Today Arabia is a scorching desert, but it has been green and lush several times in the past 8 million years. That means animals from Africa could have wandered into the area – potentially including early human relatives that were thought to have been confined to Africa.
“The deserts are turning on and off through time,” says at Griffith University in Brisbane, Australia. “We have hippos coming out of Africa. Why not hominins?”
Previous research has shown that Arabia had several rainy periods . During these wet spells, the peninsula had rivers and lakes that supported thriving grasslands and woodlands. Modern humans and other unidentified hominins moved into the area during these hospitable times.
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To extend the climate record further back in time, Petraglia’s team sampled stalagmites and stalactites from seven caves in the Umm Er Radhuma formation in central Arabia. Because these formations are created by the flow of water into caves, they contain records of past climates.
The team dated them using two methods: uranium-thorium dating and uranium-lead dating. The uranium-lead dating is a significant advance, says Petraglia, because it allows the land-based climate record to be extended much further back in time. “It’s just a game changer, because the dating could go back that far,” he says.
The caves recorded four wet periods in the past 8 million years: 7.44 million to 6.25 million years ago; 4.10 million to 3.16 million years ago; 2.29 million to 2.01 million years ago; and 1.37 million to 860,000 years ago.
We don’t yet have fossil records to reveal what Arabian ecosystems were like during these humid periods, says Petraglia. But if the past million years is any guide, he says, we can expect rich grasslands and woods around rivers and lakes.
“Suddenly we’re starting to get a handle on how habitable this area might have been,” says at the in the UK, who was not involved in the study. She says the cave records probably give a good indication of conditions throughout Arabia because rainfall there is controlled by the monsoon, which operates on a continental scale.
It is also possible that the wet conditions extended into the Sahara desert to the west, says Petraglia. If so, there may have been intermittent green corridors spanning North Africa and Arabia.
Crocker has also found : “These same arid-humid cycles, they seem to match up really nicely.”
The study’s 8-million-year timespan covers the entirety of known hominin evolution. The earliest hominins, like Sahelanthropus and Australopithecus, are known only from Africa. Only members of our genus, Homo, are known to have lived outside Africa. The oldest known is Homo erectus from Dmanisi in Georgia, from 1.8 million years ago. There are also stone tools, made 2.1 million years ago by unknown creators, .
Petraglia says it is possible that hominins, including those that came before Homo, lived in Arabia . “The humid windows are telling us there’s no reason why they shouldn’t be,” he says. The problem is that there is a huge gap in the Arabian fossil record, spanning from about 5 million to 500,000 years ago: “So far, nobody has either looked for or found sites of that age in Arabia.”
“I’m not aware of any reasons why they wouldn’t have been able to migrate if it was wet enough,” says Crocker.
More recent humid periods have allowed people to settle in areas that are now barren desert. A study published on 2 April described . They lived about 7000 years ago, during the African Humid Period of 14,500 to 5000 years ago, when the Sahara was green.
Nature
