Why there is no single family tree linking humans and viruses
A common ancestor, in the usual sense, means two lineages trace back to a single ancestral population. Humans and viruses do not fit that model because viruses are not one lineage. Some virus groups may have originated from cellular ancestors, others from independent replicating molecules, and they do not form a single branch on the tree of life. The supplied papers do not claim a shared ancestor; instead, they describe cases where viral and human genetic material have become intertwined over time.
For example, some human genomes contain DNA sequences derived from viruses, called endogenous viral elements, which are remnants of ancient viral infections that became permanently inserted into the human germline. One study found that certain individuals share an endogenous human herpesvirus 6A sequence derived from a single integration event, indicating that this viral DNA has been inherited within human populations [1]. That is gene transfer, not evidence that humans and viruses share a common ancestor.
Gene transfer and coevolution: how viruses and humans become entangled
The more accurate picture is one of occasional gene exchange and long-term coevolution. Viruses can pick up bits of human genetic material, and human genomes can retain viral sequences. A study of the Omicron variant of SARS-CoV-2 examined a unique insertion mutation and explored whether it might have originated from the human transcriptome or from co-infecting viruses, suggesting that template switching during replication could incorporate host or other viral sequences [2]. This is a possible mechanism for genetic exchange, not proof of common ancestry.
Similarly, hepatitis E viruses in rodents show a basal sister relationship to human-associated strains, meaning rodent viruses are close evolutionary relatives of human strains. The analysis indicated that human-associated hepatitis E virus likely originated in ungulates such as swine and camelids, while the deeper evolutionary origins of human- and ungulate-associated strains were projected to rodent hosts [3]. This describes cross-species transmission and host shifts, not a shared ancestor between humans and viruses.
Ancient coevolution: viruses tracking human populations
Some viruses have coevolved with humans over long periods, and their genetic variation can mirror human geographic patterns. A study of human papillomavirus type 16 from four continents found molecular variants that suggest an ancient pandemic spread of the virus and its coevolution with humankind [4]. This means the virus has been evolving alongside human populations for a long time, but it does not mean the virus and humans share a common ancestor.
The distinction matters: coevolution means the virus and host influence each other's evolution, while common ancestry means they descend from the same ancestral lineage. The evidence supports the former, not the latter. Viral sequences in human genomes and host shifts between species are signs of ongoing interaction, not a shared root.
Sources used in this answer
Virus-derived variation in diverse human genomes
Some human genomes contain endogenous HHV-6A sequences from a single integration event, showing virus-derived variation and coevolution between viruses and humans.
Omicron variant of SARS-CoV-2 harbors a unique insertion mutation of putative viral or human genomic origin
The Omicron variant's unique insertion mutation may have originated from the human transcriptome or co-infecting viruses via template switching, suggesting a possible mechanism of genetic exchange.
Ancient evolutionary origins of hepatitis E virus in rodents
Rodent-associated hepeviruses are basal sister lineages to human hepatitis E virus, and human-associated HEV likely originated in ungulates, with deeper origins projected to rodents.
Molecular variants of human papillomavirus type 16 from four continents suggest ancient pandemic spread of the virus and its coevolution with humankind
Molecular variants of human papillomavirus type 16 from four continents suggest ancient pandemic spread and coevolution with humankind.