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Coloured scanning electron micrograph (SEM) of Long-nosed cattle louse (Linognathus vituli) and egg cases.

A long-nosed cattle louse (Linognathus vituli; artificially coloured), and egg cases.Credit: Dennis Kunkel Microscopy/SPL

There was a time, at least 90 million years ago, when mammals might have been blissfully free of lice. But it wasn’t to last. An ancient mammalian ancestor of today’s elephants and elephant shrews picked up the tiny skin parasites from a bird and kickstarted a remarkable — and perhaps uncomfortably intimate — association between mammals and lice that continues today1.

That’s the conclusion drawn by biologist Kevin Johnson at the University of Illinois in Champaign and his co-authors, following a genomic investigation of the mammalian ‘tree of lice’. The research suggests that many of the lice parasitizing today’s mammals can trace their roots to a single louse ancestor on a single mammal that lived before the extinction of the non-bird dinosaurs.

A lousy tale

The story of mammalian lice is rarely told, but in some ways it is as dramatic as the story of mammals. For instance, when seals adapted to live in the ocean tens of millions of years ago, their lice adapted with them to become the only truly marine insects2. “Lice can quite intricately co-evolve with their host,” says Bret Boyd, a biologist at Virginia Commonwealth University in Richmond.

But lice also have an extraordinary ability to switch hosts when the opportunity arises. It’s this skill that helps explain why the lice found on seals, skunks, elephants and humans all seem to have descended from the same ancestor. After examining genetic data from 33 louse species that come from all the major groups of mammals, Johnson and his colleagues conclude that lice have switched between mammalian hosts at least 15 times since they first parasitized mammals.

Crawling with diversity

It’s partly because of this host-switching that the mammalian tree of lice has proved so difficult to put together — but it’s not the only reason. Just obtaining lice from a wide range of host species to extract their DNA is a logistical challenge, says Vincent Smith, a biodiversity informatics researcher at the Natural History Museum in London.

“The tree has been contested over the years,” says Boyd. “Kevin seems to have figured it out.”

However, Jessica Light, an evolutionary biologist at Texas A&M University in College Station, cautions that it might be too soon to say this is the final picture. “Future studies with increased sampling may either support or fail to support these findings,” she says.

Pinning down the tree of lice has broader implications. Biologists of the early twentieth century used lice to test their ideas on co-evolution, the intertwined evolution of two or more species, says Smith. He thinks the new study might tempt biologists interested in these broad evolutionary themes to take a fresh look at lice.

The tree of lice could also provide important insights into host-switching — a hot topic, given that the origins of some diseases including COVID-19 can be explained by host-switching from other animals to humans. Anything that gives us a deeper understanding of the mechanics of the process “might shed light on how to minimize the chance of host-switching of new diseases to humans”, says Johnson.

But the process is complicated. Boyd says that one reason blood-sucking lice can survive on mammals at all is because the parasitic insects carry symbiotic bacteria that provide them with B vitamins they can’t easily obtain from mammalian blood. However, just as the lice can switch between mammalian hosts, it seems that the bacteria can switch between lice hosts. While investigating a marine seal louse a few years ago, Boyd and his colleagues discovered that its bacterial symbionts had been acquired relatively recently3.

“Presumably the louse lost some ancestral symbiont and replaced it with this new one, so it’s very like host-switching at a deeper level,” he says. “It’s levels upon levels of complexity.”

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