear around 420 million years ago, becoming more common in the Early Devonian, which began 416 million years ago. “Under ideal conditions of ultra dry fuel and forced airflow, smoldering fires may be sustained at O2 > 10 percent, but this is not believed to be possible under natural conditions,” the paper notes. Consequently, Lenton concludes oxygen concentrations must have been around 15 percent to sustain the fires that produced these specimens.