science Watch / Saul Scheinbach
Our Long, Lousy Relationship
“Lice are part of our lives and history” – M. Ascunce
Humans and lice go way back—even before we were human. Lice have been sucking primate blood for 25 million years and we are a veritable lice ecosystem with three kinds that exclusively call us home.
Our most loyal companion, the human louse, Pediculus humanus, likes us so much it has branched into two sub-species, Pediculus humanus capitus, the head louse, and Pediculus humanus corporis, the body louse. The former clamps onto our hair shafts, while the latter makes do by clinging to our clothing. Both are obligate parasites that live on our blood. The third species we host, the crab louse (Pthiris pubis), grips our thicker pubic hairs. It diverged about three million years ago from a related louse that lives on gorillas and we can speculate on how it got to us.
The body louse diverged from the head louse over 100,000 years ago when we gave it a safe haven by wearing clothes. Although closely related, the two parasites don’t interbreed even where their habitats overlap (think neckline), but can be forced to do so in the lab. DNA sequencing shows that the nearest relative of P. humanus is P. schaeffi, which lives on our closest relative, chimps, and the two louse species diverged when we did from chimps about 5.6 million years ago.
Sequencing lice genomes adds to the insights of our evolutionary history and migration patterns gained from fossils. For example, in 2008 Dr. David Reed, an entomologist at the University of Florida, Gainesville, FL, and his colleagues, compared mitochondrial DNA, inherited only from females, of human head lice from around the world. The DNA samples fell into two groups that diverged about a million years ago when two early Homo species in Africa split. One lice lineage stayed with us, but when we left Africa much later, we became infested with the other from direct contact with a now extinct group of humans in the New World, probably Neanderthals.
Now a study in the November 8, 2023 issue of PLOS One led by Dr. Reed and Dr. Mary Ascunce, presents sequencing data of stretches of nuclear DNA from 274 head lice collected from people in 25 places around the world. The data reveal two distinct genetic clusters, suggesting human head lice arrived in the Americas twice. One cluster linked Asia, Africa and Central America with a tight linkage between Honduras and Mongolia. The second linked samples from Europe and North America.
Human head louse and egg (nit) attached to a hair shaft. Credit: Josef Rieschig
"These lice are mirroring the colonization of the Americas, the two migration waves," says Dr. Ascunce. The first cluster represents the earliest migrants to the New World from Africa through Asia about 23,000 years ago and supports the idea that they entered from Siberia. The second cluster denotes the arrival of modern European colonists. They brought their lice which spread to the first peoples already infected with their own.
Sequencing of lice nuclear DNA is in its infancy and already an enigma has arisen. The two genetic clusters show less hybridization between them than those from other regions. It could simply be that lice from the two clusters don’t readily interbreed.
Human pubic (crab) louse. Credit: PHIL, DPDx, CDC
Fossil evidence shows that Neanderthals left Africa about 600,000 years ago, much earlier than modern humans did, and that we interbred in Europe about 50,000 years ago, something Dr. Ascunce wants to examine in more detail. She had hoped the study might shed more light on Neanderthal liaisons by finding DNA derived from Neanderthal head lice, but they were missing from this study.
“New ongoing studies are being done using whole genome sequences from human lice, so stay tuned for more exciting research on that,” she said. “The team will look for any type of close contact from sharing sleeping sites to fights to interbreeding."