Beautiful tool
Last year, Wiltshire used pollen to convict three drug dealers for murdering a drug runner. “It was a spectacular case,” she says. “One of the gang hid behind an oak tree within a cypress hedge. We showed that the palynological assemblage in samples from the crime scene — from leaf litter, soil, washings from the oak tree trunk and from foliage on the cypress hedge — was very similar to that on the clothes, shoes and vehicles retrieved from the suspects.
There were a number of rare plant pollens that were in the soils at the scene that matched some on the exhibits.” All three of the suspects were found guilty, thanks in large part to Wiltshire’s pollen analysis, and sentenced to life in prison. DNA, the current darling of forensics, is faster to process than a pollen assemblage, which must go through a painstaking chemical treatment to remove the material surrounding it and the cytoplasm and sex cells inside it and then be carefully analysed through a microscope to identify each individual species. Palynologists can determine a plant group — spruce trees, for example — using a standard light microscope at up to 1,500 times magnification.
More time-consuming and expensive is the higher-resolution scanning-electron- or transmission-electron-microscope analysis that’s sometimes needed to identify a plant species. So why bother with pollen? “The problem with a single hair is, how did it get there?” explains Dallas Mildenhall, a principal scientist at the forensics lab GNS Science in New Zealand. “Couldn’t it have blown in from the window? But when you’re dealing with a large number of pollen grains, the chances are highly likely that they were transferred at a precise time.”
“For a while, DNA was God,” adds Lynne Milne, Australia’s leading forensic palynologist at Curtin University of Technology. “It almost seemed like if there was no DNA, there was no evidence. But now people are looking at it differently.” One of pollen’s greatest advantages for forensics is its microscopic size. Criminals don’t realise they’ve taken it with them or left it behind. In Milne’s first case, in 1997, a man washed his clothes after murdering his estranged wife. Laundering the clothes didn’t succeed in eliminating the trace evidence on it. Milne still found an uncommon type of acacia-shrub pollen on his shirt, clearly linking him to the woods where he had dumped his wife’s body and to the car that had been used to get there.
The coldest cases
Pollen’s cell walls are very durable, so the grains can survive for millions of years. In one case in Wales, pollen from a walnut tree that had been cut down 80 years prior linked suspects to the scene of a crime where the tree had once stood. The pollen had persisted in the soil and made its way into the suspects’ vehicle. Pollen’s longevity makes it especially useful for investigating cold cases, like the mysterious identity of a teenage girl murdered in a cornfield in upstate New York in 1979. Twenty-seven years later, in 2006, palynologist Vaughn Bryant, the director of the palynology lab at Texas A&M University, analysed her clothing and found pollen that occurs only in the southwestern US, possibly around San Diego. This supported the local police’s belief, based on other evidence, that she was from the southwest, not the northeast. “Pollen is an incredible tool and resource,” says Livingston County sheriff John York, who is still searching for the girl’s identity. “We can always suspect where a person came from, but to have definitive proof is another thing.”
The US, with its huge diversity of flora and extensive pollen records, is ideal for forensic palynology. The field debuted there in the mid-1970s, when the Department of Agriculture first used pollen to ensure that beekeepers receiving domestic subsidies were actually making their honey in the US. But it’s only been used in a handful of criminal cases here since then. “Local law-enforcement agencies don’t know about it or don’t believe it’s useful,” Bryant says. “Or they don’t want to pay for it.”
