Sand dunes of Sahara desert in Morocco. (© Alexmar - stock.adobe.com)
In a nutshell
- The Arabian Peninsula experienced multiple wet periods over the past 8 million years, transforming from desert to green corridor and enabling species migration between Africa and Eurasia.
- Cave formations reveal ancient rainfall patterns shifted over time, with older rain coming from southern monsoons and newer precipitation from Mediterranean winter storms.
- The discovery challenges the idea that Arabia has been permanently hyperarid, showing instead that its climate fluctuated in sync with global cooling and ice sheet formation.
MAINZ, Germany — Eight million years ago, hippos wallowed in lakes and crocodiles lurked in rivers across what we now know as one of Earth’s most inhospitable deserts. New research reveals that the Arabian Peninsula wasn’t always the barren landscape we see today, but rather cycled through repeated “green phases” that created vital corridors for animals and early humans migrating between Africa and Eurasia.
The study published in Nature showed how the region cycled through periods of higher rainfall when rivers and lakes dotted the landscape, supporting a diverse range of animals that would struggle to survive in the region’s current harsh conditions.
“The Saharo-Arabian Desert is one of the largest biogeographical barriers on Earth, impeding dispersals between Africa and Eurasia, including movements of past hominins,” write the researchers, who analyzed cave formations from seven different cave systems in central Saudi Arabia.
These formations, called speleothems, only develop when there’s sufficient rainfall to feed underground water systems. In today’s central Arabia, annual rainfall barely reaches 104 millimeters (about 4 inches) – far below the roughly 300 millimeters needed for these cave formations to develop.
Reading the Climate Records in Stone
The international research team identified at least eight distinct humid periods stretching from the late Miocene (about 7.5 million years ago) to the late Pleistocene (less than 200,000 years ago).
“The repetitive recurrence of wetter conditions on the Arabian peninsula is not only of climatological importance,” explains Hubert Vonhof, group leader at the Max Planck Institute for Chemistry and co-author on the study, in a statement. “As the aridification of the Saharo-Arabian desert intensified over the past eight million years, these short intervals of wetter conditions became increasingly important for enabling mammalian exchange between Africa and Eurasia, likely including dispersals of our human ancestors.”
Using advanced dating methods, the scientists determined precisely when these cave formations grew. They also extracted and analyzed tiny water droplets trapped within the calcite crystals – essentially fossil raindrops preserved for millions of years – to determine where this ancient rainfall originated.
The analysis revealed a shift in moisture sources over time. The oldest cave formations formed primarily from summer monsoon rains coming from the south, while more recent formations showed increasing influence from winter storms originating in the Mediterranean.
“We were able to study the hydroclimate of the Arabian Peninsula more comprehensively than ever before,” says Monika Markowska, the study’s lead author. “We found that during the last eight million years, a southward displacement of Monsoon rains gradually decreased rainfall during the wetter intervals. As a whole, the Arabian peninsula became increasingly drier.”
A Gateway for Ancient Life
These findings align with fossil discoveries in the region. The Baynunah Formation in the United Arab Emirates (dated to 7-7.7 million years ago) contains fossils of water-dependent animals including hippos and crocodiles. Similarly, Saudi Arabia’s Nefud Desert has yielded middle-to-late Pleistocene fossils of various water-dependent species alongside stone tools indicating human presence.
“Arabia has traditionally been overlooked in Africa-Eurasia dispersals, but studies like ours increasingly reveal its central place in mammalian and hominin migrations,” notes Dr. Faisal al-Jibrin, lead Saudi archaeologist of the Heritage Commission.
Climate Connections in a Changing World
The climate shifts tracked in this study reflect global patterns, especially the cooling of the Northern Hemisphere and growth of polar ice sheets. This cooling intensified the temperature difference between poles and equator, reshaping global atmospheric circulation and shifting rainfall patterns worldwide.
Today’s Arabian Peninsula may be among Earth’s most formidable deserts, but its past tells a different story—one of recurrent transformation that played a crucial role in the biogeographic history of Africa and Eurasia.
Paper Summary
Methodology
The research team collected speleothems (cave formations like stalactites and stalagmites) from seven cave systems within a 10-kilometer radius in central Saudi Arabia. They used two different radiometric dating techniques: uranium-thorium dating for younger samples (less than 600,000 years old) and uranium-lead dating for older samples. In total, they analyzed 22 individual speleothems and obtained 74 radiometric ages spanning from approximately 7.5 million years ago to the late Pleistocene. The team also performed stable isotope analyses on the calcite minerals and the tiny fluid inclusions (fossil dripwater) trapped within them. This included measuring oxygen and carbon isotopes in the calcite and oxygen and hydrogen isotopes in the fluid inclusions. They used microscopy and Raman spectroscopy to check for any mineral changes or alterations to ensure the samples preserved their original chemistry.
Results
The study identified multiple humid episodes in central Arabia spanning the past 8 million years, including periods during the late Miocene (7.44-6.25 million years ago), early Pliocene (4.10-3.16 million years ago), early Pleistocene (2.29-2.01 and 1.37-0.86 million years ago), and middle-to-late Pleistocene (coinciding with interglacial periods MIS 15, 11, and 7). Analysis of the water trapped in fluid inclusions revealed that older speleothems formed from southern-sourced (monsoon) precipitation, while younger speleothems showed increasing influence from northern (Mediterranean) moisture sources. The study found a progressive trend toward drier conditions over time, with more stable water availability in the Miocene and Pliocene compared to more variable and reduced rainfall in the Pleistocene. After approximately 500,000 years ago, only gypsum formations were deposited (rather than calcite), indicating even drier conditions with occasional slight increases in moisture.
Limitations
The study acknowledges potential sampling bias due to cave preservation issues, though the authors argue that dryland environments may actually preserve ancient speleothems better than tropical regions. There are also significant gaps in the record, particularly between 6.3 and 4.1 million years ago, which coincides with known regional arid periods. The dating techniques have varying precisions, with uranium-thorium dating providing higher resolution for younger samples than uranium-lead dating for older samples. Additionally, while speleothems provide direct evidence of past humid periods, they don’t indicate exactly how much rainfall occurred beyond the minimum threshold needed for formation (estimated at around 300 mm per year).
Funding/Disclosures
The study was supported by multiple institutions, including the Royal Society, the Max Planck Society, the Dr. Abdulrahman Al Ansari Award to the Green Arabia Project, the Saudi Heritage Commission, the Saudi Geological Survey, and the Leverhulme Trust. Additional funding came from the German Research Foundation through priority programs “SPP-1833 Building a Habitable Earth” and “SPP-2238 Dynamics of Ore Metals Enrichment.” The authors declare no competing interests.
Publication Information
The study, titled “Recurrent humid phases in Arabia over the past 8 million years,” was published in Nature on April 10, 2025. The research was led by Monika Markowska from Northumbria University and the Max Planck Institute for Chemistry, along with numerous international collaborators from institutions in Germany, Malta, the UK, Australia, Austria, Switzerland, Saudi Arabia, and the United States.
