There are two paths. One leads to the arm of Noah Rose, a postdoctoral research fellow at Princeton University. The other leads to a guinea pig.

For some species of mosquitoes, their preference for humans reveals something about their evolution — and the ecology of their ancestral homes. New research, published July 23 in the journal Current Biology, identifies the genetic components underlying Aedes aegypti mosquitoes’ affinity for humans and indicates that their human-seeking behavior can be traced to two environmental conditions: climate and urbanization.

The findings could explain why mosquitoes evolved human preference — and how climate change and swelling urbanization might drive greater human attraction in the future.

“Mosquito evolution is related to human history,” said Rose, the first author of the study, which was led by researchers at Princeton and included an international team of collaborators. “They evolved in response to changes in how humans live, they spread around the world in response to historical events, and they’re spreading disease in a way that reflects that.”

A small fraction of the roughly 3,500 known species of mosquitoes evolved to specialize in biting humans. It’s these species that are responsible for the vast majority of mosquito-borne illnesses suffered around the world. One example, A. aegypti, is the main vector for diseases including dengue, Zika, chikungunya and yellow fever. This species spreads diseases that make over 100 million people sick each year.

“What sets this species apart is that it specifically targets humans,” said Carolyn “Lindy” McBride, an assistant professor of ecology and evolutionary biology and neuroscience at Princeton University and the senior author of the paper.

But not all individuals within this species show the same affinity for humans. McBride, Rose and their colleagues wanted to explore what ecological factors might drive some of them to hunt for humans specifically.

To do this, the researchers gathered A. aegypti eggs from 27 locations, representing the diverse range of habitats found across the species’ native range in sub-Saharan Africa. These locations included land within seven countries, and varied from dense urban areas to remote forests. They also ranged climactically, from hot and dry to wetter and cooler areas.

The researchers created 50 lab-based colonies of mosquitoes derived from the eggs they collected. Then, they put the mosquitoes in a two-port olfactometer — a device that encourages  mosquitoes to follow their sense of smell along one of two paths: a tube leading to a human — represented by Rose’s arm— and another leading to a guinea pig. (Strategically placed screens prevented harm to Rose and the guinea pigs in this study).[…]

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