The origin of viruses is still under debate within the scientific community – however evidence suggests viruses have existed for at least 3.5 billion years

• Viruses have since evolved alongside and in tandem with other living organisms via a process known as coevolution
  

The fact that the genetic code is universal suggest that the origins of viruses and cells are connected

• The virus-first theory states that due to their simplistic structure, viruses may have predated cells as the first replicating units 
  

• The escape theory suggests that viruses originated in cells as transposable genetic elements that became encapsulated and able to be spread
  

• The regressive theory claims that viruses represent the remnants of a symbiotic cell that progressively lost cellular components
  

The large diversity of viruses suggest that there may have been independent origins for different viruses

• The characteristics shared by all viruses may be the result of convergent evolution (all viruses are obligate parasites and hence experience common selection pressures)
  

The determination of viral origins is hampered by the fact that viruses are too small and fragile to fossilize (evolutionary evidence is instead based on genomic analysis)