While parts of the Arabian Peninsula are the most inhospitable climates on Earth today, that wasn’t the case in the not-too-distant past. In fact, recent research indicates it was once a lush, green area that was used as a stepping stone by early humans moving out of Africa. Now, Owen Jarus at LiveScience reports that a multi-disciplinary research team has found evidence of 46 ancient lakes throughout the Nefud desert containing artifacts from early humans.

Jonathan Gornall at The National reports that it wasn’t until recently that archaeologists came to believe that the Arabian Peninsula went through a green phase. Instead, researchers believed the area was a an empty wasteland that wasn’t populated by humans until a few thousand years ago. But in 2001, an Oxford researcher came across the results of archaeological surveys conducted by the Saudi’s in the 1970s, revealing evidence of prehistoric inhabitants in what are now harsh deserts. That eventually led to the Palaeodeserts Project, a five-year collaboration between researchers from seven countries and a dozen institutions.

The researchers started excavations in 2013 to look at how the climate of Arabia has changed over time and what role the area played for early humans. Since then, they have found that over the millennia, hiccups in Earth’s orbit have caused annual monsoons to slip northward, causing periods of higher precipitation dubbed “Green Arabia” events. That green area lured early humans to the peninsula, which served as a staging ground for eventual migrations into Asia and Europe.

Jarus reports that in the new study in the journal Archaeological Research in Asia, researchers examined satellite, aerial and topographic maps to identify any potential paleo-lakes, or lakes that existed during Green Arabia events, in the western Nefud Desert, a 40,000-square-mile desert in the northern part of the Arabian Peninsula. The team then looked at sediment core from the lakes and excavated any human artifacts or animal remains they could find. The so-called paleolakes they discovered showed signs that humans lived along their shores, with tools dating from 1.8 million to 250,000 years ago.

Jarus reports that while it was believed early humans skirted the edge of the Nefud desert on their slow journey out of Africa, the new sites show that they moved deeper into the desert than previously thought.

Sylvia Smith at the BBC explains that researchers believe early humans followed rivers on two routes into Arabia, one across a landbridge in the modern-day Horn of Africa and another across the Sinai Peninsula. Though researchers have not found human remains of these early people, their stone tool technology is similar to that found in eastern Africa. “It is reasonable to suppose that anatomically modern humans have been present in Arabia for at least 125,000 years, and possibly a little longer,” Ali Ibrahim Al Ghabban, deputy director of the Saudi Commission on Tourism and National Heritage, tells Smith.

Gornall reports that researchers are interested in learning how the people living in “Green Arabia” coped as the climate began changing. “We now have evidence of dramatic swings through time between wet and dry, a repeated cycle,” Michael Petraglia, a co-author of the paper tells Gornall. “The big question is what happened to those populations when things got bad?”

In fact, the cyclical nature of Green Arabia means that sometime in the future the area will once again be lush and full of vegetation, though researchers aren’t sure exactly when that might happen, and whether climate change in general will put a damper on the greening.