While sheltering from Hitler’s bombs deep in the London Underground system during the Blitz, Londoners discovered a very different peril lying in wait: mosquitoes.
As they emerged covered in itchy bites, the stories of wartime blood-sucking entered into folk legend. For over 80 years, the London Underground mosquito has been assumed to be a new species, mutating to live in the dark and damp tunnels, ready to pounce on unsuspecting travellers.
New research, though, reveals a very different story. This mosquito is simply a migrant from the Middle East, albeit one which probably began its journey to Britain many thousands of years ago.
A team of biologists and evolutionary scientists have discovered the Tube mosquito is genetically identical with the insect which probably originated along the Nile Delta in ancient Egypt, when it evolved from feeding off birds to preying on humans.
Until now it had been thought that following studies in the 1990s, Culex pipiens molestus, to use its scientific name, had evolved to live in mass transit systems less than two centuries ago. It is found not just in the London Underground, but in many other tunnels around the world, and was seen as an example of how quickly species can evolve to a new environment.
What does the new research say?
Published in Science, the authors of the research say their work “debunks one of the most widely cited examples of rapid urban adaptation – an example that has captured the attention of scientists and laypeople for 25 years”.
“Rather than benchmarking the speed and complexity of urban evolution, this updated history highlights the role of early human society in priming taxa (species) for colonisation of modern urban environments.”
The international team was led by Princeton University and worked with dozens of research groups worldwide. Their work was prompted by doubts about a 1999 study that concluded that the London Underground mosquito was a new variant of the insect.
“It became a textbook example of rapid evolution in modern cities,” said Professor Lindy McBride, the senior author of the study and an expert on evolutionary biology. “But cracks in this story have long been present and our analysis of DNA sequences from hundreds of mosquitoes supports a very different history.”
They studied the genome of hundreds of living and dead mosquitoes, including many from the collection of London’s Natural History Museum going back to 1900. The conclusion was that the mosquitoes reached Britain in Roman times, around 2,000 years before construction of the London Underground.
In fact it may be much older. The molestus mosquito is a variant of Culex pipiens which progressed from feeding on birds to biting mammals and humans possibly as far back as 12,000 years ago. This was probably in Egypt, where some of the earliest agricultural societies emerged, or in Mesopotamia, now part of modern Iraq.
Where has the mosquito lived?
From there it spread into Europe, adapting to human hosts, and then even further afield, with branches in Russia, the USA and even Australia. In 2011, following a wetter than usual summer there were reports of “extra-bloodthirsty” mosquitoes escaping sewers in large numbers and “terrorising” New York’s Upper East Side, with parents “forced to take extreme measures to protect their children and their homes”, according to local CBS television.
It reached Australia in the 1940s, probably through Asia, and then spread through human populations due to troop movements in the Second World War. In the UAE, 11 species of mosquito have been identified, although not the London Underground type.
In cooler northern Europe, the mosquito has adapted to living underground where it is active all year round. Crucially it requires only pools of water to breed and no longer depends exclusively on blood for its food – except when sheltering citizens during the Blitz provided a ready source of meals.
The “human-biting form thrives in urban below-ground environments, such as subways, cellars and cesspits, and differs from its above-ground counterpart in ways that seem perfectly suited to subterranean life,” the study says.
These mosquitoes “are able to mate in confined, indoor spaces and remain active in winter. Adult females readily bite humans and other mammals. Yet if hosts are scarce, they can develop a first clutch of eggs without taking any blood”.
The new research does not just offer a historical explanation but gives insight into the spread of West Nile virus, which can cause fever in humans and, in rare cases, death. The disease is found principally in avian populations, but spreads to humans possibly because hybrid mosquitoes have emerged which feed off both birds and people.
