Ancient sediments recovered from southern Jordan provide new clues about the region's climate during a period when modern humans may have been migrating out of Africa. Published in Science Advances, the findings support growing evidence that the southern Levant — comprising desert now known as Jordan, Israel and Palestine — was once a wetland that could have supported human passage.
"The paleohydrological evidence from the Jordan desert enhances our understanding of the environmental setting at that time," said Mahmoud Abbas, a researcher at Shantou University in China and first author of the study. "Rather than a dry desert, a more humid savannah grassland provided the needed resources for human survival during their journey out of Africa and into southwest Asia and beyond."
Abbas and colleagues also found a distinctive pair of stone flakes in sediments dated to around 84,000 years ago, implying that modern humans may have populated this northern route out of Africa earlier than previously thought. Previous archaeological evidence points to present-day humans having largely descended from migrants who took a southern route out of Africa around 60,000 years ago — from the Horn of Africa across the Bab-el-Mandeb strait to the southernmost tip of Arabia, and then through southern Arabia and into Asia.
"The new ages obtained from wetland sediments containing lithic Levallois flakes adds a new insight on human occupation in the region," Abbas added. This discovery could help shape researchers' understanding of the chronology and dispersal routes of some of the earliest human migrations out of Africa. "Our study provides new evidence from archaeological, climatological and environmental perspectives to make this corridor thrive again in the scientific community."
"Successes" and "Failures" of Early Human Migration
Homo sapiens, which evolved in Africa sometime between 300,000 and 200,000 years ago, spread beyond Africa in multiple waves, following much earlier migrations by Neanderthals and Homo erectus into Europe and Asia. One wave is thought to have occurred more than 120,000 years ago along a northern corridor, through the Sinai Peninsula, Jordan River Valley and present-day northern Israel, where human remains and stone flakes have been discovered in and around caves. But these earlier waves are often considered "failed dispersals" — journeys that ceased after migrants and their progeny eventually died out.
"Despite the ages obtained from archaeological remains in Skhul and Qafzeh caves [in Israel], the absence of any evidence younger than 90,000 years in the region made it difficult to accept this route as successful," said Hua Tu, researcher at Shantou University and coauthor.
But more recent paleoclimate studies have pieced together new evidence that suggests early modern humans could have already been present in Arabia by 85,000 years ago. They could have even occupied Arabia's Nefud Desert region as early as around 120,000 years ago, in what may have been riverine savannah and grassland environments wet enough to support human passage.
"Recent discoveries in Arabia, such as footprints dated to 115,000 years [ago] and the finger bone dated to 85,000 [years ago] shed light again on this corridor," Abbas remarked. "But still, the gap exists, and it was the southern Levant that provided a wider image to the story."
Ancient Climate in the Levant
The southern Levant currently contains low, dried-out river valleys ("wadis" in Arabic), but recent studies showed that the region once harbored ancient lakes and wetlands during a pivotal time for human migration, potentially tens of thousands of years earlier than previously believed.
Like prior studies, Abbas and coauthors employed a sediment dating technique known as optically stimulated luminescence (OSL) — a method of estimating how recently sediments were last exposed to sunlight, allowing scientists to draw inferences about their ages. Abbas works in the research group of Zhongping Lai, a professor at Shantou University, an expert in OSL and luminescence techniques and study coauthor.
"We use a series of chemical and physical pretreatment to extract the pure quartz or enriched feldspar grains from the sediments, then measure the intensity of [the] luminescence signal," Lai explained. "It can provide information on depositional processes and landform evolution."
Lai noted that luminescence dating is ideal for analyzing sediments in this region because they contain abundant quartz and feldspar minerals, but they lack the organic materials often used for radiocarbon dating. Additionally, luminescence techniques can date materials to as far back as 800,000 years ago if sediments aren't contaminated by light during extraction, while radiocarbon techniques can typically date back to about 50,000 years.
The researchers extracted sediments from three sites along the Jordan River Valley: Wadi Hasa, Gregra and Wadi Gharandal. "The main idea was to establish a reliable and systematic chronological framework for these sediments, which [indicate] wetland settings in the presently harsh environment," said Abbas.
The sediments contained muddy sands, organic matter and marl — a silty carbonate material often associated with freshwater environments. Accumulated pebbles and rock debris in sediments from Wadi Hasa showed instances of flash flooding and a landslide, which may have caused a dam to form, the study noted. Luminescence dating showed that wetland sediments occurred at these sites between around 115,000 and 43,000 years ago.
"It was lucky that two stone tools interbedded with wetland sediments were [found] incidentally during our field trip," Tu said, commenting on two Levallois flakes found in the sediments at Wadi Gharandal, dated to around 84,000 years ago. The Levallois technique describes an advanced method of knapping sharp, pointed stone tools, and is characteristic of tools produced by Neanderthals and Homo sapiens who first migrated out of Africa.
Much Remains Unknown
The new findings support growing evidence that early modern humans may have inhabited the once well-watered Levant and northern Arabia much earlier than widely believed. This could reinvigorate work to investigate human presence along the northern corridor between 130,000 and 80,000 years ago, despite the current lack of human remains found from that time in the region.
When asked about the purported presence of Neanderthals in the Levant between 80,000 and 55,000 years ago, the researchers noted that Neanderthals are out of the scope of the present study, which only focuses on the environmental context of the region. More work like the current study, which fills gaps in the climate conditions during a time of potential interaction between Homo sapiens and Neanderthals, could eventually paint a more detailed picture of how the species may have coexisted in the region for thousands of years.