Evolution in action

The yellow fever mosquito is responsible for spreading yellow fever, Zika, chikungunya, and dengue viruses, leading to hundreds of millions of infections each year. Although it previously showed no interest in humans and preferred other animals, a new genomic analysis has revealed that the ancestral mosquito species split about 5,000 years ago due to a drying climate in the West African Sahel, transforming the previously prevailing grassland landscapes into a desert.

Researchers compared mosquito genomes collected across Africa and Brazil and discovered another significant migration event when the insect spread from Africa to the Americas. This likely occurred around 1800, during the period when approximately 80 000 enslaved individuals were transported across the ocean annually. Yellow fever mosquitoes laid their eggs in water barrels and fed on the people aboard the ships.

In the absence of water bodies, some mosquitoes likely adapted to laying their eggs in water storage containers in communities, subsequently developing a preference for humans as a food source due to their increased availability compared to other mammals.

The investigation also unveiled rapid and ongoing evolution within the species, allowing for more efficient transmission of diseases in African cities. These evolving mosquitoes may explain recent changes in disease transmission patterns. 

For instance, Burkina Faso experienced its first modern dengue outbreak only seven years ago, yet the problem has resurfaced every year since. Additionally, Ouagadougou's mosquitoes quickly adapted to breeding in public hand-washing services installed during the COVID-19 pandemic.

It remains uncertain whether the evolving mosquito strains will spread beyond Africa. But is is clear anyway, that scientists now have an unprecedented opportunity to observe evolution in action.

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