Humans have a skewed view of viruses because we only notice them if they cause disease. In reality, however, viruses are much more than just pathogens. In fact, they play a crucial role in the evolution of life. Viruses were not only the probable precursors of the rst cells, but they have helped shape and build the genomes of all species, including humans. Viruses are present in every species and every ecological niche and aect every organism. ey even inuence the global climate by regulating the population densities of microorganisms-and thereby nutrient availability-in the oceans. We are the invaders of the viral world, not vice versa. Viruses have also been a major factor for the evolution of all life. ey helped build the genomes (A genome contains the complete set of genetic instructions, needed to build an organism). Almost 50% of our genome is comprised of retro elements. If the shortest retro elements, 500,000 long terminal repeat promoters, were once full-length retroviruses, they would add up to the size of our genome. Are we therefore the descendants of viruses? is is impossible to prove, because viral footprints disappeared with time.