How are lice thought to be related to book lice?

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human head louse — louse Human head louse (*Pediculus humanus*).

It is generally accepted by scientists that lice (order Phthiraptera) are derived from book lice (order Psocoptera).3

Both groups are part of the paraneopterous stock of insects, which underwent an evolutionary radiation during the Permian Period. While lice are now specialized, permanent ectoparasites of birds and mammals, they share a common evolutionary history with the psocids (book lice). This relationship highlights the transition from free-living ancestors to the parasitic forms observed in modern lice species.123