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Stone Age humans once sheltered in lava tube caves
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CNN —
People who lived in the Arabian Peninsula thousands of years ago went
underground when they wanted to beat the heat. Possibly stopping there
as they traveled between oases and pastures, they ducked into vast
subterranean tunnels where molten lava had once flowed millions of
years earlier, according to a recent study.
Beginning in the Stone Age, Neolithic herders descended into and
occupied these vast tunnels, known as lava tubes, archaeologists have
discovered. Cooler air underground would have provided a welcome
respite from the sun and wind, and for thousands of years, humans
sheltered with their livestock in the tunnels. The herders left behind
objects and even carved pictures on the rocky walls, researchers
reported April 17 in the journal PLOS One.
In the Harrat Khaybar lava field, about 78 miles (125 kilometers) to
the north of Medina in Saudi Arabia is a tunnel system called Umm
Jirsan, the longest in the region. Scientists haven't yet confirmed
the age of the lava that formed this system, but a 2007 study
suggested it was around 3 million years old. Umm Jirsan spans nearly 1
mile (1.5 kilometers), with passages that are up to 39 feet (12
meters) tall and as much as 148 feet (45 meters) wide.
Archaeologists at Umm Jirsan recently found animal bones dating from
400 years to more than 4,000 years ago, and human remains ranging from
150 years to about 6,000 years ago. The research team also found cloth
fragments, pieces of carved wood and dozens of stone tools — the first
evidence that humans were using the tunnels, starting at least 7,000
years ago.
"From earlier reports we knew that fossils were preserved at the
site," said lead study author Dr. Mathew Stewart, a research fellow at
the Australian Research Centre for Human Evolution at Griffith
University in Australia.
"However, we were not expecting to find evidence for human occupation
in the form of rock art, lithic artefacts, stone structures, and
pottery," Stewart told CNN in an email. "People made use of and
occupied these lava tubes over millennia. While most research in
Arabia is focused on surface sites, underground settings such as at
Umm Jirsan offer tremendous potential to fill in some of the gaps in
the data."
This discovery highlights the significance of Umm Jirsan and other
tunnels for understanding human dispersal in the region, said
Guillaume Charloux, an archaeologist with the French National Centre
for Scientific Research. In general, knowledge about ancient climate
and humans in northwestern Arabia is limited, "particularly during the
transitional phase between the Neolithic and the beginning of the 2nd
millennium," said Charloux, who studies ancient sites in Saudi Arabia
but was not involved in the new research. .
Around this time, local people were settling around recently formed
oases; the appearance of these desert refuges would shape human
migration patterns in the region for millennia, he said via email.
"The main contribution of this innovative and major research project
seems to me to be that it brings to light the long-lasting use —
probably ephemeral occupation — of this type of cave, which had
remained unstudied, and their enormous potential, particularly for
understanding paleoenvironmental contexts."
For nearly 15 years, Stewart and his colleagues have been piecing
together evidence of ancient human life in Arabia, mostly from sites
around lake deposits, Stewart said. Beginning around 400,000 years
ago, recurring periods of humidity saturated Arabian deserts with
rainfall. During these "Green Arabia" phases, lakes and ponds abounded
and the landscape bloomed with lush vegetation, leading to waves of
migrating humans who dispersed into southwestern Asia, Stewart and
other researchers previously reported in the journal Nature.
But the last Green Arabia phase was around 55,000 years ago, and harsh
desert environments aren't kind to archaeological evidence. While
stone tools preserve well in dry deserts, bones and other organic
materials are easily degraded and destroyed by erosion and extreme
heat and cold, leaving little for researchers to interpret, Stewart
noted.
"To that end, in 2019 we decided to investigate underground settings
where organics and sediments might be better preserved," he said.
So the scientists turned their attention to Umm Jirsan. The site had
previously been mapped by the Saudi Geological Survey, and a report
from 2009 described it as a refuge for wild animals such as foxes,
wolves, birds and snakes. Caches of bones in the tunnels included
human skull fragments estimated at the time to be about 4,000 years
old. But until 2019, the tunnel system hadn't yet been closely
investigated by archaeologists, Stewart said.
"We were able to date the animal bones and sediments, which informed
us that people began occupying the cave by 7,000 years ago and perhaps
as early as 10,000 years ago," Stewart said.
Compared with other sites where humans once lived, the amount of
archaeological material at Umm Jirsan was "quite scant," suggesting
that people were visiting the tunnels as temporary refuges rather than
living there permanently, the study authors reported.
In another tunnel near Umm Jirsan, the researchers found 16 panels of
engraved rock art. The carvings appeared to be herding scenes, with
tool-wearing, stick-figure humans standing alongside domesticated
animals such as dogs, cattle, goats and sheep. Other carvings showed
animals with dramatically arching horns resembling those of an ibex;
however, these horned animals could represent a different breed of
domesticated goat, according to the study. The carvings' subjects and
their varnish coating hint that they date to a regional period known
as the Chalcolithic (around 4500 to 3500 BC), which preceded the rise
of the Bronze Age.
"Collectively, the archaeological findings at the site and in the
surrounding landscape paint a picture of recurrent use of the Umm
Jirsan Lava Tube over millennia," Stewart said. The site — which lies
along a known migratory route for Bronze Age herders — "may have
served as a stopping off point, a place of refuge protected from the
elements."
This unprecedented evidence of human occupation in ancient Arabian
lava tubes sheds light on how people adapted to live in arid
landscapes, and further investigation of Umm Jirsan and other lava
tubes promises to add even more details, Stewart added.
"These sites have tremendous potential to fill in some of the gaps in
the natural and cultural archives that persist in the Arabian
archaeological record."
_Mindy Weisberger is a science writer and media producer whose work
has appeared in Live Science, Scientific American and How It Works
magazine._
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