Mosquitoes began biting hominins 1.8 million years ago, study says

Between 1992 and 2020, a group of intrepid scientists walked deep into the forests of Sundaland, across the Malay Peninsula, Sumatra, Java, and Borneo, to collect mosquito larvae of 11 species to understand the evolutionary origins of anthropophily, or the affinity for humans.

This effort, the researchers have written in a new paper in Scientific Reports, “can provide critical insights into mitigating the impacts of novel diseases due to mosquito-borne pathogens.”

Every year, mosquito-borne diseases kill more than 6 lakh people worldwide, according to the World Health Organization. But this is hardly new: according to the new study, mosquitoes first began to ‘feed’ on humans, thus transferring disease-causing microbes into their blood in exchange, in the Pleistocene epoch around 1.8 million years ago in Southeast Asia.

Today, several species of mosquitos are highly anthropophilic as well as very efficient vectors of human malaria parasites.

The malaria-causing Anopheles Leucosphyrusgroup of mosquitoes encompasses around 20 species in Southeast Asia, including in Northeast India, and each species has different host preferences. While some feed on non-human animals in forest canopies, such as on monkeys, gibbons, and orangutans, others feed on humans on the ground, and some on both, according to the study paper.

“The establishment of anthropophily in multiple species of the … group could be attributed to the trait evolving independently multiple times following the arrival of anatomically modern humans in [Southeast] Asia 76,000-63,000 years ago,” the paper read.

“Alternatively, anthropophily may have evolved once in an ancestral species, possibly in response to the colonisation of Southeast Asia by early hominins. Conservative estimates place Homo erectus in China at least 1.6-1.7 million years ago (Mya), and possibly as long ago as 2.4 Mya. However, the timeline of hominin colonisation southwards into Southeast Asia remains contentious.”

Sniffing humans out

Later in the Pliocene epoch, open forests transitioned to savannah, forcing terrestrial mammals to adapt to new habitats. Now, the abundance of ground-dwelling host species could have triggered an “adaptive evolutionary innovation” in the way mosquitoes sought their hosts – and this shift, per the paper, could have paved the way to ‘feed’ on humans.

The researchers used phylogenomic tools to analyse the evolution of mosquitoes and their preferred hosts.

Several smell-producing volatile compounds and smell-related genes determine which hosts the mosquitoes pick to bite, and evolutionary changes in the insects’ olfactory genes were critical to their developing a preference for human blood, the team added.