By Ashley Strickland, CNN
(CNN) — Mosquitoes haven’t always had a taste for human blood — partly because the tiny yet dangerous insects have been around a lot longer than humans.
Pinpointing when mosquitoes shifted their preference to human blood could provide a novel window into the spread of early human ancestors across the globe, according to a new study.
The genetic analysis found that certain mosquitoes collected in Southeast Asia, including ones capable of transmitting malaria, likely evolved in response to the presence of our early ancestors, or hominins, in the region between 2.9 million and 1.6 million years ago, which could support some hypotheses for when prehistoric humans reached the area.
The findings, published February 26 in the journal Scientific Reports, suggest that Homo erectus may have been present in numbers abundant enough to trigger such an adaptation in some forest-dwelling mosquitoes, said study coauthor Catherine Walton, senior lecturer in Earth and environmental sciences at the University of Manchester in the United Kingdom.
Traditionally, scientists have largely relied on fossil evidence and sources of ancient DNA to map the timeline and locations of prehistoric humans as they spread out of Africa. But these physical traces are often lost to time.
Non-archaeological methods, such as DNA sequencing and computer modeling, could help track the human footprint in environments such as humid, tropical climates of Southeast Asia, where conditions accelerate the decomposition of remains.
Different groups of researchers have debated for decades whether early human ancestors like Homo erectus reached Southeast Asia around 1.8 million or 1.3 million years ago because the fossil record is sparse.
“I think it’s so difficult and so challenging to patch together that history that we really have to rely on diverse sources of information,” Walton told CNN. “What we can get from mosquitoes, fossils or human genomes, it’s all limited in its own way. So, it’s trying to bring it together and seeing when things match up that really gives us the power.”
Evolving a new appetite
Mosquitoes may be thought of primarily as pests that actively seek out humans, but human blood feeding is rare across the more than 3,500 known mosquito species, according to lead study author Upasana Shyamsunder Singh, a postdoctoral scholar at Vanderbilt University in Nashville, Tennessee.
Some mosquitoes in Southeast Asia’s Anopheles leucosphyrus group are anthropophilic, meaning they prefer human blood to that of other animals.
Unraveling the evolution of this dietary preference enables a deeper understanding of how malaria can spread from disease-causing pathogens carried by these mosquitoes today.
“We were interested to know why some members of the Leucosphyrus group are super attracted to humans, while others are attracted to biting monkeys, and we wanted to see how and when this transition happened,” Singh said.
The team sequenced the DNA of 38 different mosquitoes belonging to 11 species within the Leucosphyrus group, which had been arduously collected during fieldwork between 1992 and 2020 across Southeast Asia.
Fieldwork in Borneo offered groundbreaking insights into the behaviors of human blood-feeding mosquitoes versus those that prefer to feed on monkeys, Walton said.
Researchers tracked when and how mosquitoes, which lived in little pools of water in the rainforests, made their approach while trying to bite humans. Meanwhile, many fruitless nights were spent sitting in trees trying to collect other mosquitoes that preferred monkeys. Because these mosquitoes wouldn’t fly nea