Early humans passed through the Jubbah Oasis in northern Saudi Arabia repeatedly during periods of increased rainfall over hundreds of thousands of years.

Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news on fascinating discoveries, scientific advancements and more.

CNN  — 

Stone tools and ancient animal fossils have revealed that early humans were in Arabia 400,000 years ago. Prehistoric climate change may have encouraged their travels across what are now vast stretches of desert, according to new research.

This is the oldest dated evidence for humans in Arabia, which includes Saudi Arabia and other countries across the Arabian Peninsula.

Saudi Arabia’s deserts are some of the driest regions in the world, but it was a different story hundreds of thousands of years ago. Environmental changes occurred after periods of heavy rainfall in the desert, creating lush grasslands that served as the perfect backdrop for early human ancestors migrating to and from Africa.

In the hollows between large dunes, researchers found evidence of ancient lake formation at the Khall Amayshan 4 archeological site and Jubbah Oasis in the Nefud desert, located in northern Saudi Arabia. Between 400,000 and 55,000 years ago, these periodical lakes formed and filled at five different times that were associated with the discovery of stone tools.

The stone tools help document how these early human cultures and their materials shifted over time. The oldest tools belong to cultures that relied on handaxes, like early human ancestors Homo erectus and Homo heidelbergensis. This transitions to more developed stone tool technology that could have belonged to early Homo sapiens and Neanderthals.

Archaeologists survey the Nefud Desert in northern Saudi Arabia. Ancient lakes formed in the hollows between dunes thousands of years ago during periods of increased rainfall.

The research is a “breakthrough in Arabian archaeology,” according to lead study author Huw Groucutt, head of the Max Planck Extreme Events Research Group in Jena, Germany, based at the Max Planck Institute for Chemical Ecology.

“Arabia has long been seen as an empty place throughout the past,” Groucutt said. “Our work shows that we still know so little about human evolution in vast areas of the world and highlights the fact that many surprises are still out there.”

Exploring ‘Green Arabia’

Hollows between sand dunes in the Nefud desert would catch and pool water during “pulses” of heavy rainfall thousands of years ago, creating small lakes, wetlands and rivers bordered by grasslands. This environment supported the migration of both animals and early human ancestors.

The Arabian Peninsula in Southwest Asia is the only land bridge that connected Africa to Eurasia. While this area has been of interest to researchers, the deep arid interior of the peninsula isn’t easy to access.

“In order to penetrate deep into the desert we have to have jeeps and enough supplies to conduct our surveys and excavations, and occasionally we have to employ the use of a helicopter to get to the archaeological sites in the interior of the deserts,” said study coauthor Michael Petraglia, project leader and professor at the Max Planck Institute for the Science of Human History, via email.

The site of Khall Amayshan 4 in northern Saudi Arabia showed the remains of ancient lakes.

Satellite imagery allowed the researchers to see there were once about 10,000 ancient lakes across Arabia, and surveying the archaeological sites revealed stone tools and animal fossils.

Massive wild cattle, ostriches, gazelles and even hippos lived along the shores of lakes and were surrounded by grasslands and savannahs. “The presence of hippos is extraordinary, as that tells us that there were enough rivers, lakes and wetlands to sustain them,” Petraglia said.

A storm arrives during archaeological excavation of an ancient lake.

“These ‘Green Arabia’ events drew in animals and the hominins that were pursuing them,” he said. “However, green events were not long-lasting, as they were followed by decreased rainfall and the formation of deserts. We do not know what happened to hominins during these arid periods, but we suspect that they either moved on or went extinct.”

The findings have suggested that human migrations trekked through the heart of Arabia, rather than taking coastal routes as suggested by previous research.

A place to gather

The stone tools recovered also shed light on the people who made them, even if their remains can no longer be found in the desert. And these tools tell a diverse story of cultures coming together on migration routes.

This is a 400,000-year-old handaxe stone tool from the Khall Amayshan 4 site.

“Arabia is the staging ground for migrations between Africa and Asia,” Petraglia said. “It is the geographic bridge between continents that has been ignored by scientists for decades. It is clearly a key region for the migrations of multiple hominin (early human ancestor) species over time, likely coming from different directions including Northern Africa, the Levant and possibly other parts of Asia.”

It’s possible that Arabia also served as a place for different species of humans to interact. Some of the tools dated to 55,000 years ago are similar to those made by Neanderthals, which would make this the southernmost discovery of Neanderthals so far.

“Arabia may be one of the most important places for understanding where humans mixed and mated,” Petraglia said. It is potentially the staging ground for interbreeding among species. We must find human fossils to prove this, and so the search is on.”

“It also serves as a reminder of the profound ways in which environmental change has affected, and continues to affect, the survival of life on our planet,” said study coauthor Tom White, a senior curator of Non-Insect Invertebrates at the Natural History Museum in London, in a statement.

Now, the researchers want to fill in the remaining gaps in the history of how ancient humans eventually migrated across the globe.

“We know that archaic humans made it to China by 2.1 million years ago,” Petraglia said. “This means that we are missing an archaeological record in Arabia, and there is a current gap of more than 1.5 million years.”

This question could be answered by the oldest sites in Arabia – scientists just have to identify them first.