Fossils may not tell the complete story of early humans. Mosquitoes could fill in the gaps

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 near humans, the researchers had to collect larvae from the ground beneath the trees.

The team reconstructed an evolutionary history of the Leucosphyrus group using DNA, genetic mutation estimates and computer models.

The results showed that while the mosquitoes began by feeding on nonhuman primates, a preference for human blood feeding evolved once within a subset of the group between 2.9 million and 1.6 million years ago in a region called Sundaland that includes Java, Sumatra, Borneo and the Malay Peninsula.

Sundaland was once likely covered in tropical rainforests that persisted for millions of years, providing the perfect habitat for mosquitoes.

“And then in the last 2 million years, there’s a lot of environmental change that’s taking place, which also feeds into the human story, where you start getting periodic climatic fluctuation,” Walton said.

Cooler, drier conditions may have resulted in seasonal forests and grasslands, potentially enabling the migration of early hominins across Sundaland and triggering a decision for mosquito species: stay in the rainforest, or adapt to new environments and food sources, the study authors wrote.

“Hominins must have been relatively numerous, certainly compared to nonhuman primates, to have driven that change in mosquitoes,” Walton said.

Laurent Husson, researcher at the University of Grenoble Alpes’s Institute of Earth Sciences in France, said the study showcases the relationships within what he calls the Earth system: how shifts in the planet, climate and vegetation can play roles in changes for individual species, like mosquitoes and early humans.

“Unraveling these delicate relationships is extremely stimulating, and it seems that there are large avenues opening to investigate what can be referred to as the Earth system,” Husson wrote in an email.

While Husson was not involved in the new study, he authored previous research suggesting the presence of Homo erectus in Sundaland 1.8 million years ago.

An intertwined history

Studying pests that rely on humans can reveal our evolutionary history, said Dr. David L. Reed, interim director for the Florida Museum of Natural History.

“Written in their DNA is a whole other accounting of our history,” Reed wrote in an email. “Using the right parasites, those that track humans closely, and modern tools like genomics will undoubtedly continue to fill in gaps of our understanding of human evolution.”

Reed was not involved in this study, but he has worked on research that showed how the genetic diversity of head lice is connected to the migratory movements of humans in the Americas over time.

Going forward, the authors of the new study want to trace how other genes, such as olfactory ones used to detect the chemical signatures of hosts, evolved in mosquitoes over time and determine whether they changed sequentially or if there was a rapid burst of adaptation in response to the presence of new hosts.

Tracking the history of micropredators like mosquitoes is the kind of out-of-the-box thinking that can play a part in filling gaps where the hominin fossil record is almost nonexistent and complement future archaeological findings, said Dr. Fernando A. Villanea, assistant professor in the department of anthropology at the University of Colorado, Boulder. Villanea did not participate in the study, but he served as one of the peer reviewers for the original manuscript.

“Using the emergence of mosquito species that feed uniquely on humans today to infer the timing of arrival of hominids to Southeast Asia is brilliant,” Villanea wrote via email. “Only time will tell if the fossil evidence will support an early arrival of Homo erectus — or other archaic hominids — to Southeast Asia, but the article is an important contribution to broaden our theoretical horizons, and for paleoanthropologists to keep that possibility in mind.”

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