L'Arbre du Ténéré and the Saharan Pump

There’s a strange scrap-metal sculpture deep in the southern part of the Sahara—the Ténéré Desert of Niger—with a base made of metal drums filled with concrete and covered with graffiti. A steel pole rises from it for about fifteen feet, and its top is a spiky accumulation of fake branches. It looks like the offspring of a street sign and a terminally ill tree, and the resemblance isn’t coincidental: it’s a memorial to the now-deceased Arbre du Ténéré, an umbrella thorn acacia tree that once stood there, the only tree for more than 200 kilometers. For at least a century it was a landmark of the approximate halfway point of the azalai, the salt-and-millet caravan between the towns of Agadez and Bilma.

That stretch of desert is as dry as any in the world. The water table is 35 meters below the surface, as discovered when the French government (which ruled Niger as part of French West Africa) dug a well for caravaners back in the 1930s. While doing so they discovered that the roots of the tree had delved that far as well, keeping it alive despite the dessicated conditions. That leads to the big question about L’Arbre du Ténéré: what did it do for water until it reached that level? The effort involved is clearly a sufficient barrier to prevent any other tree from growing in the region, which leads to the related question of how a seed even got several days journey from any possible parent let alone manage to take root.

That the Tree is there, and that it’s alone, are clues to the solution of a lot of ecological mysteries. Start with the fact that human beings emerged from Africa some 130,000 years ago and spread to the rest of the world. This was once a controversial statement since creatures very much like human beings—Homo erectus—left Africa and colonized Asia and Europe in a very similar fashion two million years ago. It was once thought (and some anthropologists still argue) that we evolved from those original settlers, but DNA studies in the last few decades now strongly suggest that we replaced them after repeating the cycle of expansion they first introduced.

Either explanation runs up against the same problem, though. There’s evidence that the Sahara Desert is at least seven million years old. How did primitive people get across it? They had no way of knowing that Asia and Europe were on the other side and were nice places to live. The idea that we might have evolved from H. erectus does at least have the advantage of only requiring this unlikely event to happen once. Now the new evidence says it happened at least twice. Something is going on.

Don’t focus entirely on Homo sapiens sapiens—granted, since likely many of you reading this are H. sap. as well, they’re particularly interesting. But dwelling too much on our own ancestors will keep us from realizing that there are numerous other species that have apparently developed south of the Sahara and made it north to the rest of the Old World. Lions are originally African, but spread north at about the same time as H. erectus; they were found in southern Europe and the Middle East until humans hunted them to extinction. Ostriches had a similar distribution, with the last known member of the northern species dying in Jordan in 1966. Oryx antelopes live on the southern and northern fringes of the Sahara, as well as Arabia. Old world cotton species are native to sub-Saharan Africa and, more familiarly, spread from the Indus River valley (in modern Pakistan) after the invention of cotton textiles.

So instead of one or two species of human making the trek across the Sahara, somehow all of these species made it. Some of them are plants, which are not noted for trekking. How did they do it?

The answer seems to be that they didn’t cross the desert at all. The Sahara is not always stone-dry, instead cycling back and forth between relatively hospitable savannah (note that the stereotypical tree under which savannah-dwelling lions lounge is an acacia) and the desert-like conditions seen today. Even now the desert is approaching a peak: it’s an open question whether or not Lake Chad will disappear entirely in the next few decades. But for four thousand years until about 3000 BC the Sahara was in a greener phase, the Neolithic Subpluvial, that fed the lake. Since then the grasslands have been falling back, but the Ténéré Desert is not too far from the modern-day southern edge of the desert. The water table was higher than it is now, and some time in the 19th century it would have been acceptable—just barely—to a particularly drought resistant acacia seedling. There would have been others like it relatively nearby, but even under the best of conditions acacias are widely spaced. As the others would have been older too, they would have succumbed to age or drought over time, eventually leaving the Tree of Ténéré to its isolation.

This current climate cycle is not the only one, or even the only one named. Before the Neolithic Subpluvial there was the Mousterian Pluvial. Prior to it was the Abbassia Pluvial, which began 120,000 years ago. In other words, modern human beings left Africa during a dry period between the Abbassia Pluvial and the wet period that was in turn before it. This is the Saharan Pump in action: cycle after cycle, the changing climate sucks in sub-Saharan species then expels them into Asia. When northern Africa has enough moisture, it’s colonized by species from the south. When the rains go away some return south, but others are forced north. All the indigenous societies of Europe and Asia (and via Asia, the Americas and Australia) came to be because refugees from the Sahara had to escape in one direction or another when the rains failed them, and some of them picked the way opposite to the one from which their distant ancestors had arrived.

In the end, L’Arbre du Ténéré was killed and had to be replaced by a monument because of the Saharan Pump. It just wasn’t from lack of water as one might expect, but by a cause more indirect. After the pump pushed Homo sapiens into the north all those millenia ago, those people eventually developed agriculture with the species they found there, then moved on to cities, writing, mathematics, and more. And in the north those people eventually developed industrial technology. A piece of this was the automobile, and the camels of the Agadez-to-Bilma azalaïs were replaced by trucks. In 1973 a driver ran his truck into the Tree, killing it. Its dead remains are now on display in a museum in Niamey, the capital of Niger.

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