• Viruses have existed since living cells first evolved (2)
• They do not form fossils so molecular techniques have been used to compare the DNA or RNA of viruses to investigate their origin (3)
• There are 3 main hypotheses that explain the origins of viruses (4)

Regressive

• Viruses may have once been parasitic cells of other cells
• Over time they lost the genes not required by their parasitism
• aka. degeneracy or reduction hypothesis

Progressive

• Viruses may have evolved from bits of DNA or RNA that "escaped" from the genes of larger organisms
• Escaped DNA may have come from plasmids or transposons

Virus-first

• Viruses may have evolved from complex molecules of protein and nucleic acid at the same time the first cells appeared on earth
• They would have therefore been dependent on cellular life for billions of years

• Viruses are currently recognized as ancient and therefore have origins that predate the split of life into 3 domains (5)