inoculating and maybe conquering new hosts and territories (estimated 800 million/
day/m
2
), with numbers that are significantly higher compared to deposition of
bacteria (Reche et al. 2018). Their small size allows viruses to be swept up higher
than it can occur for larger organisms, leading to a longer residence time in the
atmosphere, and thus viruses potentially can be transported longer distances, which
possibly explains why genetically closely related viruses have been detected in very
distant and different ecosystems (Short and Suttle 2005).
Different Origin Hypothesis First, the fact that viruses can infect species in all
domains of life suggests that they have to be ancient (Cornish-Bowden and Cárdenas
2017; Gaia et al. 2018). Viral genome sequencing and comparisons revealed the vast
diversity of viral genomes with five classes of viral genes currently recognized.
These included dsDNA viruses (Baltimore classification group I, e.g., Herpesviruses), ssDNA viruses (Baltimore group II, e.g., Parvoviruses), dsRNA viruses
(Baltimore group III, e.g., Reoviruses), +ssRNA viruses (Baltimore group IV, e.g.,
Coronaviruses), -ssRNA viruses (Baltimore group V, e.g., Orthomyxoviurses). Two
additional classes included viruses that possess the enzyme reverse transcriptase
(ssRNA-RT, Baltimore group VI, e.g., Retroviruses, and dsDNA-RT viruses, e.g.,
Hepadnaviruses, Baltimore group VII) (Baltimore 1971; Simmonds et al. 2017). No
single phylogenetic marker has been detected that could serve as a marker for all
viruses. Recent research also confirmed that lateral gene transfer occurred among
unrelated viruses complicating taxonomical grouping (Diemer and Stedman 2012).
This observation led to the hypothesis that viruses evolved from a variety of different
ancestors and did not originate from one LUCA (Forterre 2017). This is also
supported by the diversity of different structures of capsids, shapes in general, and
mode of entry to the host cell, especially between phages and animal viruses, as well
as the existence of RNA and DNA genomes. Structural analyses of capsid proteins
have led to the current hypothesis that viruses formed at least during 20 independent
events (Krupovic and Koonin 2017). It is currently assumed that with a few
exceptions, viral proteins have no homologues in modern eukaryotic cells which
contradict the idea that viruses have pickpocketed cellular genes. Therefore, it is now
believed that viral genes originated in a so-called virosphere during viral gene
replication influenced by genetic shift, with many genes recruited from cell-lines
long extinct, or both (Iranzo et al. 2016; Zhang et al. 2018).
Unfortunately, the fossil record for agents smaller than microorganisms, such as
bacteria and fungi is non-existent, but the presence of viral agents can be determined
indirectly based on cytopathic effects on their host. For example, we know the virus
that causes smallpox, a now eradicated viral illness which haunted humanity for
hundreds of years and more, has left its marks on mummies such as Ramses V who
ruled from 1149–1145 BC as discovered in a tomb in Egypt in 1898 (Strouhal 1996).
It is being discussed if virions could be re-activated from mummified victims, then
spreading into the environment upon meeting a suitable host, thus allowing the
recurrence of epidemics with re-emerging and yet unknown viral entities. For
example, due to climate change, the wooden vault full of nineteenth-century smallpox victims mummified in the permafrost of the Arctic Circle near Koltsova, near
28 Viruses, Underestimated Drivers of Ecology and Evolution of Life
617
day/m
2
), with numbers that are significantly higher compared to deposition of
bacteria (Reche et al. 2018). Their small size allows viruses to be swept up higher
than it can occur for larger organisms, leading to a longer residence time in the
atmosphere, and thus viruses potentially can be transported longer distances, which
possibly explains why genetically closely related viruses have been detected in very
distant and different ecosystems (Short and Suttle 2005).
Different Origin Hypothesis First, the fact that viruses can infect species in all
domains of life suggests that they have to be ancient (Cornish-Bowden and Cárdenas
2017; Gaia et al. 2018). Viral genome sequencing and comparisons revealed the vast
diversity of viral genomes with five classes of viral genes currently recognized.
These included dsDNA viruses (Baltimore classification group I, e.g., Herpesviruses), ssDNA viruses (Baltimore group II, e.g., Parvoviruses), dsRNA viruses
(Baltimore group III, e.g., Reoviruses), +ssRNA viruses (Baltimore group IV, e.g.,
Coronaviruses), -ssRNA viruses (Baltimore group V, e.g., Orthomyxoviurses). Two
additional classes included viruses that possess the enzyme reverse transcriptase
(ssRNA-RT, Baltimore group VI, e.g., Retroviruses, and dsDNA-RT viruses, e.g.,
Hepadnaviruses, Baltimore group VII) (Baltimore 1971; Simmonds et al. 2017). No
single phylogenetic marker has been detected that could serve as a marker for all
viruses. Recent research also confirmed that lateral gene transfer occurred among
unrelated viruses complicating taxonomical grouping (Diemer and Stedman 2012).
This observation led to the hypothesis that viruses evolved from a variety of different
ancestors and did not originate from one LUCA (Forterre 2017). This is also
supported by the diversity of different structures of capsids, shapes in general, and
mode of entry to the host cell, especially between phages and animal viruses, as well
as the existence of RNA and DNA genomes. Structural analyses of capsid proteins
have led to the current hypothesis that viruses formed at least during 20 independent
events (Krupovic and Koonin 2017). It is currently assumed that with a few
exceptions, viral proteins have no homologues in modern eukaryotic cells which
contradict the idea that viruses have pickpocketed cellular genes. Therefore, it is now
believed that viral genes originated in a so-called virosphere during viral gene
replication influenced by genetic shift, with many genes recruited from cell-lines
long extinct, or both (Iranzo et al. 2016; Zhang et al. 2018).
Unfortunately, the fossil record for agents smaller than microorganisms, such as
bacteria and fungi is non-existent, but the presence of viral agents can be determined
indirectly based on cytopathic effects on their host. For example, we know the virus
that causes smallpox, a now eradicated viral illness which haunted humanity for
hundreds of years and more, has left its marks on mummies such as Ramses V who
ruled from 1149–1145 BC as discovered in a tomb in Egypt in 1898 (Strouhal 1996).
It is being discussed if virions could be re-activated from mummified victims, then
spreading into the environment upon meeting a suitable host, thus allowing the
recurrence of epidemics with re-emerging and yet unknown viral entities. For
example, due to climate change, the wooden vault full of nineteenth-century smallpox victims mummified in the permafrost of the Arctic Circle near Koltsova, near
28 Viruses, Underestimated Drivers of Ecology and Evolution of Life
617