Out of Africa and into Arabia
Surprising as it may seem, a quarry near Al Ain has brought scientists closer to answering one of history’s most fascinating questions: when did people first leave Africa?
Modern humans are thought to have sprung from Africa about 200,000 years ago, with their earliest traces in the fossil record coming from what are now Ethiopia and Kenya, in the east of the continent.
Until now, different researchers have held widely divergent views on when these early humans left the continent.
According to one scenario, it was only within the past 60,000 years that people travelled into Arabia, from where they continued on to South Asia.
From here, some continued on to South-east Asia and Australia, while other populations colonised Europe.
A competing hypothesis suggests people left Africa as far back as 130,000 years ago.
A major new study, published in the journal Geology and based on fieldwork carried out at the Al Sibetah quarry, 15 minutes’ drive north-west of Al Ain, favours the second hypothesis by indicating that Arabia at this time had a much more benign climate than it does today.
“People could have migrated across the landscape because it was wet. If you have rivers, it’s more vegetated and there’s more fauna,” says Professor Adrian Parker of Oxford Brookes University, one of the authors of the study.
It is, he says, part of a “growing body of evidence” that favours the earlier dates.
The evidence comes in the form of what the study’s lead author, Dr Ash Parton, a postdoctoral research assistant at the University of Oxford’s School of Archaeology, describes as “an extraordinary sedimentary sequence of ancient river gravels and soil”.
Typically, the harsh climate of the UAE erodes softer sediments or buries them beneath sand. But in this case, the sediments – located within a large area of the UAE and Oman featuring material deposited from the Hajar Mountains – had recently been exposed by quarrying.
Measuring tens of metres in height, the sequence was analysed in painstaking detail: every few centimetres, Dr Parton and Prof Parker collected samples of material, which were sent back to laboratories in Oxford and Nottingham in the United Kingdom so a complex series of chemical, magnetic and other tests could be carried out.
Analyses of carbon in the samples indicated when vegetation was growing in the area, while certain types of sediment pointed to the presence of active river systems.
“You can have silt [that is] very fine grained, or some more wind blown, and gravels, which means there’s been quite a big river there. It’s studying through time. The different sediments tell you what the climate was like,” says Prof Parker.
The climate fluctuated dramatically, partly due to changes associated with glacial periods and interglacials. However, superimposed upon these fluctuations, there are thought to have been several periods since 155,000 years ago when there would have been rivers and vegetation in the area. The wetter conditions were due to rains from the Indian summer monsoon, which today touches Arabia only in southern Oman’s coastal areas around Salalah.
Previous studies suggested, according to Dr Parton, that for much of the past, the landscape of Arabia had mostly been too harsh to support human populations and that potential “windows of opportunity” to expand across the peninsula occurred only every 100,000 years.
“Our findings have shown that this happened far more frequently and that, approximately every 23,000 years a vast river network extended from the Hajar/Oman mountains towards the Gulf coast,” he says.
“These active wadis would have been surrounded by a lush green landscape of savannah grasslands and trees.”
Dr Parton has been visiting the UAE, and other parts of Arabia, for the past eight years, typically conducting fieldwork during winter months. The potential of the Al Sibetah site, which has been used for waste disposal, was identified by chance by experts from the British Geological Survey carrying out mapping in the area.
There are no traces of ancient human activity at the site but the environmental findings tie in with archaeological evidence from elsewhere in the Arabian Peninsula, in particular the Jebel Faya rock shelter site in Sharjah. Four years ago, in the journal Science, a group of researchers announced the discovery at this location of stone tools thought to date from about 125,000 years ago.
“At that site there’s very strongly dated evidence for human occupation,” says Prof Parker, who was a co-author also of the ground-breaking Science paper.
He says that the Al Sibetah study provides “a framework from climate and environment against which the archaeology can be set”.
The researchers are continuing to work at other sites in Arabia, hoping to uncover further evidence, both archaeological and environmental, about the first movements of people out of Africa.
“In the past, the trouble would be that there’s one site that’s the environmental site and the other site that’s archaeological, and it’s hard to correlate them. We’re trying to look at sites from multiple disciplines,” says Dr Huw Groucutt, another author of the study on Al Sibetah, and, like Dr Parton, a postdoctoral research assistant in the University of Oxford’s School of Archaeology.
Particular areas of interest in the Arabian Peninsula include the An Nafud desert, in northern Saudi Arabia, and parts of the Empty Quarter. Some of the places being analysed, especially those in the An Nafud desert, are much more remote than the Al Sibetah site, and to reach them researchers must drive for several hours across large sand dunes.
“It’s a very big project; we have sites all over the place. It’s a huge area and almost nothing was known about it until the past few years,” Dr Groucutt says.
“In terms of studying climate and archaeology, it’s one of the most exciting areas to be involved in.
“It’s really pioneering-stage research. It’s been fantastic.”
So, in the years to come, Arabia is likely to yield more clues about the movements of early humans – and perhaps the results will further strengthen the conclusions to have come out of a humble quarry near Al Ain.