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The earliest human traces

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The Origins of Humanity · PREHISTORY

Determining the exact age of remains several hundred thousand years old poses a particular scientific challenge: no written record allows these traces to be dated directly. Archaeologists therefore rely on indirect physical and geological methods, applied to sediments and to the objects themselves, to place sites such as Lézignan-la-Cèbe or Pont-de-Lavaud in time, among the oldest known deposits in France.

🔬 How to date remains a million years old

Paleomagnetism exploits a property of ferromagnetic minerals: as a rock cools, they record the direction of Earth’s magnetic field at the moment of their formation. This field periodically reverses over geological time — the magnetic north pole flips to the south, and vice versa. The most useful boundary for early prehistory is the one separating the Matuyama and Brunhes periods, which occurred around 780,000 years ago: remains found beneath a layer of reversed polarity are therefore necessarily older than that date.

Electron spin resonance (ESR) complements this approach over a wider span, from 20,000 years to one million years. This method relies on the fact that certain crystals — tooth enamel, quartz, calcite, stalagmites — progressively trap electrons under the effect of natural radioactive radiation present in their environment. This accumulation, reset to zero by heat or exposure to light at the moment of deposit, acts as a clock that researchers can later “read” in the laboratory. This is the method that notably made it possible to date Pont-de-Lavaud.

🏛️ Lézignan-la-Cèbe, a site excavated since 2008

The site of Bois-de-Riquet, in the commune of Lézignan-la-Cèbe (Hérault), has been excavated every year since its discovery in 2008, under the direction of Laurence Bourguignon, a researcher at INRAP and a native of the commune, assisted notably by L. De Weyer. This continuity, rare for such an ancient site, has made it possible to refine the understanding of the deposit year after year.

Successive campaigns have confirmed the site’s exceptional richness: the twenty-three tools in quartzite and basalt found in the same layer as more than four hundred faunal remains make it one of the rare European deposits where lithic industry and datable fauna coexist so clearly for such an early period, comparable to the sites of Pirro Nord (Italy) and Kozarnika (Bulgaria). Its dating combines several complementary approaches: paleomagnetism, biochronology — based on comparison with lagomorph fossils already known from geological layers of equivalent age in Spain — and radiometric dating of an overlying basalt flow, which bounds the site’s maximum age at 1.57 million years.

🏞️ Pont-de-Lavaud, a complex stratigraphy

The site of Pont-de-Lavaud, in the commune of Éguzon-Chantôme (Indre), was spotted in 1982 during surveys conducted as part of the P.13 research program, after being exposed by road works. A detailed survey of the visible section was carried out as early as 1983, before a limited excavation, taken down to the bedrock, was undertaken in 1984 under the direction of Denis Vialou, with Águeda Vilhena Vialou as co-director.

This excavation revealed an unusually complex stratigraphy for a site of this age: eight distinct layers, spread across three sedimentological sequences, bearing witness to several successive phases of deposition. It is in this complex fluvial context, dated by electron spin resonance to around 1.1 million years, that the industry on quartz pebbles, based on the bipolar-on-anvil technique, could be uncovered and precisely placed within its geological context.

🧠 Key points

  • Without written records, archaeologists date very old sites using physical methods: paleomagnetism (reversals of Earth’s field, Matuyama-Brunhes boundary at 780,000 years) and electron spin resonance (20,000 years to 1 million years)
  • The site of Lézignan-la-Cèbe (Bois-de-Riquet) has been excavated without interruption since 2008 under the direction of Laurence Bourguignon (INRAP)
  • Pont-de-Lavaud, discovered in 1982 and excavated by Denis and Águeda Vilhena Vialou, presents a stratigraphy of eight layers across three sequences
  • These indirect methods make it possible to date remains more than a million years old with enough precision to reconstruct the history of settlement