Forensic science techniques are of incredible benefit in criminal investigations. But while they can help reveal who a piece of evidence came from or how it got there, there is a major capability gap within the forensic sciences – and it’s all to do with time. When did a person die or when was a stain deposited? Answers to such questions are crucial, in particular for defence lawyers trying to establish alibis.
Luckily, research is constantly throwing up new results. Over the last few years, a series of new findings have made great contributions to the area of “temporal forensics”, some of which could vastly improve our understanding of what happens to our bodies after we die.
An unofficial law of forensic investigations is to “always consider at least one alternative hypothesis”. This is when the time aspect comes in to play. If a blood stain belonging to a suspect is found at a murder scene, this appears to be very strong evidence against them. However, if he or she states that they were at the crime scene previously for legitimate reasons and had a nose bleed, then this would effectively negate the evidence.
A key part of such a murder investigation is finding out exactly when the victim died. Unfortunately, “post mortem interval”, otherwise known as time of death estimation, is a field that can be highly subjective. It is currently estimated using traditional techniques, such as measuring body temperature and monitoring rigor mortis (body stiffness). For short intervals, such as hours, these can be reliable, but it gets harder with time. That’s because both internal factors within the body (its size and the presence of medication) and the external environment (hot, cold, wet) affect how the body decomposes.
Recent research has shown that some genes become more active following death by producing RNA transcripts (small genes that carry information from the DNA to make proteins). Scientists think this change in RNA production could be a cellular repair mechanism in response to a drop in oxygen in the blood (hypoxia) following death – declining with time as decomposition kicks in. If researchers can show that this follows a predictable pattern, then it would be of enormous benefit for time of death estimation. In theory, once a body has been found, tissue samples could be taken and analysed to measure the amount of specific RNA genes.
Proteins in muscles
Proteins are another fascinating area of research when it comes to post mortem determination. We already know that proteins break down following death – this is how meat rots. But what was only recently revealed was that such proteins break down in a predictable fashion, some faster than others. For example, one completely breaks down within eight hours and one breaks down within 20 hours. Therefore, if the second protein was detected in a tissue but not the first, then the post mortem interval would be between eight and 20 hours. The research has so far only been done on pigs and will need to be conducted on humans before it can be reliably applied in forensic casework.
Stain age prediction
Research on how to predict the age of a stain is scarce but has great potential impact – in particular when it comes to rape cases between previously consenting partners. So, while the allegation is one of rape at a particular time, the suspect could argue that any physical evidence obtained, such as semen, comes from the earlier consensual intercourse. Currently, investigators use “semen persistency data”, which show how long sperm can remain in different parts of the body, to tackle such issues. For example, you would not expect to find a large amount of semen in the vagina after 28 hours. Therefore, this can be used to say whether or not the incident was recent or a few days ago.
However, this can only be applied in certain situations and using intimate swabs, so it cannot be applied to semen left in condoms, bedding or clothing. Researchers are working on ways to quantify the amount of genetic material such as RNA molecules within a body fluid stain. A study monitoring messenger RNA and ribosomal RNA over a period of 150 days demonstrated that they do indeed change reliably over time. When we have identified enough such markers, of which some will degrade faster than others, we should be able to estimate how old a stain is.
Another key area of temporal forensics is age prediction. This is where the age of the deceased or a suspect is predicted from tissue or bloodstains left behind at the crime scene, respectively. In addition, such techniques can be used to verify claims made by individuals as to their ages – for example for child asylum seekers and child refugees. This can be a lot more difficult than it sounds as everyone grows, matures and ages at different rates through genetic and environmental influences. Although there are a few different techniques that can be used to predict the ages of individuals, these become very limited once the individual has reached maturity. For example, forensic odontology can be useful for estimating the age of children.
One successful research area is based on “DNA methylation”, a mechanism used by cells to control gene expression – whether (and when) a gene is turned on or off. DNA methylation is environmentally influenced and involves compounds called “methyl groups” being added to the DNA as somebody ages. Researchers have shown that the presence of such compounds can be used to estimate the age of an individual, give or take 3.8 years. This may seem very wide, but it represents a vast improvement in our capability to predict age and there is still a great body of work to come.
Determining how long a wound has been around for – whether the person is alive or dead – is also difficult for forensic investigators. In abuse cases, for example, it is often unclear whether injuries were caused over a sustained period of time or by a one-off incident. Wound age prediction could also be used to estimate the sequence of multiple injuries, which is especially important in a murder case. While such techniques are of limited use in forensic investigations at the time, there’s some promising research being done on wound healing – such as how bruising progresses as it heals – which may soon make them much better.
Hopefully, these latest advances in science will make it far more difficult to get away with murder in just a few years’ time.