biostudies-literatureSwali PCancer Research UKEuropean Research CouncilMedical Research CouncilWellcome Trusteadr2147https://www.ebi.ac.uk/biostudies/studies/S-EPMC7617810biostudies-literatureUnknown388(6749)Several bacterial pathogens have transitioned from tick-borne to louse-borne transmission, which often involves genome reduction and increasing virulence. However, the timing of such transitions remains unclear. We sequenced four ancient <i>Borrelia recurrentis</i> genomes, the agent of louse-borne relapsing fever, dating from 2300 to 600 years ago. We estimated the divergence from its closest tick-borne relative to 6000 to 4000 years ago, which suggests an emergence coinciding with human lifestyle changes such as the advent of wool-based textiles. Pan-genome analysis indicated that much of the evolution characteristic of <i>B. recurrentis</i> had occurred by ~2300 years ago, though further gene turnover, particularly in plasmid partitioning, persisted until ~1000 years ago. Our findings provide a direct genomic chronology of the evolution of this specialized vector-borne pathogen.Science (New York, N.Y.)Ancient &lt;i&gt;Borrelia&lt;/i&gt; genomes document the evolutionary history of louse-borne relapsing fever.PMC7617810852558CC2109217223FC001595217223/Z/19/Z220457Williams MMadgwick RTait FMullan GTeoaca AIrish JDValoriani SWilson LSpeidel LKing MBorrini MBuckberry JAnastasiadou KCarlin RPetchey FBricking ABooth TGilardet AKelly Mvan Dorp LSkoglund PPeto JGutierrez MGCootes KSwali PSilva MBarrington CTan CCSBuster LMcCabe JArmit IGlocke IfalseAncient &lt;i&gt;Borrelia&lt;/i&gt; genomes document the evolutionary history of louse-borne relapsing fever.Several bacterial pathogens have transitioned from tick-borne to louse-borne transmission, which often involves genome reduction and increasing virulence. However, the timing of such transitions remains unclear. We sequenced four ancient <i>Borrelia recurrentis</i> genomes, the agent of louse-borne relapsing fever, dating from 2300 to 600 years ago. We estimated the divergence from its closest tick-borne relative to 6000 to 4000 years ago, which suggests an emergence coinciding with human lifestyle changes such as the advent of wool-based textiles. Pan-genome analysis indicated that much of the evolution characteristic of <i>B. recurrentis</i> had occurred by ~2300 years ago, though further gene turnover, particularly in plasmid partitioning, persisted until ~1000 years ago. Our findings provide a direct genomic chronology of the evolution of this specialized vector-borne pathogen.2025-01-01T00:00:00Z2025 May2026-06-01T12:02:41.322Z2026-04-08T12:00:05.702ZS-EPMC76178104040306710.1126/science.adr2147