Shaun
A new study of stalagmites in caves in central Saudi Arabia provides strong evidence that the region was lush and green for much of the last eight million years—a phenomenon known as “Green Arabia,” but until now only a hypothesis.
The study also indicates that the central band of the world’s “barrier” deserts— from the Sahara in the west, across Arabia and to India’s Thar Desert in the east—were at times well-watered, savannah-like landscapes that encouraged the migrations of primates and other animals out of Africa, including Homo sapiens and some of our hominin ancestors.
“The sand seas that we are used to seeing have not always been the case,” says archaeologist Michael Petraglia, the director of the Australian Research Centre for Human Evolution at Griffith University. “That has had a huge effect on human evolution.”
How Scientists Retrace Timelines?
Petraglia is a senior author of the study, published in Nature. He has worked on the Green Arabia theory since 2010, mainly using studies of sediment cores taken from the region’s ancient dried-up lakes.
The cores contained traces of the plants that once grew there and the types of sediments produced by the climate, and showed that Arabia—and likely the Sahara and the eastern deserts—had been humid for long periods.
But the sediment cores only dated back to the last half million years or so.
The new climate data from the central Saudi Arabian caves, however, was used to reconstruct the region’s climate over the last eight million years—a dramatic advance.
The key data comes from seven caves at As Sulb, an eroded limestone plateau northeast of Riyadh in central Saudi Arabia, where 22 rock samples were taken in 2019.
The samples were mostly from stalagmites, which grow upward from a cave floor as mineral-infused water slowly drips on them (their counterparts, stalactites, grow down from the cave ceiling.)
University of Malta archaeologist Huw Groucutt, one of the study authors, says stalagmites more often contain the evidence that scientists are looking for, including the traces of lead, uranium, and thorium used to date them.
Uranium-thorium dating works by comparing radioactive traces of uranium in the samples to the thorium it decays into. This method provides accurate dates over the last 600,000 years.
But uranium-lead dating is a relatively new technique that compares the uranium isotopes to the lead they decay into over a longer period. This can date a stalagmite over a much longer time—around 7.44 million years ago, in this case.
Often overlooked
The new climate record agrees with evidence Petraglia has championed in other studies about Green Arabia, which suggest the entire band of deserts that separate most of Africa from Eurasia—including the Sahara and deserts farther east—were verdant for long stretches of time.
He says Arabia is often unnoticed in diagrams that purport to show the routes animals and early humans used to disperse from Africa.
But a green Arabia could have been a key route for such migrations.
And when Arabia was humid, Petraglia says, the Sahara and other deserts would have been humid too—climate changes that may have been caused by periodic variations in Earth’s orbits around the sun.
“These findings have been spectacular,” Petraglia says. “This is an entirely new source of [climate] information, not only for Arabia but for many places around the world.”
Groucutt explains that the stalagmites grew larger only when the environment was wet enough for water to seep through the ground and into the cave, while the isotopes in the rock samples revealed when these times of growth had occurred.
The study shows that Arabia’s humid periods in length from more than a million years to tens of thousands of years—long enough for waves of animals and early humans to wander into Arabia from northern Africa and continue on into the fertile lands beyond.
And there is more to come. Petraglia explains that scientific research on caves in the region has been conducted for several years, but it has all been unpublished until now: “we’re just at the beginning of cave work in Saudi Arabia.”
Groucutt describes expeditions that have already taken place to retrieve samples from caves in the northern parts of the Arabian peninsula, which could help refine the geographical extent of the humid phases.
Blown awayPalaeoclimatologist Paul Wilson of the University of Southampton in the United Kingdom, who was not involved in the new study, says he was “blown away” by the reconstructed climate record, which had far greater detail than any that had existed until now.
His own research with deep-sea sediments showed they contained less dust from the Sahara during several wet periods over the last 11 million years that also correlate with variations in Earth’s orbit around the sun.
But while Africa has been well studied in recent years, Arabia has mostly been overlooked, he says: “This is a really powerful validation of some of the things that we’ve long expected.”
