Our species began as a sort of arboreal rat when dinosaurs ruled the planet, and arguments abound over where the simian line branched off, but it was apparently around the time the dinosaurs declined. The general consensus is that the genus Homo began its evolutionary story in Africa and began spreading beyond the continent at least 2 million years ago. Not agreed is which route post-ape hominins, and then humans, took – if we can call it a route.
The general assumption is that they left Africa via Egypt and the Sinai Peninsula, then the “Levantine corridor,” including Israel. Some have postulated another route via the Horn of Africa, possibly when sea levels were low, before the passage through Arabia.
Now a study published in Nature by Huw Groucutt and Michael Petraglia of the Max Planck Institute for the Science of Human History, with a large international team, has shown that Arabia was occupied by multiple waves of hominins and humans over the last 400,000 years, at least. But they didn’t arrive via the Horn of Africa, they got there through Egypt and Sinai, Petraglia says.
When did these early humans live in Arabia, and not only on the coast but inland? In the interludes when it was green, Groucutt and the researchers say.
Clues among the dunes
Prehistoric archaeological evidence in Arabia is scanty. Israel has rich evidence of human evolution but has been under sophisticated archaeological investigation for over a century; Saudi Arabia and the rest of the Gulf have been blessed with interdisciplinary archaeological investigations for a little over 10 years, Petraglia explains.
There is ample evidence that Israel was on at least one route out of Africa. There are extensive Homo-related remains, mainly stone tools but also some bones in caves. In the Nefud Desert of Arabia, exactly one prehistoric bone has been found from early human history – a finger bone dating to 85,000 years ago, identified as Homo sapiens, and there are no caves to speak of, Petraglia says. But there may be more to be found.
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“Arabia is the great unknown,” he says, meaning from the perspective of human evolution. “There hasn’t been a lot of on-the-ground research and certainly no interdisciplinary research to find and date stratified [archaeological] sites.”
Now that there has been interdisciplinary research, the team reports on the discovery of no less than 10,000 lost paleo-lakes in geologic basins in Arabia that rose and fell, so to speak, with the recurring greening of the land. Each such green phase was relatively short. In association with the paleo-lakes, the researchers found evidence of at least five Homo colonizations going back almost half a million years. The pulses of human occupation were short but unmistakable.
The team’s evidence isn’t bones but stone tool assemblages associated with the ancient lakes, and they weren’t made by hippos.
The stone tool assemblages date to roughly 400,000, 300,000, 200,000, 130,000 to 75,000 years ago, as well as a site from 55,000 years ago. Each phase of occupation was characterized by a distinct material culture, which indicates that each time, Arabia was colonized by a different type of hominin.
Arabia, unlike Africa, evidently did not feature continuity of the population. That said, the researchers note strikingly different tool kits in different areas from the same period, suggesting that different Homo species coexisted there during Arabia’s green phases – when the land was lush with lakes and rivers amid grasslands where the deer and the antelope played.
This is a good place to note that Arabia’s greening was not precisely cyclical – it’s not that it happened every X thousands of years, it happened when it happened, though orbital cycles are one factor. Also, we can’t be sure that the Sahara and Arabia “greened” at the same time. They may have been slightly out of sync. Anyway, when Arabia dried out again, becoming like the desert we know today, the romping deer and antelope and hominins disappeared again.
“Arabia has long been seen as an empty place throughout the past,” Groucutt says. “Our work shows that we still know so little about human evolution in vast areas of the world and highlights the fact that many surprises are still out there.”
Blessed stratification at an ancient lake
The sites where paleo-lakes were found in association with Homo expansions are the Khall Amayshan and Jubbah basins in the Nefud Desert. Khall Amayshan is unique in Saudi Arabia in being a stratified site. Today it’s sand dunes but the researchers found evidence of six phases of lake formation in the basin.
“Khall Amayshan contains a stacked sequence, almost like pancakes,” Petraglia says: “overlapping” remains of paleo-lakes of different ages. For most of them the team found associated stone tool assemblages that were all different from one another.
“It’s the only place [in Arabia] where we found a sequence of archaeological finds in one spot,” he says.
Thus we learned about the intermittent human presence going back at least 400,000 years. And when Arabia dried out again, the hominins and humans either died or moved on, some returning to Africa and some heading north to Eurasia. And possibly Arabia was reached by early humans trekking indirectly from Eurasia back to Africa, bearing a smattering of Neanderthal genes.
Of course, as Petraglia points out, early humans went where the weather and sustenance took them; they didn’t aim to reach Europe or Asia. They had no GPS; it was only see river, nice, eat animals, no more animals, move on, see lake, see fish, nice, eat fish, water and fish gone? Move on.
See the Neanderthal
The big picture created by accruing evidence indicates that actually all of “southwest Asia,” which includes the Middle East, Israel, Arabia, Iran and Iraq, was a stomping ground of early humans. There was no “route.” Biogeologically, Petraglia explains, southwest Asia is simply an extension of Africa.
Hominins and humans went back and forth, and no, he doesn’t believe they got to Saudi Arabia via the Horn of Africa.
“We think that’s wrong,” Petraglia says. “I think when the Sahara went green, hominins moved into the Sahara then simply walked along the Sinai, moved into the Negev and then moved into green Arabia. It was just a simple walkway out of Africa.”
In geological timescales, Arabia featured more African animals than Eurasian ones, and the Homo line was part of that biogeography, Petraglia adds. He also believes more evidence of early humans and hominins will be found in Arabia, and of earlier ones too.
“Hominins have been reaching Asia for at least 2 million years, so we’re missing a record of 1.5 million years,” he says. “I think it’s going to be found in Arabia, but we haven’t been looking at the right places yet.”
Where are the right places? Petraglia says he has hunches. Well, as for the sites they found dating to 400,000, 300,000, 200,000 and 130,000 to 75,000 years ago, and then a site from 55,000 years ago, if they were occupied by different hominins, who were they?
“The two older sites, 400,000 and 300,000 years, are very clearly Acheulian sites, hand ax cultures,” Petraglia says. There are no fossils, but the tools’ similarities with contemporaneous African sites indicate that the residents were Homo heidelbergensis.
The Middle Paleolithic sites from 200,000 to 75,000 years were used by early Homo sapiens – again going by the tools, the team suggests. And then there is the site from 55,000 years.
Again, no bones have been found. But going by the tools, the team thinks the site was occupied by Neanderthals coming not from Africa but from the north.
Israel was occupied by Homo sapiens from at least 200,000 years ago until 100,000 years ago; then they disappeared, and from 70,000 to 50,000 years ago, Israel was thronged by Neanderthals. The tools found at the 55,000-year-old Saudi site are typical of the Levantine Neanderthals, Petraglia notes.
If Neanderthals really did penetrate that far south, Arabia and Israel could both be part of a vast area where sapiens and Neanderthals met and interbred, which we know definitely did happen – certainly Neanderthals were known to have persisted in Iraq and Iran until at least 45,000 years ago. “It could have happened all over the place,” Petraglia adds.
And there we have it. When the climate was clement, wandering hominins and humans arrived and took advantage of the hospitable land. When they left Africa, not all took the Levantine corridor to Eurasia. Some evidently turned right into the lovely land that was Arabia. As long as they had water and animals to eat, that is.
Given that the region is experiencing accelerated aridification due to human-caused climate change, and that this time the desertification process may be irreversible, the Arabian deserts may never experience lakes and animal migrations again.