Origin of Viruses
- Viruses can infect nearly all living species and are estimated to have been on Earth 3.5 billion years before humans evolved
- There is evidence that viruses evolved alongside other species, a process known as coevolution
- Around 8 % of the human genome contains small segments of viral DNA thought to be left over from ancient infections
- These DNA fragments are called endogenous retroviruses (ERVs) and have been passed along and modified over millions of years of evolution
- The origin of viruses is still under debate in the scientific community and among virologists
- One issue is that viruses are not found in fossils so there is limited evidence for their evolution
There are three key theories as to the origin of viruses:
Escape theory
- Viruses arose from genetic elements, such as DNA and RNA, that gained the ability to move between cells
- These genetic elements became surrounded by an outer boundary forming a virus particle
Regressive/reduction theory
- Viruses are remnants of cellular organisms or were once small cells that became parasites of larger cells
- Over time the cellular structures that were no longer needed were shed , leaving behind just viral structures
Virus-first theory
- Viruses predate their current cellular hosts
- During evolution we expect simpler organisms to give rise to more complex organisms, so the simple nature of virus particles could indicate that viruses evolved first
Theories of virus origin diagram
There are multiple theories for the origin of viruses
- Viruses are diverse and this diversity suggests that there may have been different origins for different viruses
- It is possible that all of the above theories or correct, or indeed that none of them are correct and that a different process occurred
- There are some features that are common among many viruses which indicates that convergent evolution may have occurred
- All viruses have a capsid protein outer boundary and no cytoplasm contained within this boundary
- All viruses have genetic material, either DNA or RNA
- The genetic code is the same as that used by other organisms
- All viruses are parasitic in nature and cannot replicate or or carry out their functions without a host cell