Head Lice Help Scratch the Surface of Human Evolution
Head lice are more than annoying parasites; they are satellites of human evolution.
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Realizing your child has head lice is perhaps one of the less joyous moments of being a parent. Cue a trip to the supermarket to purchase copious amounts of treated hair conditioner and a fine-toothed comb, followed by the labor-intensive process of trying to eliminate the itchy parasites.
Head lice are very common in young children and their families, and archaeological studies suggest they have been for a long time; “nits” – headlice eggs – dating back thousands of years have been discovered during digs in South America, for example.
The human louse (Pediculus humanus) needs its host to survive, feeding on blood by biting the scalp. While it might be an inconvenience to the general population, for scientists, it can offer a window into human evolution.
"Given the intimate relationship between the parasite and the human host, the study of human lice has the potential to shed light on aspects of human evolution that are difficult to interpret using other biological evidence,” scientists at the United States Department of Agriculture (USDA) Agricultural Research Service (ARS), led by Dr. Marina Ascunce, molecular biologist at USDA-ARS, write in PLOS ONE.
Head louse populations are strongly structured geographically
Ascunce and colleagues analyzed genetic variation in 274 human lice obtained from 25 sites across the world, including Asia, Europe, America and Africa. To measure diversity, they used nuclear microsatellite loci and female-inherited mitochondrial DNA.
What are nuclear microsatellites?
Nuclear microsatellites can be likened to “tags” in an organism’s DNA, made up of repeating sequences of nucleotides. To analyze genetic diversity, scientists count how many times the repeating sequence is different in each individual, and the more differences there are, the greater the level of diversity.
The researchers’ analyses showed that within the 274 lice analyses, there were two distinct clusters: Cluster II was found in Europe and the Americas, while Cluster I had a global distribution. Lice sampled from the Americas were the only group to have ancestry from both Cluster I and Cluster II. This, Ascunce and colleagues say, could be the result of lice mixing from populations that arrived with the First People, and those that were carried over during the Colonial period.
“Both this current study and our past work support the idea that head louse populations are strongly structured geographically,” the researchers say.
A genetic relationship was found between lice samples from Asia and Central America, which Ascunce and colleagues say supports the notion that people from East Asia migrated to North America, becoming the first Native Americans. These individuals later spread into Central America, where the lice sampled carry a genetic mark from their Asian ancestors.
Ascunce and colleagues note that, “due to the use of microsatellites that are known to be fast-evolving markers, our analyses are more suited to recent events” and slower evolving markers might be utilized to study ancient events. They also suggest that future studies analyzing whole genome and epigenome data could provide further insight into louse and host evolution.
Reference: Ascunce M, Toloza A, González-Oliver A, Reed D. Nuclear genetic diversity of head lice sheds light on human dispersal around the world. PLOS ONE. 2023. doi: 10.1371/journal.pone.0293409
This article is a rework of a press release issued by the United States Department of Agriculture (USDA) Agricultural Research Service (ARS).
