Scientists have long wondered what gave Earth enough oxygen to support the rise of animals and people, and researchers said Monday they believe it all started with moss.

The report in the Proceedings of the National Academy of Sciences said mossy ground cover began to proliferate about 470 million years ago, giving our planet its first stable source of oxygen and allowing intelligent life to thrive.

"It's exciting to think that without the evolution of the humble moss, none of us would be here today," said co-author Tim Lenton, a professor at the University of Exeter.

"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."

Oxygen first appeared in Earth's atmosphere about 2.4 billion years ago, in what was known as the Great Oxidation Event.

But oxygen did not reach its current levels until some 400 million years ago.

Some scientists have theorized that forests gave rise to increasing oxygen levels, but the PNAS study disagrees with that notion.

Using computer simulations as a way of peering into the past, researchers estimated that lichen and moss could have generated about 30 percent of Earth's oxygen by about 445 million years ago.

As moss proliferated, it increased the amount of organic carbon in sedimentary rocks, driving up oxygen levels in the air.

This oxygen boost "allowed large, mobile, intelligent animal life, including humans, to evolve," said the study.