Dozens of life-sized rock engravings of camels and other desert-dwellers in Saudi Arabia have revealed that humans thrived in the ultra-dry region 12,000 years ago.
The carved images, on massive boulders, suggest fresh water sources started to appear in the previously inhospitable area around the time, allowing people to move in.
The rock art sites, described in a study published in Nature Communications, also feature stone tools which may hint at the people who made the art.
Study co-author Michael Petraglia, an archaeologist at Griffith University, said the discovery "fundamentally" changed our understanding of human movement in the area.
According to Professor Petraglia, the rock art is the oldest discovered so far in the Arabian Peninsula.
"We have a very early date for this rock art, but also a unique date for the human penetration of the heart of Arabia, once environments started to improve," he said.
Hugh Thomas, an archaeologist at the University of Western Australia who wasn't involved in the study, said the finding helped to fill an "archaeological hole" in central Arabia.
"Much of our current understanding of this period is derived from research conducted in the Fertile Crescent and the coastal areas of the Arabian Peninsula," Dr Thomas said.
"This study … demonstrates that the Arabian peninsula has a rich history at this time, even if little trace of it has been found to date."
Carvings found in the Nefud Desert
Saudi Arabia is dry now, but it used to be much drier — and, at other times, much wetter.
Hundreds of thousands of years ago, it went through very damp periods, where it became a crossroads for animals and early humans.
But as the last ice age reached its peak 25,000 years ago, rainfall stopped and the place became so arid it was uninhabitable.
Several 10,000-year-old archaeological sites have been found dotted across the region, showing it had become more amenable to life by then.
But researchers didn't know whether people had started repopulating the area even earlier.
Professor Petraglia and his archaeologist colleagues followed up reports from local amateur archaeology enthusiasts of huge rock carvings at three sites at the southern end of the Nefud Desert.
They found and excavated a total of 176 engravings, mostly of camels, aurochs, gazelles, ibex, and wild horses.
Some carvings were more than 2 metres high.
"We have found the most spectacular concentration [of carved images]," Professor Petraglia said.
The team also excavated artefacts including 532 stone tools from around the sites, which the researchers believed belong to the people who made the carvings.
Art dated to around 12,000 years old
While the carvings could not be directly dated, the researchers could estimate the age of the tools and sediment layers around the carved boulders using carbon dating, and another technique called luminescence dating.
These dating methods gave them an age of 11,400 to 12,800 years for the carvings, some 2,000 years older than the previous earliest evidence of humans repopulating the area.
"We've added a couple thousand years onto that history, and that's really important because we have these hunter-gatherers moving into the area before domestication," Professor Petraglia said.
This meant that 12,000 years ago, the desert hosted ephemeral water sources, which could fill up in wet years and support life.
"Once you have the formation of these ponded areas and wetlands, wild animals would have been attracted to it too, as well as the hunter gatherers," Professor Petraglia said.
"Not only would they have been a sort of [diet] source, but they were being adorned on the walls of these large boulders."
Phillip Edwards, an archaeologist at La Trobe University who wasn't involved in the research, said the desert, which seems inhospitable to us, would not have been impassable for experienced hunter-gatherers.
"The distance from north Syria to the area of the Dead Sea in Israel and Jordan is no more than Aboriginal men would travel each year from the channel country of Queensland to fetch prized Flinders Ranges ochre," he said.
"Particularly in arid areas, hunter gatherers, as in Australia, do tend to travel on phenomenal distances very quickly."
He called the rock carving find "exciting".
"It adds to the very exciting, innovative finds that have been emanating from Saudi Arabia over the last decade or so because of the enormous expansion of archaeological work there."
He emphasised that the art itself had not been directly dated, and while he believed it most likely the carvings were 12,000 years old, they may have been as young as 8,000.
So who made the enormous rock carvings?
The researchers aren't sure, but Professor Petraglia said the artefacts found at the site were very similar to those found further north, in the Levant.
This suggests that the people who made the tools were originally a northern population.
Dr Edwards agreed, but also said some of the tools from more recent layers resembled those from later Egyptian periods.
"It's not impossible that you have two sets of connections [to different populations]," he said.
