When most of us think of Arabia, we think of rolling sand dunes, scorching sun, and precious little water. But in the quite recent past it was a place of rolling grasslands and shady woods, watered by torrential monsoon rains.

The finding could help settle how and when modern humans first left Africa, where our species evolved. If Arabia was once lush and fertile, it would have been an ideal place to migrate to.

"There were more windows of opportunity for humans to leave Africa than previously thought," says lead author Ash Parton of the University of Oxford in the UK.

Our hunter-gatherer ancestors "wouldn't have been able to exist in many areas of Arabia as it is today," says Parton. "At present the Indian Ocean Monsoon just clips the very southern edge of the peninsula," so the rest of Arabia is desert.

His team's findings suggest that the monsoon pushes further into Arabia every 23,000 years, allowing plants and animals to flourish. The findings are published in the journal Geology.

Modern humans evolved in Africa around 200,000 years ago. Later on, some of them left Africa for Europe and Asia, and from there spread all around the world.

But it is not clear when they left Africa, or what route they took.

The most widely-accepted notion is that they left around 60,000 years ago, travelling along the coastline of Arabia into southern Asia. That would mean they were stuck in Africa for 140,000 years.

Other archaeologists think they left much earlier, perhaps as much as 130,000 years ago. "We have evidence that humans managed to expand out of Africa and into the Middle East between 130,000 and 90,000 years ago," says Parton. "But a lot of people have believed that this was a dead end, this was as far as they got, because of Arabia and the deserts."

Parton and his colleagues have now shown that Arabia went through several periods of heavy rainfall. This would have created savannahs and woodlands, making it much more habitable. That supports the idea of an earlier migration.

Parton studied dried-up riverbeds in south-east Arabia. Found in a quarry, the Al Sibetah site preserves the silt and sediments from the bottom of the rivers, going back 160,000 years.

They found evidence of five wet phases, during which the rivers flowed and silt was deposited. During the dry times, there was little water flow and less silt was laid down.

The first wet phase happened between 160,000 and 150,000 years ago, and the most recent was around 55,000 years ago. Each was an opportunity for people to move out of Africa towards Asia.

Previous studies had suggested that rainfall increased during these periods, but it was unclear how much. In a scorching desert, a little extra rainfall doesn't make much difference. The new study suggests that the increases were big enough to support rich ecosystems.

"The environmental record I've got fits perfectly with the archaeological record," says Parton. "There was a whole series of movements of humans into Arabia."