Earth’s unique lifeforms evolved because of the relatively high oxygen content in the atmosphere. Today, the air we breathe sports an oxygen concentration of 21 percent — vastly outclassing pitiful levels of less than one millionth of a percent on Mars and 0.1 percent on Venus. 

In the past, scientists speculated that Earth’s oxygen levels varied between 15 to 35 percent. Now a new study in the journal Nature Geoscience, which uses the ashes of ancient burnt vegetation, provides the first real proof of the blue planet’s extreme oscillations in O2. 

Ian Glasspool of the Field Museum in Chicago and colleagues used concentrations of charcoal from peat deposits to reconstruct the last 419 million years of atmospheric oxygen concentrations. 

Glasspool exploited the strong correlation between atmospheric oxygen and flammability. During periods of low levels of oxygen, around 15 percent, it is difficult for even the driest plants to catch on fire. But when oxygen levels rise above 25 percent, even wet plants can burn, Glasspool told Discovery News.

At between 30 and 35 percent, wildfires become rampant. 

This is exactly what Glasspool imagines the Late Paleozoic was like, around 280 million years ago, with oxygen levels peaking at approximately 30 percent.  

Named the Carboniferous period, this era is characterized by extreme plant diversification, including the evolution of conifers (like those shown in the image above) and the prominence of giant insects, including dragonflies with 2-foot wingspans. 

Scientists suspected that insects, which get oxygen more directly than mammals through air tubes in their tissues, rather than through their blood, grew in size to take advantage of the greater oxygen concentrations.  

But, before this study researchers could not rule out other explanations, like the lack of predators, for explaining insect gigantism during the Carboniferous.

As for what drove the changes in oxygen, Glasspool said, “The dramatic increase in atmospheric oxygen levels in the Early Carboniferous is almost certainly due to the rise to dominance and the ecological diversification of land plants."

In addition to explaining the strange creatures of the Carboniferous, the study puts an end to any questions concerning whether lower oxygen levels set the stage for mass extinctions at the Permian-Triassic boundary 251 million years ago and the Triassic-Jurassic boundary 200 million years ago. 

According to the charcoal deposits, low oxygen levels lagged behind the initial extinction events, rather than preceding them. And in the case of the Triassic-Jurassic extinction, atmospheric oxygen remained high, at around 30 percent. That would seem to rule out oxygen decline as an extinction driver.  

Only one big question remains unanswered: why did oxygen concentrations level out to around 21 percent nearly 50 million years ago? For now, it remains a mystery.