Scientists have found the reason why mosquitoes, which can spread deadly diseases, are attracted to humans.

A study conducted by researchers at the Rockefeller University reveals that mosquitoes were not always attracted to humans and did not depend on human blood for their own and their eggs' survival. Thousands of years ago, they sucked blood from hairier and furrier animals. However, they made an intelligent move and started biting humans.

Leslie B. Vosshall, Robin Chemers Neustein professor, head of Rockefeller's Laboratory of Neurogenetics and Behavior and a Howard Hughes Medical Institute investigator, says that mosquitoes made a smart move by switching to human blood. Humans provided an ideal lifestyle suitable to mosquitoes.

Mosquitoes are attracted to humans because humans always had water around them, which is required for mosquitoes to breed, and humans were less hairy than other animals.

The study examined the genes of some mosquitoes. The researchers found that some mosquitoes have acquired preference for human body scent, which leads them to bite humans.

Black mosquitoes or Aedes aegypti formosus normally lay eggs outdoors and also prefer to feed on forest animals. Brown mosquitoes called Aedes aegypti aegypti prefer breeding in water and biting humans.

To understand the genes of mosquitoes, which attract them to humans, the researchers crossbred the two types of mosquitoes and created genetically diverse grandchildren. The researchers sorted the mosquitoes based on their preference to odor then compared the two groups.

The study discovered 14 genes that were strongly associated with liking humans. However, an odor receptor gene called Or4 stood out.

"It's very highly expressed in human-preferring mosquitoes," says Vosshall referring to the Or4 gene.

The researchers found that the gene was responsible for the mosquitoes becoming attracted to sulcatone, which is an important ingredient of human odor. Mosquitoes started to recognize sulcatone and associated it thousands of years back as a sign of being near a food source.

The researchers indicate that they are trying to understand the transition of mosquitoes from harmless animal-biting buggers to transmitters of deadly human diseases, such as dengue and yellow fever.

The study is published in the journal Nature.

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