How Chilean arsenic eaters vindicated a classic work of crime fiction
A little while ago I wrote about the in Dorothy L Sayers’ 1930 novel . The premise of the murder mystery is that two people sit down to eat an arsenic-laced dinner but only one of the pair dies.
I argued that, according to 1930s scientific understanding, Sayers was completely right. But modern scientific theories of arsenic poisoning would have meant that either both or neither died from their arsenic exposure in the tainted meal.
I was wrong.
When Sayers was writing Strong Poison, it was thought that developing an arsenic tolerance was entirely achievable by eating small mounts of the poison over a period of weeks or months. One of the diners would then be able to eat a normally lethal dose of arsenic without cause for concern.
was something of a fashion in 19th century Europe as it gave glossy hair and flawless complexions. There would still have been damage to health, but this may have taken months or years to appear. Some of these arsenic eaters, wheeled out in front of audiences like fairground attractions, would eat quantities of arsenic normally considered fatal with apparent immunity.
But more recent science has dismissed the idea of building up a tolerance to arsenic. by the arsenic eaters, or the samples of arsenic taken were impure. If this was the case the Strong Poison plot would not have worked. It has also been suggested that these arsenic-consuming individuals were in fact eating large lumps of arsenic that would not have been absorbed completely into the body and excreted largely unchanged. This would not have helped Sayers’ poisoner either, as large gritty lumps of arsenic in a meal would have been noticed and the victim unlikely to eat it.
But science moves on. New observations are made, more data are collected, known facts are brought into a new light and better theories are developed. Now it seems that scientists have found evidence for a way that Sayers’ poisoner could have managed to get away with their fiendish plan.
Humans do have some tolerance to arsenic, but it is very, very low. Arsenic is often present in soil and plants grown in that soil will naturally absorb some arsenic. If we then eat these crops we will in turn ingest some arsenic. The quantities encountered in most people’s everyday lives are minute and not worth worrying about.
With many toxic substances, our body protects us from damaging effects by making chemical changes that render them less toxic and easier to excrete. In the case of arsenic, the body uses enzymes to . MMA is still quite toxic but DMA, produced by the enzyme AS3MT, is far less dangerous, as well as being more readily excreted.
If humans could improve the performance of AS3MT, more arsenic could be converted into DMA and the body would be able to tolerate ingesting more of the poison.
It seems that a single letter mutation to the DNA that codes for AS3MT really can offer dramatic improvements in performance, though the mechanism is still unclear. Whatever the reasons for the improvement, it means that anyone with this genetic mutation has a much better chance of surviving an arsenic-laced meal. This beneficial mutation is a relatively rare occurrence in the general population. Perhaps Sayers’ murderer in Strong Poison was particularly lucky to be naturally blessed with this metabolic advantage.
However, . Researchers at the University of Chile found that 68% of the population in the Quebrada Camarones region of the Atacama Desert in Chile have the genetic code for the improved version of AS3MT. This is compared to 8% of Chileans living in San Juan de la Costa in the south of the country. The difference between the two populations is the amount of arsenic in their drinking water.
The Camarones Valley has levels of arsenic in its water 100 times greater than the World Health Organisation’s safe limits. There are few alternative sources of drinking water. This should mean poor health for anyone living in the area, including problems such as increased infant mortality and cancer. Surely no one could live, let alone thrive, in such an environment? And yet people do, and have done for 7,000 years.
If you are worried about being poisoned by arsenic, or have plans to to poison someone else Dorothy L Sayers-style, you might think a long holiday in northern is the answer. Unfortunately, it isn’t that simple.
The necessary genetic mutation cannot be acquired by a few weeks or even months of consuming small quantities of arsenic, like those who tried it in the 19th century. You have to be born with the mutation, and even such a small genetic change takes time to spread through a population.
Newcomers to the area, drinking the local water, are very likely to suffer ill-health from their exposure to the elevated arsenic levels. In the 1960s the water supply in Antofagasta was switched from a non-contaminated source to the Toconce river, which has a high level of arsenic, though still not as high as the Camerones water. The result is a high level of lung and bladder cancer even forty years after the contamination has been removed.
The population in Quebrada Camarones has fared better because they have been exposed to the poison for much longer – 7,000 years longer, in fact. The first settlers to the region all those years ago are likely have suffered poor health. The scientists who carried out the recent study believe that this poor health pressured the population into evolving the widespread AS3MT mutation that is seen today. A few early settlers thrived in the region and passed on their genetic advantage to future generations.
Other areas with naturally high levels of arsenic, such as San Antonio de los Cobres in Argentina, also have populations with a high incidence of the mutation. It may be that these populations, both in South America though geographically very distant, are genetically more closely related, may share common ancestors. Or it may be that the adaptations have occurred independently. Either way, it is a fantastic example of humans evolving to exist in extremely hostile environments.
Thanks to for pointing me towards the story of .