Humid phases once turned Arabian desert into a lush paradise: study
The region called Arabia sits at the heart of the earth's driest deserts, stretching from the Sahara in the west to India's Thar Desert in the east. It holds the distinction of being the largest biogeographical barrier on the planet. Over millennia, the arid conditions of the Saharo-Arabian Desert are expected to have prevented hominins and wildlife from migrating between Africa and Eurasia.
Research has found that this arid barrier has existed for at least 11 million years.
Then again, fossil evidence from the late Miocene and the Pleistocene epochs has suggested that water-dependent animals like crocodiles, hippopotamuses, and horses roamed the region as recently as 74,000 years ago.
The two facts raise a curious idea: could Arabia, the formidable wall of dryness, have once been a more nourishing land?
Mineral deposits uncovered recently in the caves of central Saudi Arabia have bolstered this possibility: that Arabia was indeed once part of a lush green landscape that, among other things, allowed animals — including our ancestors — to migrate through as they spread out of Africa.
According to the researchers who studied the deposits, the cause of this lushness was intermittent phases of humidity the region experienced in the last eight million years, which gradually turned a desiccated landscape into a well-watered grassland. Their findings were recently published in Nature.
In search of Green Arabia
'I visited Saudi Arabia as part of a Fulbright award. I was curious why no one was integrating Arabia into the Out of Africa story and wanted to assess the situation firsthand myself. At the time, I was working in India and I had hypothesised that movements out of Africa would have been across the Arabia-India zone,' Michael Petraglia, director of the Australian Research Centre for Human Evolution, leader of the 'Green Arabia' project, and a coauthor of the study, said.
The research team itself was international and was supported by the Saudi Heritage Commission in the kingdom's Ministry of Culture.
For years, researchers believed that the Arabian Peninsula had always been a barren land with a foreboding environment and where humans had only settled a few thousand years ago, especially once they had managed to domesticate animals like camels and goats. This belief kept Arabia firmly out of theories of 'Out of Africa' — a popular model that suggests modern humans originated in Africa and then migrated to the rest of the world.
The 'Green Arabia' hypothesis cuts through this belief and suggests that this now-arid land had the occasional humid or rainy phases that transformed it into a wet and verdant terrain, crisscrossed with rivers and lakes and capable of sustaining diverse plant and animal life.
From a decade-long quest to accumulate evidence for the 'Green Arabia' idea, Petraglia singled out the Jubbah Oasis, a remnant of an ancient lake in Saudi Arabia. 'As soon as we arrived, we found buried archaeological sites on old lake beds! We have archaeological sites going back 500,000 years now and so we know early human ancestors, and our species, were crossing the area during times of high rainfall. We have now documented a network of rivers and about 10,000 ancient lakes of every age,' he said.
The hypothesis is important because the Arabian Peninsula sits at a crossroads between Europe, Africa, and Asia. Researchers have expressed hope that understanding past climate in the region can help answer fundamental questions: How did early humans and their ancestors spread across continents? And how did changing environments shape their evolution?
Let it drip
When water flows through the ground, it sweeps up minerals in the soil along its path. When this mineral-rich water slowly drips into caves, it forms icicle-like formations that hang from the ceiling and rise up from the cave floor. These formations are called speleothems.
The researchers collected 22 speleothems from seven cave systems located in central Saudi Arabia. The mere presence of these formations, they said, was evidence of a wetter past because they form in two conditions: sufficient regional rainfall and enough vegetation and soil carbon dioxide to form carbonic acid (which dissolves limestone and triggers speleothem formation).
As a result, they added, any changes in humidity, groundwater, and vegetation would have altered the speleothems' structure. These changes are then preserved as the next mineral layer is deposited.
Another advantage of using speleothem archives to reconstruct past climates is that they can be accurately dated using routine radiometric methods, such as uranium-thorium and uranium-lead dating.
Uranium is a radioactive element and decays into thorium and lead at a fixed rate. This allows scientists to calculate how old a speleothem is by measuring the ratio of uranium to thorium in a sample.
Radiometric dating of the mineral formations revealed that over the last eight million years, central Arabia had gone through cycles of wet and dry periods. The earliest sign of a humid phase was between 7.44 million and 6.25 million years ago, and the most recent ones were between 530,000 and 60,000 years ago. These wet phases were often relatively short-lived, lasting only thousands to tens of thousands of years at a time.
'The findings highlighted that precipitation during humid intervals decreased and became more variable over time, as the monsoon's influence weakened, coinciding with enhanced Northern Hemisphere polar ice cover during the Pleistocene,' Monika Markowska, the lead author of the study, said in a statement.
The researchers have proposed that these wetter conditions played a pivotal role in helping mammals and early humans migrate between Africa and Eurasia and that the Arabian Peninsula served as a hub of continent-scale biogeographic exchange.
The past and the future
Archaeological evidence throughout history has indicated that human populations expanded when the climate was wetter and that during dry periods, they either moved to more hospitable areas, even if they were also geographically restricted or simply went extinct.
Will history repeat itself as 21st century humankind faces a climate crisis? This is a hard question to answer because human society today is highly technologised. For example, without air-conditioners, people may have already migrated out of regions suffering extreme heat today.
That said, the past is more than just about humans. For the last 15 years, another interdisciplinary team of researchers (also involving Petraglia) has been documenting ancient lakes and archaeological sites from 200,000 years ago to the present to track a changing climate and its effects on ecosystems.
'The past holds many lessons for us, as climate and a warming earth is a serious concern to humanity today,' Petraglia said.
Sanjukta Mondal is a chemist-turned-science-writer with experience in writing popular science articles and scripts for STEM YouTube channels.
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