The ‘tree of lice’ identifies the earliest animal with an infestation

An adventurous parasite travelled from a bird to an ancestor of contemporary elephants more than 90 million years ago.

At one point, at least 90 million years ago, lice may not have been a problem for animals. But it did not last. An ancient ancestor of elephants and elephant shrews acquired small skin parasites from a bird, initiating a fascinating — and maybe uncomfortably close — relationship between mammals and lice that persists to this day.

Following a genetic analysis of the mammalian ‘tree of lice,’ biologist Kevin Johnson of the University of Illinois in Champaign and his co-authors reached this result. The findings indicates that many of the lice now parasitizing mammals may trace their ancestry back to a single louse that lived on a single animal before the demise of the non-avian dinosaurs.

A terrible story

Rarely recounted, the history of mammalian lice is in some respects as spectacular as the history of mammals. When seals evolved to living in the water tens of millions of years ago, their lice also adapted, becoming the only genuinely aquatic insects. Bret Boyd, a scientist at Virginia Commonwealth University in Richmond, states, “Lice may co-evolve closely with their hosts.”

But lice also possess a remarkable capacity to move hosts when the chance presents itself. This ability helps explain why the lice found on seals, skunks, elephants, and humans all seem to share a common progenitor. After analyzing genetic data from 33 species of lice originating from all of the main mammal groups, Johnson and his colleagues believe that lice have switched mammalian hosts at least 15 times since they first began parasitizing mammals.

Abounding in variety

This host-switching is largely to blame for the difficulty in constructing the mammalian tree of lice, but it’s not the entire explanation. Vincent Smith, a specialist in biodiversity informatics at the Natural History Museum in London, notes that acquiring lice from a variety of host species in order to harvest their DNA is a logistical problem.

Boyd states that the tree has been debated throughout the years. “It seems like Kevin has worked it out.”

Jessica Light, an evolutionary scientist at Texas A&M University in College Station, warns that it may be premature to conclude that this is the ultimate image. “Future research with a larger sample size may confirm or refute these results,” she adds.

Immobilizing the tree of lice has far-reaching effects. According to Smith, early twentieth-century biologists used lice to test their theories on co-evolution, the interwoven development of two or more species. He believes that the new findings may entice scientists interested in these broad evolutionary issues to examine lice in a new light.

The tree of lice may also provide light on host-switching, a subject of great interest due to the fact that the origins of some illnesses, particularly COVID-19, may be explained by host-switching from other animals to humans. According to Johnson, a better knowledge of the process’s mechanics “may throw insight on how to limit the risk of novel illnesses transferring hosts to people.”

However, the procedure is complex. Blood-sucking lice are able to thrive on mammals, according to Boyd, because they contain symbiotic bacteria that supply them with B vitamins they cannot readily receive from mammalian blood. Nonetheless, just as lice are able to migrate between mammalian hosts, it seems that bacteria may also switch between lice hosts. While examining a marine seal louse a few years ago, Boyd and his colleagues determined that its bacterial symbionts were recently acquired.

“The louse probably lost an ancestral symbiont and replaced it with this new one, so it’s similar to host-switching on a deeper level,” he explains. There are several tiers of complexity.

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