Scientists have discovered a single fingerprint that could help solve a 2,000-year-old mystery

Here’s what you’ll learn as you read this story:

• Researchers have found a fingerprint on Scandinavia’s oldest plank boat, used by sea raiders 2,000 years ago.

• A boat builder left the seal in caulking tar.

• Experts still hope that the fingerprints can reveal who the attackers were and why they attacked Als Island.

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The mystery of why a group of sea raiders attacked the Danish island of Als 2,000 years ago is one step closer to being solved. Researchers scoured Scandinavia’s oldest plank boat (probably used by the raiders) and discovered fingerprints pressed into the tar caulking material. Although this is an admittedly small piece of evidence, archaeologists still hope it may lead them closer to discovering where the invaders came from.

The invasion of the island of Als took place in the 4th century BC, when an armada came down. Although four boats were attacked, the islanders were able to defend their home, sinking one boat in the process (probably as a thank you offering for the victory). This boat – which was first discovered in the 1880s and excavated in the 1920s – is now known as the Hjortspring boat, and is the only example of a prehistoric plunk boat found in Scandinavia.

“Where these sea raiders might have come from, and why they attacked Aals Island, has long been a mystery,” Lund University archaeologist Mikael Fauvelle said in a statement.

With the Hjortspring boat on display at the National Museum of Denmark, the university research team still unexpectedly found parts of the boat that had not been chemically preserved, and studied them with modern equipment. “The boat was waterproofed with pine pitch, which was amazing,” Fauwell said. “This suggests the boat was built in an area with abundant pine forests.”

Previously, scholars thought that the invaders must have originated in modern-day Hamburg, Germany. But the researchers have published their findings in the journal Investigating the Lost Boat Fragments. PLoS OneNow say the clues point to the Baltic Sea region as the point of origin of the attack. The authors wrote that the use of pine pitch meant that the boats must have been built in areas with pine forests, such as the Baltic Sea region east of Rügen and Scania.

“If the boat came from the pine forest-rich coastal areas of the Baltic Sea, it means that the warriors who attacked Als Island chose to launch a sea attack hundreds of kilometers over the open sea,” Fauve said.

The researchers used all the tools to investigate the find, including carbon dating lime bast cordage, which confirmed its pre-Roman Iron Age dating. They also used X-ray tomography for high-resolution scans, created a 3D model of the fingerprint, and used chromatography and mass spectrometry to help see how the caulking tar was produced.

“We’re also hoping to be able to extract ancient DNA from the tar on the boat, which could give us more detailed information about the ancient people who used the boat,” Fauvel said.

Analysis of intact cordage pieces and cordage impressions on caulking material helped experts decipher the sewing and rope-making techniques originally used in construction, and partial human fingerprints provide a direct link to ancient mariners. “Together, these results shed new light on the methods and materials used to build Scandinavia’s first plank boats,” the authors wrote, “and raise new questions about our understanding of early maritime societies in northern Europe.”

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