DNA traces in soil link a suspect to a crime scene

By Jennifer Young

During my PhD at the Australian Centre for Ancient DNA (ACAD) I developed a new soil DNA technique for use in real-life forensic casework. The study, now published in Forensic Science International, has shown that DNA identification of the fungi, plants, and bugs in small samples of soil can link a person to a particular location.

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Like a fingerprint, the life invisible to the naked eye reveals the original location of soil.

The project, co-funded by the Australian Research Council and the Australian Federal Police, created a mock crime scene to mimic the disappearance and recovery of a woman’s remains. A shovel was used to dig a shallow grave before being placed into a car boot alongside shoes worn at the time.

Six weeks later, the DNA of the fungus, plants, and bugs living in the soil stuck to the shoes and shovel was recovered, and compared to DNA detected in soils from multiple other locations across South Australia. The unique signature of fungus, plants, and bugs placed the soil samples recovered from the shoes and shovel just meters from the crime scene. This study is one of the first to demonstrate that new genomic methods can be used in real-life applications to track criminals weeks later and accurately place them at crime scenes. Our murderer has a lot more explaining to do!

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Quarter of a gram is all that’s needed. Credit: The Garden Geeks

Using soil is not new to police investigations. Soils can be examined using a number of different physical and chemical analyses (e.g. colour, particle size, and mineral content). However, geographic precision of these techniques is often limited. Our result showed soil DNA sequencing could precisely distinguish between physical locations where current methods offer limited resolution. Furthermore, the new method can deal with practical issues, such as seasonal variation of rainfall and temperature, that could otherwise impact forensic case work. Our work is the first step towards validation of powerful, new soil DNA sequencing techniques for real-world forensic case work. Continuing this preparation, we will assess further crime scene scenarios and develop a reference database describing soils around the state. A comprehensive database and track record will be crucial for integrating this technique into routine casework.

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