Researchers at the University of Washington have determined that "some" microbial life migrated from the Earth's oceans to land some 2.75 billion years ago.
However, a number of scientists continue to hypothesize that such land-based life was limited - at least initially - because the ozone layer that shields against ultraviolet radiation didn't form until hundreds of millions years later.
Nevertheless, University of Washington researchers now believe early microbes might have been widespread on land, effectively producing oxygen and weathering pyrite, an iron sulfide mineral, which released sulfur and molybdenum into the oceans.
"This shows that life didn't just exist in a few little places on land," said UW doctoral student Eva Stüeken.
"It was important on a global scale because it was enhancing the flow of sulfate from land into the ocean. In turn, the influx of sulfur probably enhanced the spread of life in the oceans."
According to Stüeken, sulfur could have been released into sea water by other processes, including volcanic activity.
But evidence that molybdenum was being released at the same time suggests both substances were being disseminated as bacteria slowly disintegrated continental rocks.
"If that is the case, it likely means the land-based microbes were producing oxygen well in advance of what geologists refer to as the 'Great Oxidation Event' about 2.4 billion years ago that initiated the oxygen-rich atmosphere that fostered life as we know it," she said.
"In fact, the added sulfur might have allowed marine microbes to consume methane, which could have set the stage for atmospheric oxygenation. Before that occurred, it is likely large amounts of oxygen were destroyed by reacting with methane that rose from the ocean into the air."
Indeed, this scenario supports the theory that oxygen was being produced for several hundred million years before the Great Oxidation Event - although it took time some time for it to reach higher concentrations in the atmosphere.
"[Our] research examined data on sulfur levels in 1,194 samples from marine sediment formations dating from before the Cambrian period began about 542 million years ago... The data has [actually] been out there for a long time, but people have ignored it because it is hard to interpret when it is not part of a large database," she added.