It’s hair-raising news for criminals on the run. Scientists behind a new study, published in the journal PLOS ONE, have worked out how to identify individuals solely based on the proteins in a single strand of their hair.
While prosecutors already look at DNA from hair as part of investigations, the technique is far from ideal: DNA is easily degradable, meaning it can only be analysed within a certain time period after the crime. But the new technique could even be used to solve historical or archaeological cases, making it far superior to DNA sequencing in many ways.
DNA degradation depends on several environmental factors including temperature, humidity and pH. It is also affected by the activity of bacteria and other microorganisms. In contrast, actual hairs can survive for a long time – sometimes centuries. After bones and teeth, hair is in fact one of the most resistant structures of the human body. The physical flexibility and robustness of the hair structure is due to proteins with a high degree of intermolecular bonds. A single hair is associated with some 300 different proteins.
In order to understand how the protein can be used to identify individuals, it is important to understand that proteins are coded by DNA. This means that a certain level of the genetic variation that we see in different people’s DNA passes into their proteins. In fact, genetic information in the DNA is translated into amino-acid chains that make up proteins.
The authors focused their attention on hair samples obtained from four different groups of people. Three samples were collected from American-European, American-African and Kenyan living subjects. The fourth was collected from two British archaeological excavations, one in London and the second in Kent, dating from about 1750 to 1853 respectively. They also analysed hair samples from 76 living humans of American-European and African descent.
Hairs from different subjects were milled and then biochemically processed with a solution of urea, the major organic component of human urine; dithiothreitol, a detergent; and a substance called trypsin, which can cut chains of amino acids. This resulted in mixtures of peptides (short amino-acid sequences), which were then analysed using a liquid chromatography mass spectrometer, which separates compounds of particular masses so that we can identify them.
In this way, the researchers managed to pin down around a hundred protein markers that can help identify someone using a single hair. The most common proteins, both in the modern and archaeological samples, were structural compounds like keratins – the main component of hair – and related proteins. The accuracy of the technique is such that we can reliably identify an individual among 12,500 people in the European population (the value was considerably lower in the African population).
But this accuracy can be greatly improved by analysing more and more peptides and discovering further protein markers. The current accuracy can be compared to that at the beginning of the DNA typing era. But nowadays, DNA sequencing can reliably identify a person among 1013 unrelated individuals.
The method not only allows for human identification but it can also reveal how old the sample is and what region it comes from, so that we can distinguish between current and ancient samples. But while the discovery is exciting, the technique is not quite polished enough to be used in the court room.
The main task now will be to analyse hair samples from all over the world, which will make it a lot more reliable. It may one day be a great complement to DNA sequencing and may even be the first point of call in many criminal investigations. But, unlike DNA analysis, it won’t be able to distinguish between twins.
This is the latest in a number of studies showing how useful hair can be in forensic investigations. Studies are also showing that morphological and toxicological analyses of hair may provide useful information on the circumstances of death. Hair lesions may give indications of blunt and sharp force trauma, and may give away culprits who are hair fetishists.
The new discoveries will boost the significance of hair as evidence in courts, at a time when some forensic disciplines have been criticised in the US. It will also be of great use in archaeology.
In the landmark forensic science book A fly for the Prosecution (2000), Professor Lee Goff set out how tiny insect evidence can help to solve major crimes. Today’s discovery suggests the title for what could be another influential book: A Hair for the Prosecution.