Mosquitoes Developed a Taste for Human Blood Recently
An experiment with disease-carrying mosquitoes in Africa showed a surprising result: they switched to human blood just thousands of years ago.
The photo in Science Magazine’s article about mosquito evolution is ready for a horror movie: a close-up of an adult mosquito emerging from its watery nursery. Even if they won’t attack Manhattan, these small creatures often carry smaller, yet no less deadly, weapons: agents for dengue fever, yellow fever, and chikungunya. Why do they suck human blood? Elizabeth Pennisi writes about research at Rockefeller and Princeton Universities that shows a recent switch by one variety, Aedes aegypti aegypti, from its close cousin, A. aegypti formosus. The latter variety bit guinea pigs in the lab, but showed no attraction to humans; the former was the reverse. One particular gene named Or4 differed between them. A press release from Rockefeller explains how they isolated the scent that the disease-carrying mosquitoes preferred:
The researchers guessed that Or4 must be detecting some aroma in human body odor. To figure out which one, they asked volunteers to wear pantyhose for 24 hours. And then they placed those stinky stockings in a machine designed to separate their scent into the hundreds of individual chemicals that make up body odor. The researchers came up with one match, a chemical called sulcatone that was not found in pantyhose worn by guinea pigs.
Humans have a lot of sulcatone, but guinea pigs do not. It appears that a variant in the Or4 gene caused the mosquitoes to be attracted to sulcatone. When did this occur?
The switch from preferring animals to humans involves a variety of behavior adjustments: Mosquitoes had to become comfortable living around humans, entering their homes, breeding in clean water found in water jugs instead of the muddy water found in tree holes. “There’s a whole suite of things that mosquitoes have to change about their lifestyle to live around humans,” Vosshall says. “This paper provides the first genetic insight into what happened thousands of years ago when some mosquitoes made this switch.”
Another insight into mosquito attraction to odors is found in a PNAS paper, where researchers tried to figure out why DEET works as a repellant.
We have discovered an odorant receptor in the southern house mosquito, which is essential for repellency, thus unravelling how DEET works. Additionally, we have identified a link between this synthetic repellent and methyl jasmonate, thus suggesting that DEET might work by mimicking defensive compound(s) from plants.
More knowledge about the chemicals that attract and repel mosquitoes can lead to better repellants.
Pennisi started with an interesting fact: “Out of the millions of species of insects, only about a hundred suck human blood.”
Olfactory organs in animals are tremendously complex. Individual olfactory receptors on the cilia of olfactory neurons have to be able to fit a wide variety of chemicals. Genes control which olfactory receptors are produced in the olfactory neurons. It is certainly conceivable that slight changes in the “lock and key fit” of odorants to receptors can cause changes in the sensitivity of the animal to other organisms. Changes in sensitivity could lead to changes in pre-existing behavioral responses.
This story points out that the curse on creation in Genesis 3 did not necessarily have to involve complete overhauls of plants and animals, but at least in some cases, slight changes to existing structures and functions. Mosquito mouthparts are very complex. It is hard not to imagine them as designed. What we see here is that they could have been designed to work in the ecology in a beneficial way, perhaps with non-human targets that would not be harmed. As diseases began to develop, they could be carried by these miniature flying robots. When a mutation altered existing structures and functions to aim at odorants humans produce in abundance, another aspect of the curse came on with a vengeance. The scientists did not say how many thousands of years ago this happened; we don’t know (probably on one knows), but it’s conceivable it was since the time Adam and Eve left Eden.