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Ancient humans repeatedly entered the Arabian peninsula from Africa during the past 400,000 years. A single archaeological site in Saudi Arabia holds evidence of five separate occupations, according to a new study.

A second study suggests that each out-of-Africa migration was made possible by a shift to a wetter climate, creating green corridors. It simulated changes in the region鈥檚 climate over the past 300,000 years, and found that there were several periods when conditions were ideal for people to move from Africa to Asia.

鈥淚t just establishes how closely early human migrations were linked to climate change,鈥� says at the Max Planck Institute for Chemical Ecology in Jena, Germany.

Groucutt and his colleagues excavated a site called Khall Amayshan-4 in the Nefud desert in what is now northern Saudi Arabia. In a single hollow between sand dunes, the team found the preserved remains of several lakes that had formed during spells of wetter climate and then dried up.

鈥淭his was a really exceptional site,鈥� says co-author Paul Breeze at King鈥檚 College London. The team also re-examined the nearby site of Jubbah.

The lakes at Khall Amayshan-4 existed about 400,000, 300,000, 200,000, 130,000-75,000 and 55,000 years ago. In each case, the team found stone artefacts left by hominins. But no two assemblages were the same. The oldest two contained mostly hand-axes, but of different designs. The three more recent ones featured stone flake tools, which are evidence of fairly complicated tool manufacture, but again the designs were significantly different. The team argues that each group of artifacts represents a separate migration into the area, by a different group 鈥� and perhaps different species.

The most recent occupation, about 55,000 years ago, is around the time when our species (Homo sapiens) expanded out of Africa into Europe and Asia, and even to Australia. This happened between 80,000 and 40,000 years ago, says co-author Eleanor Scerri at the Max Planck Institute for the Science of Human History in Jena, Germany. The people at Khall Amayshan-4 55,000 years ago may have been part of that diaspora, she says.

It isn鈥檛 certain who any of the occupants were, though, because no hominin bones were found. The occupation 400,000 years ago was before H. sapiens evolved. Some assemblages may represent African hominins moving into the Arabian peninsula, but some of the artefacts look Neanderthal 鈥� suggesting Neanderthals came in from Eurasia, says Groucutt.

The temporary lakes formed when the Arabian climate temporarily became wetter, so rivers flowed between the sand dunes and lush vegetation grew. In a second study published last week, a team led by at the University of Cambridge simulated the climate of the Arabian peninsula and north-east Africa over the past 300,000 years, to figure out when conditions were best for hominins to migrate from Africa to Asia.

Manica says his team took 鈥渢wo approaches鈥�, which 鈥渆nded up giving us pretty much the same answer鈥�. The first asked how much rainfall humans need to survive. 鈥淚f you look at the distribution of where hunter-gatherers are right now, they completely disappear below 100-90 millimetres of rainfall per year,鈥� he says. Similarly, areas with less than 100 millimetres of rainfall tend to have very few grazing animals, which hunter-gatherers rely on for food.

The team created maps of rainfall at different times in the past 300,000 years and looked for periods when continuous corridors of rainy climates existed, which humans could have expanded into and thus reached Asia. The simulations identified several windows when the climate was suitable for human populations to expand from Africa into Asia.

The windows largely matched the occupations found at Khall Amayshan-4. 鈥淭heir episodes line up remarkably well with ours,鈥� says Manica. For instance, the Nile and Sinai regions were liveable between 246,000 and 200,000 years ago 鈥� in line with an Arabian peninsula occupation about 200,000 years ago. This area reopened between 130,000 and 96,000 years ago, potentially explaining the occupation 130,000-75,000 years ago.

The Arabian peninsula was only intermittently inhabited, says Scerri, in contrast to other regions with more equable climates. That may mean it was a boundary where different hominin groups could sometimes meet.

For Groucutt, this suggests that humans and Neanderthals may have interbred in Arabia. Today, everyone whose ancestry is mostly non-African carries some Neanderthal DNA. This suggests that the two groups met shortly after the out-of-Africa expansion began. Exactly where is uncertain. Thanks to Khall Amayshan-4, says Groucutt, Arabia is now a place where we have Neanderthal-style tools and human-style tools very close in time, which suggests that the two species were there at similar times, and so might have been able to interbreed.

The two studies also have implications for the route hominins took when moving from Africa to Asia. The two main possibilities are a northern route through the Nile and Sinai, and a southern route across the Bab-el-Mandeb strait: the narrowest point of the Red Sea between Africa and the Arabian peninsula. Manica鈥檚 team looked at both. 鈥淲e find that quite often they are both available,鈥� he says.

For Groucutt, finding multiple occupations in north-west Arabia 鈥渜uite strongly supports the northern route鈥�.

Manica argues that both routes may have been used at different times. 鈥淭here is no reason why the same route would have been taken every time,鈥� he says. In his simulations, the northern route was 鈥渜uite challenging鈥� about 60,000 years ago, whereas the southern route was ideal. One possibility is that people used the northern route for the earlier migrations, but by 60,000 years ago they had developed boats or rafts, and could use the southern route. Manica points out that modern humans reached Australia at least 50,000 years ago, which requires sea crossings far more challenging than the Bab-el-Mandeb strait.

For Manica, the baffling thing is why it took so long for humans to successfully expand out of Africa, when the climate repeatedly enabled it. The earlier wet spells were actually better than the one about 60,000 years ago. 鈥淭hey came out at a period that was OK but not the ideal period,鈥� he says.

Whatever enabled the successful final migration, it wasn鈥檛 the climate, says Manica. It may be that Eurasian hominins like Neanderthals and Denisovans were in decline and therefore there was less competition 鈥� but that just raises the question of why they were in decline.