3,500 years of sheeppox virus evolution inferred from archaeological and codicological genomes

By sequencing 21 ancient sheeppox virus genomes from Bronze Age remains and medieval parchments, researchers traced the virus's 3,500-year evolutionary history, revealing its divergence timeline, early gene inactivation events, and its long-standing threat to Eurasian food security.

The Gist

Imagine you are a detective trying to solve a mystery that has been cold for thousands of years. The crime? A devastating disease that has been plaguing sheep flocks across Europe and Asia since the Bronze Age. The suspects? Tiny, invisible viruses. And the evidence? Not fingerprints or DNA from a modern lab, but ancient viral genomes hidden inside old sheep teeth and medieval book pages.

This paper is the story of how scientists cracked this case, revealing that sheep have been fighting a losing battle against "Sheeppox" for over 3,500 years.

Here is the breakdown of their discovery, translated into everyday language:

1. The Time Travelers: Finding Viruses in Old Stuff

Usually, when we think of ancient history, we think of bones, pottery, or gold. But viruses are tiny and fragile; they usually rot away instantly. However, this team of scientists found a way to catch them in a "time capsule."

• The Teeth: They dug up sheep teeth from Bronze Age settlements in Kazakhstan and Russia (dating back to around 1,700 BCE). Inside the roots of these teeth, they found the virus's DNA. It's like finding a letter written by a soldier in a war, sealed inside a bone.
• The Books: This is the real magic trick. They found the virus in medieval manuscripts (old books made of animal skin). In the Middle Ages, scribes wrote on parchment, which is made from the skins of calves, goats, or sheep. If a sheep was sick with Sheeppox when it was skinned, the virus DNA got trapped in the leather. The scientists treated these ancient books like crime scene tape, swabbing the pages and finding the virus's genetic fingerprint.

2. The Family Tree: Who is Related to Whom?

The scientists built a "family tree" for the virus. They compared the ancient Sheeppox virus to its cousins: Goatpox (which infects goats) and Lumpy Skin Disease (which infects cows).

• The Big Surprise: For a long time, scientists thought the virus that infects cows (Lumpy Skin Disease) was the "grandparent" of the family. But this study suggests that Sheeppox is actually the oldest branch.
• The Analogy: Imagine a family where the grandfather (Lumpy Skin Disease) is the oldest, and the father (Goatpox) and son (Sheeppox) are younger. This study says, "Wait a minute, the son (Sheeppox) actually split off from the family tree first, before the father and the other cousin even existed." It turns the whole family tree upside down.

3. The "Software Update": How the Virus Learned to Adapt

Viruses are like hackers trying to break into a computer (the animal's immune system). To succeed, they often delete parts of their own code that they don't need anymore.

• The Discovery: The scientists looked at the ancient DNA and found that the virus had already "deleted" (inactivated) nine specific genes by the time the Bronze Age sheep were alive.
• The Metaphor: Think of the virus as a smartphone app. When it first appeared, it had a lot of extra features (genes) that were clunky and unnecessary. By the time the Bronze Age rolled around, the app had already been "updated" to remove the bloatware. It had optimized itself specifically to infect sheep.
• The Takeaway: This means the virus adapted to sheep very quickly—within a few thousand years of sheep being domesticated. Once it figured out how to infect sheep, it didn't change much for the next 3,000 years. It was a "perfect" infection for sheep, and it stayed that way.

4. The Economic Nightmare

Why does this matter? Because Sheeppox is a disaster for farmers.

• Then and Now: In the Bronze Age, if your whole flock got sick, you lost your meat, your wool, and your milk. It was a financial catastrophe.
• The Medieval Connection: The fact that they found the virus in so many medieval books suggests that Sheeppox was a constant, recurring nightmare for European farmers. It wasn't just a one-time event; it was a frequent visitor that killed animals and ruined economies for centuries.

5. The "Parchment Paradox"

One of the weirdest findings was that they found Sheeppox DNA in books made from calfskin and goatskin, even though the virus is supposed to only infect sheep.

• How? The scientists think the virus didn't infect the cows or goats. Instead, it was likely a "contamination." Imagine a slaughterhouse where they are processing a sick sheep. The virus gets into the water or the tools. Then, they process a calf. The virus gets stuck on the calf's skin. When that skin is turned into a book page, the virus DNA is preserved there, even though the calf never got sick. It's like finding a drop of red paint on a white canvas because the artist dipped the brush in red first.

The Bottom Line

This paper is a time machine. By reading the genetic code trapped in ancient teeth and medieval books, we learned that:

1. Sheeppox is ancient: It has been threatening food security for over 3,500 years.
2. It adapted fast: The virus figured out how to infect sheep very early in history and stayed that way.
3. History is written on skin: Even old books can tell us about the diseases that shaped our past, not just the kings and queens who owned them.

It's a reminder that while human history is written in ink, the history of our survival is often written in the microscopic DNA of the animals we depend on.

Technical Summary

1. Problem Statement

Sheeppox virus (SPPV) is a highly contagious and economically devastating pathogen affecting domestic sheep, causing high mortality and morbidity. While historical records describe outbreaks dating back to antiquity, the deep evolutionary history of SPPV, its divergence timeline relative to other capripoxviruses (Goatpox virus [GTPV] and Lumpy Skin Disease Virus [LSDV]), and the timing of its adaptation to the sheep host remain poorly understood due to a lack of ancient genomic data. Furthermore, the phylogenetic relationships within the Capripoxvirus genus are contentious, with conflicting hypotheses regarding which lineage diverged first.

2. Methodology

The study employed a multi-disciplinary approach combining archaeology, paleogenomics, and bioinformatics to reconstruct the evolutionary history of SPPV.

• Sample Collection & Screening:

  • Researchers screened 754 ancient and historical samples from Eurasia, including sheep teeth from Bronze Age settlements in Kazakhstan and Russia, and parchment/paper manuscripts from medieval Europe (UK, France, Austria, Germany).
  • Metagenomic screening was performed using KrakenUniq against a custom database to identify Capripoxvirus reads.
  • Samples with sufficient viral DNA representation underwent shotgun sequencing or in-solution RNA bait enrichment (hybrid capture) using custom 80bp RNA baits targeting Capripoxvirus.

• Genomic Assembly & Authentication:

  • De novo assembly was performed using SPAdes for high-coverage samples, followed by scaffolding with RAGTAG.
  • Authentication of ancient DNA (aDNA) was confirmed via characteristic damage patterns (C→T substitutions at 5' ends), short read lengths (<100 bp), and high pairwise similarity to SPPV compared to GTPV/LSDV.
  • Host species identification for parchment samples was determined via mitochondrial DNA analysis (competitive alignment to sheep, goat, and cattle references).

• Phylogenetic & Evolutionary Analysis:

  • Phylogeny: Maximum Likelihood (ML) trees were constructed using IQ-TREE2, and time-calibrated Bayesian phylogenies were generated using BEAST v1.10.5.
  • Molecular Clock: The study utilized uncorrelated relaxed clock models and shrinkage-based random local clocks to estimate divergence times, incorporating radiocarbon dates and historical manuscript dates as priors.
  • Gene Integrity: Comparative genomics assessed the status of 156 genes across Capripoxvirus species to identify inactivation events (frameshifts, premature stop codons) using MACSE and BLASTn.
  • Selection Analysis: Tests for positive selection were conducted using PAML (codeml) to detect antigenic drift.

3. Key Contributions

• Novel Ancient Genomes: The study reports 21 novel ancient SPPV genomes, extending the molecular record of the virus by ~3,400 years. This includes the oldest known Poxviridae genomes recovered to date (Bronze Age, ~1,700 BCE).
• Codicological Discovery: This is the first successful recovery of pathogen genomes from medieval parchment and paper manuscripts, demonstrating that viral DNA can persist in animal-skin-based documents for over a millennium.
• Phylogenetic Resolution: The study provides robust evidence for the root of the Capripoxvirus tree, challenging previous assumptions about the order of divergence among SPPV, GTPV, and LSDV.
• Gene Inactivation Timing: The research pinpoints the timing of critical gene inactivation events that define the host specificity of SPPV and GTPV.

4. Key Results

• Temporal and Spatial Distribution:

  • Bronze Age: Two high-quality genomes were recovered from sheep teeth in Kazakhstan (MYRZ180, ~1,875–1,645 BCE) and Russia (RUB002, ~1,336–1,130 BCE). These samples represent the basal lineages of the SPPV clade.
  • Medieval Europe: 17 SPPV-positive parchment samples were identified, primarily from the 8th to 14th centuries CE. Notably, SPPV was detected in high-status manuscripts (e.g., the York Gospels, Corpus Glossary).
  • Host Diversity in Parchment: SPPV was found in parchment made from cattle (calfskin), goats, and sheep. This suggests either cross-species infections (rare for SPPV in cattle) or viral contamination during the parchment production process (e.g., soaking pits or gelatin sizing).

• Evolutionary Divergence:

  • Root Placement: Phylogenetic analyses (supported by Bayesian Root Posterior Probabilities) indicate that SPPV is the most basal lineage within Capripoxvirus, diverging first from the common ancestor of GTPV and LSDV.
  • Divergence Dates: The study estimates the divergence of the Capripoxvirus genus between 11,500 and 3,700 years ago. Specifically, the SPPV lineage diverged from the GTPV/LSDV clade approximately 5,000–5,200 years ago, coinciding with the domestication and translocation of sheep from the Eurasian steppe to Europe.
  • Coalescence: The most recent common ancestor (tMRCA) of the sampled SPPV diversity is estimated at ~4,080 BP.

• Gene Inactivation:

  • Rapid Early Inactivation: Nine genes known to be inactivated in modern SPPV were found to be already inactivated in the oldest Bronze Age genomes (MYRZ180). This implies these inactivation events occurred rapidly (within ~1,600 years) after the SPPV lineage emerged, likely driven by adaptation to the sheep host.
  • Parallel Inactivation: The study found no shared disruptive mutations between SPPV and GTPV for these inactivated genes, suggesting parallel, independent inactivation events in the two lineages rather than inheritance from a common ancestor.

• Evolutionary Dynamics:

  • The SPPV dataset shows a strong temporal signal (R² = 0.96).
  • No significant signals of positive selection were found in modern SPPV compared to ancient strains after multiple testing correction, suggesting the virus has reached a stable equilibrium with its host since the Bronze Age, with no further major gene losses observed.

5. Significance

• Food Security History: The findings reveal that sheeppox has threatened the food security and economic stability of Eurasian pastoralist communities for over 3,700 years, impacting milk, wool, and meat production since the Bronze Age.
• Host Adaptation Mechanism: The study demonstrates that host-specific adaptation in poxviruses can occur rapidly via gene inactivation, with the critical genetic changes for sheep adaptation occurring within the first few millennia of the virus's emergence.
• Codicological Paleovirology: The recovery of viral DNA from parchment opens a new frontier for studying the history of livestock diseases, suggesting that historical manuscripts are a vast, untapped reservoir for ancient pathogen genomics.
• Phylogenetic Clarity: By establishing SPPV as the basal lineage, the study resolves long-standing phylogenetic conflicts, suggesting that the shared gene inactivation patterns between SPPV and GTPV are a result of convergent evolution driven by similar immune pressures in sheep and goats, rather than shared ancestry.

In conclusion, this study reconstructs a 3,500-year evolutionary trajectory of SPPV, linking its emergence to the domestication and migration of sheep, and highlighting the virus's long-standing impact on human societies and the potential of historical documents as sources of ancient viral genomes.