Humans have been eating hazelnuts for at least 6,000 years

Humans’ early ancestors in Europe may not have spent their days eating Nutella on toasted bread, but hazelnuts were a valuable resource thousands of years ago. The way this vital source of energy was cultivated and harvested evolved as the landscape changed as giant glaciers retreated. Isotope analysis of the carbon in archaeological traces of hazelnuts in southern Sweden show that the nuts were harvested in progressively more open environments, according to a study published February 29 in the journal Frontiers in Environmental Archaeology. The findings paint a more detailed picture of what the landscape looked like as hunter-gathering gave way to farming. 

[Related: Neanderthals and modern humans intermingled in Europe 45,000 years ago.]

A shifting forest landscape

Around 14,000 BCE slowly melting glaciers allowed for more vegetation to grow and created open woodlands with pine and birch trees in the area for the first time. By the Mesolithic era (about 8,000 BCE) hazel trees started to become one of the dominant woodland species throughout the southern part of Sweden. Along with pine trees, the hazel forests formed a unique woodland that does not have any known comparison today, according to the study. More broadleaved trees such as oak and linden began to fill in, but hazel remained important as farming began in the Neolithic era around 4,000 BCE. 

“Farming started in southern Sweden and marked a transition to more open areas with grasslands,” Karl Ljung, a study co-author paleoecologist at Lund University in Sweden, tells PopSci. “Hazel continued to be an important species in this progressively more open landscape and was likely favored by people.”

The hazel trees provided a source for both raw materials and food, similar to seaweed. The nuts are a good source of protein and energy and have a long shelf life. Hazelnut shells can also be used as fuel in fires.

‘Plants act as time capsules’

Hazel trees and all plants contain carbon, which exists on Earth in various forms known as isotopes. Conducting stable isotope analysis of what isotopes are present at archaeological sites can give scientists valuable data on long gone environments.

“Plants act as time capsules of the environmental conditions that they experience when they grow,” Amy Styring, a study co-author and archaeological chemist at the University of Oxford in England, tells PopSci. “When we recover the remains of plants on archaeological sites, the chemistry of these plant remains can tell us about the water availability, soil fertility, and light intensity at the site where the plant grew. Given that hazelnuts are so frequently found on archaeological sites, we thought they were the perfect candidate to test whether they record environmental information in their chemistry.”

The proportions of different carbon isotopes is changed by the ratio of how much carbon dioxide is concentrated between leaf cells and their surrounding environment. For hazel and other plants, the ratio is affected by the amount of sunlight and water available to them. Regions near the poles like Sweden see nearly 24 hours of light during the summer months and almost no sunlight in the winter. This means that the sunlight affects the isotope ratio more than water, since water is not quite as scarce. 

“This means that a hazelnut shell recovered on an archaeological site provides a record of how open the environment was in which it was collected,” Ljung said in a statement. “This in turn tells us more about the habitats in which people were foraging.”

Digging into shell fragments

In the study, the team gathered hazelnuts from trees growing in various light levels at three locations in southern Sweden. They analyzed the variation in their carbon isotope values and the relationship between those values and how much light they were exposed to. 

[Related: Archery may have helped humans gain leverage over Neanderthals.]

Next, they looked at the carbon isotope values of hazelnut shells unearthed from archaeological sites in southern Sweden. The shell fragments came from four Mesolithic hunter-gatherer sites and 11 sites ranging from the Neolithic up to the Iron Age. Some of these sites had also been occupied during more than one period.

They combined the archeological and modern data and ran a model to assign the hazelnut samples to one of three categories based on where they grew–closed, open, and semi-open. 

They found that the nuts from the Mesolithic had been collected from more closed environments with more tree cover. 

“The biggest surprise was probably that light levels have such a strong effect on the carbon isotopes in hazelnut shells! Biology can be so noisy that the effect of a single factor is not always so clear,” says Styring.

By the Iron Age, most of the hazelnuts appear to have been gathered in an open area and not a woodland like the ones that existed as the glaciers retreated. Their microhabitats had entirely changed.

“Forests are dynamic places, shaped by the establishment of new species after the glacial period, diseases like the elm disease, that provided diverse environments for foraging,” say Ljung and Styring. “But people also modified the landscape, the most dramatic form being the clearing of trees to make way for fields of crops once farming became widespread.”

In future studies, the team would like to directly radiocarbon date and measure the carbon isotopes of hazelnut shells from other archeological sites and environments. These deeper looks could provide more detail into past woodlands and ecosystems and help us better understand how humans have shaped our environment over time.