The Stone Age was also the Wood Age. But, until now, there has not been so much talk about that. While the lithic tools and shells used by prehistoric humans have stood the test of time, the same cannot be said for objects made of organic matter, which require very specific conditions to avoid decomposition. On Wednesday, September 20, an international team announced the discovery of the oldest wooden structure ever unearthed in Nature. And it's 476,000 years old, an age when modern humans had not yet appeared.
The find took place in northern Zambia, not far from the Kalambo River Falls. "The site lies on the banks of the river, which floods regularly, bringing with it sediment. It is thanks to this permanent humidity that the wood was preserved," explained Veerle Rots, Professor of Prehistory at the University of Liège (Belgium) and co-author of the study. Excavated as early as the 1950s by British archaeologist John Desmond Clark (1916-2002), the site "had already yielded some interesting remains, but we didn't know how to date them. The chronological framework remained vague," added Rots.
In 2019, a collaboration between the universities of Liverpool, Aberystwyth (UK) and Liège revived excavations at Kalambo Falls. In the waterlogged sands, the team discovered several wooden objects, including an astonishing cruciform assemblage. Two superimposed logs were embedded by a U-shaped notch, over 10 centimeters wide, "clearly man-made," in Rots' words, who based her assertion on a series of marks left in the wood by stone tools.
A tricky challenge remained: determining the period in which this structure had been produced. Carbon-14 dating was not useful, as it does not go back further than 50,000 years. To get their answer, the researchers used a technique that dates not the object itself, but the sediments in which it is buried: luminescence dating.
It exploits the ability of certain natural crystals, such as quartz and feldspar, to act as dosimeters, thanks to small structural defects that act as electron traps. "Under the effect of natural radioactivity, these crystals accumulate energy, which they release when heated or illuminated in the laboratory," explained Christelle Lahaye, Professor of Geochronology at Bordeaux-Montaigne University and Director of the Archéosciences laboratory, which specializes in the study of archaeological heritage materials.
Whenever sunlight strikes them, these grains of quartz or feldspar see their "internal clock" reset to zero. It restarts and accumulates energy when it's in the dark – in other words, when the sediments are buried. The amount of energy stored is proportional to the time spent in the dark. "What we're going to measure with this technique," Lahaye summed up, "is the time that has elapsed since these crystals were last exposed to light."
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