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A series of ancient lake beds discovered in Saudi Arabia is showing how the now dry, arid deserts were once lush, green wetlands teeming with life.
Not only is this revealing how the changing climate over the past 400,000 years allowed wildlife to flourish in Arabia at certain times, but is also giving hints as to how and when humans may have migrated out of Africa.
How and when modern and archaic humans first expanded out of the African continent has long been debated.
As the only land bridge between Africa and Eurasia, attention has long been focused on the Arabian Peninsula. While this region today is often characterised by a hot, arid climate, this has not always been the case.
By looking at ancient lake beds preserved in the deserts of Saudi Arabia, at sites known as Khall Amayshan 4 (KAM-4) and Jubbah Oasis, a team of scientists have started to narrow down the windows of opportunity for ancient humans to move out of – and then back into – the African continent.
Dr Tom White is the Senior Curator of Non-Insect Invertebrates at the Museum. He has been involved in studying these ancient lakes and the animals that lived in them, helping to reveal that these bodies of water were once permanent features that provided stopping off points for a variety wildlife, including roving hominins.
'One of the most remarkable things about the KAM-4 site is that rather than just representing a single lake sequence within an interdunal basin, it's actually four lakes stacked on top of each other,' explains Tom. ‘Such long sequences are very rare in places like Arabia and provide a unique opportunity to see patterns over long periods of time’.
'Another remarkable thing is that each of these ancient lakes is associated with different assemblages of stone tools, representing different populations and possibly even species of hominin coming to KAM-4 at different times.'
The KAM-4 sequence has enabled Tom and his colleagues to show roughly when the deserts of the Arabian Peninsula were wetter, and therefore habitable for humans and other animals passing through the region.
Modern humans were not the first species of hominin to have migrated out of Africa and expand into the rest of the world. Fossil evidence shows that a number of different lineages made it as far north as the UK and as far east as China and potentially even the islands of Southeast Asia.
The obvious route for these exploratory populations, and later on our own roaming ancestors, is to have crossed what is now Saudi Arabia. The question is how they did this, either by migrating directly across the peninsula or hugging the coast and going around it.
'Arabia has been a huge barrier for human expansion out of and back into Africa,' says Tom. 'And this is true not just for humans: many other animals have migrated in and out of Arabia over the last few million years, when the conditions were right.'
One of the biggest issues is that of freshwater, or lack thereof. But in addition to this, the current dry environment of the region means that fossils which could give answers to these questions are few and far between.
But the researchers looking in the Nefud desert of northern Saudi Arabia have found a series of ancient lake beds, known as palaeolakes. Each palaeolake is stacked one on top of the other, spanning the last 400,000 years.
'We now have a chronology for periods of greening in the desert,' explains Tom. 'Each lake represents a relatively long, stable period of climate during which the conditions in the interior of the Arabian Peninsula were not hyper arid, but much more like the savannah seen in Africa.
'We know that the lakes were permanent because they sustained animals like freshwater molluscs and hippopotamus, which can't live without permanent freshwater.'
The KAM-4 lake sequence shows that permanent lakes existed in the Nefud at least five times over the last half a million years. The earliest of these 'green windows' has been dated to 400,000 years ago, then 300,000, 200,000, 130-75,000 and finally around 55,000 years ago.
The research also provides some of the oldest evidence for hominin species being present in the Arabian Peninsula to date.
What is more extraordinary is that each of these five palaeolakes are associated with distinctive stone tools, suggesting that populations arrived in the area from multiple directions and sources.
While the lack of hominin fossils means that the exact makers of these tools cannot be identified, a broader scale picture of cultural and technological change has emerged. The first two - and therefore the oldest - lakes are associated with Lower Palaeolithic Acheulian 'handaxe' technology typically associated with earlier hominin species.
The later three lakes are then associated with different kinds of Middle Palaeolithic stone flake-based technologies associated with more recent species, namely neanderthals and Homo sapiens.
This diverse array of stone tools demonstrates that different hominin populations, and probably even different hominin species, were expanding into the region at different times, although who was moving when and where is still not possible to pin down definitively.
'The most important thing about these findings is that we are starting to fill gaps in the hominin map,' explains Tom. 'There's so much we don't know about human evolution, particularly the potential for interactions between different hominin species, but we can now see when there were opportunities for things to happen.'