She would later testify that a couple of weeks afterwards, on 4 August 1994, Schmidt had come to her home while she was asleep. Schmidt told her he was there to give her a shot of vitamin B12. He’d previously given her vitamin injections to boost her energy levels, but that night she told him she didn’t want one. Before she could stop him, he’d stuck a needle in her arm. None of the previous injections had hurt, but this time the pain spread right through the limb. At which point, Schmidt said he had to leave to go to the hospital.
The pain continued overnight, and in the weeks that followed, she became ill with flu-like symptoms. She made several trips to the hospital, but test after test came back negative. One doctor had suspected hiv, but didn’t test for it. He later said that his colleague – one Dr Schmidt – had told him that Trahan had already tested negative for the infection. Her illness continued, and eventually another doctor ordered a new set of tests. In January 1995, Trahan finally received the correct diagnosis: she was hiv positive.
Back in August, Trahan had told a colleague she’d suspected that the ‘shot in the dark’ wasn’t B12. There was no doubt that hiv was a recent infection: she’d given blood several times and her most recent donation – made in April 1994 – had tested negative for hiv. According to a local hiv specialist, the progression of her symptoms was consistent with an early August date of infection. When police searched Schmidt’s offices, they found evidence that blood had been drawn from an hiv patient on 4 August – just hours before he’d allegedly injected Trahan – and the procedure hadn’t been recorded in the usual way. However, Schmidt denied visiting her and giving her the injection.[1]
Perhaps the virus itself could provide a clue about what had happened? At the time, it was already common to use DNA testing to match suspects to crime scenes. However, the task was trickier in this case. Viruses like hiv evolve relatively quickly, so the virus found in Trahan’s blood wouldn’t necessarily be the same as the one in the blood that infected her. Faced with a charge of attempted second-degree murder, Schmidt argued that the hiv virus that infected Trahan was too different to the original patient’s virus; it just wasn’t plausible that this had been the source of her infection. Given all the other evidence pointing to Schmidt, the prosecution disagreed. They just needed a way to show it.
On 20 june 1837,the british crown passed down the royal family tree, from William IV to Victoria. Meanwhile, a short walk away in Soho, a young biologist was also thinking about family trees, albeit on a much grander scale. Back in England after his five-year voyage on HMS Beagle, Charles Darwin would end up outlining his theories in a new leather-bound notebook. To help clarify his thinking, he sketched out a simplified diagram of a ‘tree of life’. The idea was that the branches indicated the evolutionary relationships between different species. Just like a family tree, Darwin suggested that closely related organisms would be closer to each other, while distinct species would be much further away. Tracing each of the branches would lead to a shared root: a single common ancestor.
Darwin’s original tree of life sketch. Species A is a distant relative of B, C, and D, which are more closely related. In the diagram, all the species evolved from a single starting point, labelled (1)
Darwin started by drawing evolutionary trees based on things like physical traits. On his Beagle voyage, he categorised bird species by features such as beak shape, tail length, and plumage.[2] This field of research would eventually become known as ‘phylogenetics’, after the Ancient Greek words for ‘species’ (phylo) and ‘origin’ (genesis).
Although early evolutionary analysis focused on the appearance of different species, the rise of genetic sequencing has made it possible to compare organisms in much more detail. If we have two genomes, we can see how related they are based on the amount of overlap in the lists of letters that make up their sequences. The more overlap there is, the fewer mutations are required to get from one sequence to the other. It’s a bit like waiting for tiles to appear in a game of Scrabble. Going from a sequence ‘AACG’ to ‘AACC’, for example, is easier than getting from ‘AACG’ to ‘TTGG’. And like Scrabble, we can estimate how long the evolutionary process has been running based on how much the letters have changed from their original sequence.
Using this idea – and plenty of computational power – it’s possible to arrange sequences into a phylogenetic tree, tracing out their historical evolution. We can also estimate when important evolutionary changes may have happened. This is useful if we want to know how an infection may have spread. For example, after sars sparked a major outbreak in 2003, scientists identified the virus in palm civets, a small mongoose-like animal. Maybe the disease had been routinely circulating in civets before spilling over into the human population?
Analysis of different sars viruses suggested otherwise. Human and civet viruses were closely related, indicating that both were relatively new hosts for the virus. sars had potentially jumped from civets into humans a few months before the outbreak started. In contrast, the virus had been circulating in bats for much longer, making its way into civets sometime around 1998. Based on the evolutionary history of the different viruses, civets were probably just a brief stepping stone for sars as it made its way into humans.[3]
During Richard Schmidt’s trial, the prosecution used similar phylogenetic evidence to show that it was plausible that Trahan’s infection had come from the hiv patient who’d visited Schmidt. Evolutionary biologist David Hillis and his colleagues compared the viruses isolated from the pair with other viruses found in hiv patients in Lafayette. In his testimony, Hillis said the
