Saudi Arabia's vast desert was
once a lush, green paradise
A new study reconstructing the Arabian Peninsula’s ancient past adds clues to how early humans
left the African continent.
https://www.nationalgeographic.com/environment/article/green-arabia-desert-lush-ancient-
landscape
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.”
