I wonder if that statement was talking about endogenous viruses:
> Approximately 8% of the human genome is made up of endogenous retroviruses (ERVs), which are viral gene sequences that have become a permanent part of the human lineage after they infected our ancient ancestors.
> ..their expression has been implicated in diseases like autoimmune disorders and breast cancer.
> they can also be extremely useful for human survival. For example, they play a very important role as an interface between a pregnant mother and her fetus by regulating placental development and function.
> It has been suggested that viruses are not only necessary for the existence of placental mammals, but also for the existence of life in general.
The above is from a Medium article by a non-specialist: https://medium.com/medical-myths-and-models/the-human-genome...
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It cites a paper presenting a "virus-first" perspective on the evolution of life:
Viruses and the placenta: the essential virus first view - https://onlinelibrary.wiley.com/doi/full/10.1111/apm.12485
> viruses were involved in most all major transitions of host biology in evolution
I'm learning as I read it, but quoting some passages that seem relevant:
> viruses are fully competent agents and editors of all host systems of instruction (DNA, RNA, epigenetic, translational etc.). Thus, they provide the host with new sources of instruction systems (not errors).
> In addition, they promote network formation by providing coherent societies (quasispecies populations) of agents able to edit host code content (and add new identity) in a diffuse, distributed manner, which promotes the creation of and editing of host regulatory networks.
> Thus, a viral role in the origin of the placental regulatory network can be expected. Viruses possess all the advantages of evolution relative to host: extreme genetic adaptability, extreme diversity, extreme numbers, extreme rates of genetic exchange, tolerance for ‘unfit’ variation, and the ability to reassemble from cryptic or ‘dead’ parts. They can transition between the chemical and living world.
> ..most initial genetic and selective events that transform host regulatory complexity are usually ‘pushed’ by virus action in a general direction of increasing complexity.
> Hence, we must always consider how any virus action on host will affect virus–host survival in their respective virosphere or virus habitat (e.g., reproductive tissue).
> A most significant development would be the emergence of a stable persistent relationship between virus and host as this represents a virus–host symbiosis that now protects the host form the same and often other lytic (disease causing) viruses.