Homo erectus is the first hominin known to have gone global, with fossils found from South Africa to Spain and Indonesia. The question is how it achieved that.
Erectus is believed to have evolved in Africa around 2 million years ago and to have reached Europe at least by 1.8 million years ago, going by fossils found in Dmanisi, Georgia. The general assumption is that they left Africa through the “Levantine corridor,” which includes Israel’s Negev, today one of the world’s driest deserts. But no one had demonstrated that such hominin migration was plausible from the perspective of the paleoclimate back then.
Now a new study by an international team with scientists from Israel, Spain and Iceland, published last month in Frontiers in Earth Science, has done just that. Analyzing the magnetic properties of the sediments left by a “fossil lake” strongly indicates that southern Israel had been much wetter 1.8 million years ago.
This combination sedimentologic, paleontologic and magneto-stratigraphic study was done on deposits of Kuntila Lake, a 30-meter-thick (nearly 100 foot) section of sediments in the Nahal Paran river basin.
The evidence, including shells, indicates that these sediments accumulated in a long-lasting, long-gone, large freshwater lake 1.8 million years ago, at the northern edge of the Sahara-Arabian desert belt.
One thing: A study published in 2018 identified stone tools in Shangchen, China, dating to 2.1 million years ago, implying an earlier exit from Africa than indicated by Dmanisi.
Asked how long Kuntila Lake existed and whether the Negev was wet in that time frame too, co-author Nicolas Waldmann – a geologist with the University of Haifa – explains that they aren’t sure, but believe it existed for several tens of thousands of years. “The wet Negev created a window of opportunity in which early humans could cross through this region,” he says.
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Separate research does indicate that the geological basin necessary for a lake to form already existed around 2 million years ago, he added.
So the new study doesn’t address 2 million or even 1.9 million years ago, but can show the lake was there 1.8 million years ago and the data has wide margins of error. All in all, it lends support to the Levantine corridor hypothesis for the Homo spread.
Ancient Ethiopian sediment, off Haifa
The inferred wetter Negev concurred with periods of expanding lakes in East Africa and clusters of short-lived expansions of the savanna throughout much of the Sahara.
Other studies showed that the Negev area once had lakes and marshes, but the sediments had not been dated, Waldmann explains. In fact, the first sign of the Negev’s wetter history was discovered in the 1970s, but the technology didn’t exist at the time to pinpoint the age of the sediments.
There is actually quite a debate in scientific circles as to whether the area of Kuntila was more lake or more marsh. Either way, the Negev would have had to be wetter than it is today to sustain either a lake of any depth or a marsh, Waldmann says. Let’s say lake – and the highlight of their paper is to date the lake: 1.8 million years, when Homo erectus was on the move.
Why was the Negev wetter? Orbital cycles, of which there are several. During the period in question, the researchers believe, seasonal rain from the tropics was much stronger and much broader. Call it more sustained rain – “even as much as 10 times more than today. The Negev was basically wet, or at least wetter. Not jungle,” he hastens to qualify.
Studies done in the deep Mediterranean Sea support the thesis, Waldmann adds. “When conditions were wetter in the Ethiopian highlands, more water flowed from the Nile into the Mediterranean,” he says. Sediments driven by the Nile brought the signal far into the Mediterranean: On the bottom of the sea off Haifa, 1,400 meters down, there is clear signal of sediments from when it rained really heavily in Ethiopia.
Another beauty of the Kuntila sediment study is its clarity. In more hospitable areas of the Mediterranean, the signal is noisy or difficult to interpret, Waldmann says.
We cannot say, at least at this point, how deep Lake Kuntila might have been. But Waldmann explains that, apparently during specific periods, it was deep enough to sustain bivalve shells.
Might it have sustained larger animals such as crocodiles? “We don’t know – we didn’t find any,” he replies. Importantly, though, they could deduce that Lae Kuntila consisted of running, not standing, water.
“The Sea of Galilee is running water. It is fed by one river and exits by another: a freshwater lake with water flowing through it,” he says. “Flowing water means a constant supply: it was raining, seasonally, in a way that was sustainable.”
When poles reverse
So how did the team date the Kuntila sediment layers? By paleomagnetostratigraphy: analysis of the magnetic signal in the different layers of rock.
Long story short is that Earth’s magnetic field is generated by the movement of iron in the planet’s core, scientists believe. Magnetic north as we know it actually moves quite a bit, oscillating at small angles around northern Canada. Right now, the magnetic north pole appears to be moving by about 55 kilometers a year toward Russia, according to the U.S. National Oceanic and Atmospheric Administration. And when the core gets frisky, or for whatever reason – we don’t actually know – the magnetic poles reverse.
All rocks on our planet are composed of minerals, and most have a certain degree of magnetism. When a rock is consolidated, these iron-bearing minerals act like nano-compasses indicating the Earth’s magnetic field during the time of their formation. In fact, millions and billions of years later, scientists can detect the direction in which those nano-compasses are pointing, and know if the magnetic north was in what we think of as north, or reversed and pointing to Antarctica.
By now, geologists have some idea when the magnetic poles switched. So if we take a given bed of sediment going back hundreds of thousands and millions of years, and check the direction of magnetism in the rocks in the different layers, and compare it with the known “table” of polar flips, we can have a ballpark idea of how old that sediment is. And thus they could deduce the age of the wet period at Kuntila: 1.8 million years ago. Ta da!
To be clear, this is not the same site as Kuntillet Ajrud, an enigmatic archaeological site in the Sinai Desert famed for the 3,000-year-old possible picture of Yahweh with a wife.
The bottom line of the effort to date the evidence of major climatic changes long, long ago is to validate the theory of hominin exit through the Levantine corridor: it was environmentally plausible.
“We had optimal conditions for hominins to be there, to drink the water and maybe catch a fish,” Waldmann says, adding that stone tools attesting to hominin presence were found in the area. Indeed – though the tools aren’t from that time, the Israel Antiquities Authority clarifies.
Waldmann and the paleomagnetostratographic team is now looking for the window of opportunity, possibly 2.1 to 2.5 million years ago in the Israeli part of the Levantine corridor.
It’s also possible there were later human exits through what is today Saudi Arabia, based on a human fingerbone found there dating to 85,000 years ago. However, that is a much, much later time than the Kuntila evidence and relates to the exit of Homo sapiens itself, almost 2 million years after erectus had started leaving Africa, going boldly where no known hominin had gone before. Why the ancient hominins trekked out of Africa in the first place is another question entirely.