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London

WHEN Eileen and Derrick Severs disappeared suddenly from their home in
central England, the police suspected murder. But they had no bodies—and
no case against the chief suspect, the couple’s son Roger. After a massive and
fruitless search, police faced the daunting prospect of dredging Rutland Water,
the largest man-made lake in Western Europe. First though, they called in the
botanist.

A botanist? You’d better believe it. Detectives can get crucial leads from
plants and their fragments. Sawdust in a safe-cracker’s turn-ups, roots growing
through a grave and pollen trapped in a stash of cannabis—all can link a
criminal with the scene of the crime, show when a body was buried or link two
batches of drugs to the same source. Pollen might even have laid to rest the
lingering doubts over who killed O. J. Simpson’s ex-wife.

Outside New Zealand, where forensic botany was pioneered, police rarely call
on the expertise of plant scientists. But word is spreading fast. The UN is
relying heavily on botanical evidence in its Yugoslavian war crimes tribunal.
Such high-profile, international exposure might be the break botany needs to
boost it into the forensic mainstream.

Why do plants leave behind such great clues? Pollen is virtually everywhere
in the summer air, to the lasting misery of hay fever sufferers. Each type of
plant has pollen with its own distinctive shape, and every place has a
characteristic “pollen fingerprint” created by its local vegetation. Plants also
turn up in the mud on our shoes, in the food we eat—and in the stuff we
use to make buildings. All this makes plants the perfect “supergrass”.

So when Tony Brown, an Exeter University botanist, analysed the mud from the
Severs’s car, he found green leaf fragments, probably from a recent off-road
foray. The mud was rich in both grass and tree pollen, which suggested a
relatively undisturbed topsoil near a woodland edge. And the biggest clue? Two
per cent of the pollen was from a horse chestnut tree, relatively uncommon and
stingy with its pollen because insects do the job of spreading it about. So
wherever the car had been, the trees must have stood very close by. Together
with the soil type, that focused the search down on just five candidate woods.
Within two days, police found the bodies in a shallow grave in the wood nearest
the Severs’s bungalow. The bodies yielded enough non-botanical evidence to
convict Roger Severs of murdering his parents.

Plants can also help solve crimes when the trail has gone cold, because they
contain lots of long-lived cellulose. Growth rings on roots growing through a
body or the foliage above a grave can tell you roughly when someone died even if
only their skeleton remains. Pollen evidence, too, can easily give investigators
a lead into cases years or even decades old.

In 1997, for example, police in Western Australia were hunting a serial
rapist who’d been preying on women for several years. They had DNA samples, a
description, and a pair of shoes left at one of the crime scenes. They had a
hunch the rapist was an itinerant worker, but no idea where to look for him.
Would they have to wait until he struck again?

Later that year Lynne Milne, a pollen researcher at the University of Western
Australia, was giving a lecture to Western Australia Police on the potential of
pollen evidence in forensics. “After the lecture, one of the guys in the
audience came up to me and said, ‘Could you get pollen from shoes that have been
in an exhibit bag for three years?'” she recalls. The officer hoped the pollen
might tell him where the rapist lived or worked. Milne agreed to give it a
try.

She found a mixture of pollen types typical of the south-western part of the
state. And remnants of hay stubble implied that the suspect had walked over a
recently cut field. As it happens, the south-western town of Northam is the
centre of Western Australia’s chaff-cutting industry—and a major employer
of itinerant farm workers. The police tracked the suspect down in a matter of
weeks. He confessed and was convicted in 1998.

Then there’s the UN’s Yugoslavian war crimes tribunal—the massive
investigation into the atrocities in Bosnia and Croatia during the war. Some
17,000 people are missing in north-east Bosnia alone. The perpetrators tried to
hide the scale of the killings by moving bodies from large mass graves to
numerous smaller sites. But Brown has pinpointed the original burial sites from
the minerals and pollen in soil on the bones at the new sites. “I’ve analysed
more samples from Bosnia than the whole of the rest of my forensic career,” says
Brown. His evidence points towards several mass executions rather than numerous
dispersed small-scale killings.

Pollen is just the start, though. Jane Bock and David Norris at the
University of Colorado, Boulder, have pioneered the analysis of semi-digested
plant remains. Their first case was the murder of a young woman in 1982. She was
staying at her uncle’s home in Denver, Colorado, while working at a local radio
station. When she failed to return home from work one night, the alarm was
raised and her body was found in a field near a road. She had been raped and
stabbed over 40 times.

The victim’s last known meal was a takeaway burger, but the police wanted to
know whether she had eaten again before her death. The contents of her final
meal were not obvious from a gross examination of her stomach contents, so an
imaginative investigator asked Bock to have a look. At first, says Bock, the
idea made her feel quite queasy. “But then I thought, what if this had been one
of my students?”

Without reference collections to work from, Bock and Norris had to compare
the sample against slides of food they’d chewed themselves. They found that they
could recognise groups of cells by the architecture of their resilient cell wall
and the way the cells fitted together. The woman’s stomach contained red
cabbage, kidney beans, onions, tomatoes and red peppers: very different from a
burger menu. Was her last meal a salad, perhaps eaten with her killer? The
police spoke to staff at nearby restaurants and got lucky. One remembered
serving the woman and gave a description of her male companion. He was tracked
down and found guilty of her murder.

But botanical evidence can do more than guide a search—it can also
place a suspect at the scene of the crime if the pollen profile on the suspect’s
clothing matches the scene. Of course, that doesn’t prove they committed the
crime. “The best defence against environmental evidence is to say that you
visited the crime scene on a previous occasion,” says Brown. But even then
pollen can sometimes help.

Early on New Year’s Day 1997, a woman was raped in a churchyard in Auckland,
New Zealand. The man she identified as the rapist, Robert Alexander Low, didn’t
deny being there but claimed he’d tried to help the woman after her attacker had
left. The defence suggested that in her frightened state, she must have confused
Low’s face with that of the real rapist.

The police turned to forensic pollen expert Dallas Mildenhall of the
Institute of Geological and Nuclear Sciences in Lower Hutt. He took samples at
the scene and from Low’s clothing. The woman fought with her attacker underneath
Powis Castle bushes (Artemisia arborescens) lining the alley. Parts of
the bushes broke off in the struggle. The rapist would have the pollen all over
his clothes, even if he’d picked off the twigs and leaves—and the bush
isn’t common in New Zealand. Sure enough, 39 per cent of the pollen in the dirt
on his jeans was Artemisia. The only explanation was prolonged contact
with the bushes. “These percentages can’t get onto clothing randomly or by
accidental brushing against the plants,” says Mildenhall, whose evidence was
instrumental in Low’s conviction.

But where botanical evidence is used in court to convict a suspect as in this
case, not just to guide a police investigation, we have a right to ask how
robust the science behind it really is. With pollen analysis, for example, what
are the odds that the mixture of pollen in the mud on a suspect’s shoes could
have come from somewhere other than the crime scene?

No one knows yet, but it should be possible to work out how precise pollen
evidence is. Brown is testing out a statistical technique called the “likelihood
ratio” which compares the fit between the pollen sample and the crime scene
against other sites. Ideally, you’d have a database of pollen profiles from
around the world to make it work—but there isn’t one. Limiting your area
of enquiry is the next best option. Where does the suspect claim to have been?
If they could have visited many places with a similar pollen profile, then
pollen is not going to be much help. However, if the pollen profile where the
body was found is unique in a hundred-kilometre radius, you can be pretty sure
your suspect was there.

Still, the true test of forensic botany’s rigour can only come in court. Even
in New Zealand—the only country to use plant evidence widely—the
science behind pollen analysis has never been challenged in court. The same is
true in the US, although it has rarely been used there. But there are signs that
the US justice system is becoming more sceptical of scientific evidence in
general.

In 1993, the Supreme Court laid down more stringent criteria for a legally
acceptable scientific technique. Crucially, it requires a forensic discipline to
demonstrate its effectiveness and quantify its error rate in specifically
designed scientific tests, not merely legal precedents. Before that ruling, a
science simply had to show that it was widely accepted. “It is no longer a
question of a popularity contest, a show of hands at a scientific conference,”
says Edward Imwinkelried, an expert on science and the law at the University of
California, Davis.

Since then, lawyers have successfully challenged the scientific basis of
document examination, bite marks and, most recently, fingerprinting. In the
fingerprint case the judge, Louis Pollack, let prosecutors present similarities
between the fingerprint of the suspect and a print found at the scene. But he
did not allow the expert to give an opinion on whether the two matched, because
no one has rigorously measured the error rate of such comparisons.

To be credible in court, forensic botany is going to have to prove
scientifically that it can do what it claims. Many questions still need to be
answered. For example, how long does clothing hold different pollen types? Do
loose and dense weaves collect pollen differently? How long do different pollen
grains persist in the environment? How unique are pollen fingerprints to
specific regions? With so few botanists doing legal work, these answers could be
a long time in coming.

Some basic research is continuing, though. Ed Golenberg at Wayne State
University in Michigan is working on techniques for extracting DNA from pollen
grains. Tricky stuff, because the samples are so small and the DNA tends to
break down within days. And scientists know comparatively little about the
genetic variability of plants. Still, if detectives could trace pollen to
individual plants, they could place a suspect at the scene more confidently and
accurately.

Promising as it is, botany frequently gets overlooked in forensic
investigations. “So often, law enforcement people are totally unaware of how it
can be used,” says Norris. Sometimes they will call in the botanist only when
all else has failed. By then, the evidence is often too contaminated to say
anything useful. Also, police are rarely trained in how to take samples. Sterile
implements are a must to avoid contaminating samples with other pollen.

Botanical evidence might even have helped resolve some of the most
controversial crimes of the day. Take the case of the murder of O. J. Simpson’s
ex-wife Nicole Brown and her friend Ron Goldman in 1994. Although Simpson was
acquitted of the crime, he has faced lingering suspicion that he really did it.
But whoever killed Brown and Goldman hid in ambush under a flowering willow tree
and would have been showered with huge amounts of pollen from the tree, says
Milne. A pollen sample from Simpson’s hair and clothes might have added to the
evidence against him—or banished the suspicion for good. But no one
thought to look for it.