A Vertical History Book
The Trévenez peat bog, located in the Yeun Elez marshlands of Brittany, is a detailed archive of environmental history. Peat bogs form in waterlogged, low-oxygen conditions where plants, particularly sphagnum moss, accumulate instead of fully decomposing. Over millennia, these layers of partially decayed organic matter build up, creating a vertical timeline of the surrounding ecosystem. Each layer entraps microscopic evidence from its time, most notably the durable pollen grains released by plants.
Scientists use a technique called palynology, the study of pollen and spores, to read this archive. By drilling a core deep into the bog, they can analyze the pollen composition at different depths. Deeper, older layers reveal the plant life of the distant past, while layers closer to the surface show more recent vegetation. The chemical properties of pollen grains make them incredibly resistant to decay, allowing them to be preserved for thousands of years. This process makes the Trévenez bog a record of vegetation shifts that have occurred since the last ice age.
The Neolithic Fingerprint
The pollen record from Trévenez and other bogs in Brittany shows the transition from Mesolithic hunter-gatherer societies to Neolithic farming communities. The evidence for the arrival of agriculture appears in southern Brittany's pollen records between 6,000 and 5,500 BCE. In northern Brittany, widespread deforestation and cultivation practices become visible in the pollen record from 4500 to 4000 BCE.
Before this transition, the pollen spectra are dominated by trees like oak, elm, and hazel, indicating dense forest cover. Then, the core samples show an abrupt change. There is a sharp decline in tree pollen and a corresponding explosion in the pollen of herbaceous plants. Specifically, scientists find the first appearance of cereal-type pollens, such as wheat and barley, which were not native to the region.
Alongside these new crop pollens, there is a surge in pollen from so-called ruderal species—weeds that thrive in disturbed ground. The appearance of species like plantain (Plantago lanceolata) and sorrel (Rumex) indicates human activity, signaling that forests were being cleared to create fields for agriculture and grazing land for livestock. This distinct shift in the microscopic pollen record marks the moment early farmers permanently altered the environment of western Europe.