The breakthrough sprang from the recognition that the geological strata contain preserved environmental DNA (eDNA) from prehistoric ecosystems, also known as fossil DNA or ancient environmental DNA (aeDNA). Environmental DNA comprises microscopic fragments of DNA from animals and plants that have been shed into soil, water and ice through faeces, hair, pollen or dead tissue.

Under the right conditions, these fragments can be preserved for a very long time, effectively turning the soil layers into genetic archives of the development of ecosystems over time. By analysing the DNA fragments, scientists can see what species used to live in a certain location. For example, if mammoth DNA is found in a soil sample, that is strong evidence that mammoths once lived there.

In their initial studies, from 2003, Willerslev’s team demonstrated that environmental DNA not only indicates that animals were present in an area but can also be used to identify which species. In Siberia, the researchers documented that mammoths, musk oxen, bison and steppe horses were present some 10,000 years ago.

Later, Willerslev’s laboratory found mammoth DNA in samples that were around 3,900 years old, supporting evidence that some mammoth populations survived much later than previously believed from mainland fossil records. Until then, the scientific consensus was that mammoths became extinct about 10,000 years ago.

These early discoveries had a profound impact on researchers’ ability to reconstruct earlier ecosystems. Instead of only working with traditional traces, such as bones and pollen, they could now include genetic fragments from the soil to gain access to a much wider section of prehistoric life.

This allowed for more holistic reconstructions of landscapes and communities of species, even in areas where traditional finds are sparse. Since then, analyses have become increasingly accurate and have been applied to samples from caves, the sea floor and the bottoms of lakes, which has expanded the scope significantly, both geographically and in terms of time.

Willerslev’s research has also greatly enhanced our knowledge of human migration. Analyses of ancient DNA have shown that the first humans reached North America earlier than previously assumed, probably via a route along the Pacific coast. Other studies have found that modern-day Inuit are not descendants of the first humans in the Arctic but from a later migration wave.

In Australia, Willerslev’s research has documented that indigenous people have lived on the continent for at least 50,000 years. These findings have given indigenous peoples greater weight in the debate about their historical presence and have changed key ideas about the global dispersion of humanity.

Another remarkable result was announced in 2022, when Willerslev and his colleagues extracted environmental DNA from two-million-year-old sediment in northern Greenland. They found that the area was once forested and home to wildlife, including mastodons. This finding once again moved the bar for how old DNA can be and provided new knowledge about the earth’s climate during a warmer past.

Today, eDNA is used not just to understand the past but also to monitor nature today. The method is applied in surveying endangered species, following the spread of invasive species and monitoring ecosystems in near real time. This has given researchers and authorities a precise and efficient tool for nature management.

Environmental DNA is also the focus of Willerslev’s latest mission-driven research initiative, the Ancient Environmental Genomics Initiative for Sustainability, which focuses on genetic climate adaptation and interactions in ecosystems. The goal is to improve future food security in a world affected by climate change.

The chapter is written by Kristian Sjøgren.