Yakutsk in Russia has been investigated (Stone 2002), a nightmarish scenario
indeed. One only needs to be reminded that smallpox killed more than three million
people in a single epidemic when Hernando Cortéz and his conquistadores introduced smallpox to the New World (Patterson and Runge 2002).
28.3 Evolution of Viruses
Researchers generally agree that viruses evolved long before eukaryotes arose on our
planet as outlined by the “virus-first theory” (Forterre 2006). However, evidence
exists that viruses have multiple evolutionary origins (Durzyńska and GoździckaJózefiak 2015; Koonin and Yutin 2018). Were viruses able to live an independent
life in the past, meaning, were they once alive? This debate received new attention
with the recent discovery of giant viruses or giruses (Colson et al. 2017). Some of
these recently detected giant DNA viruses (e.g., Mimivirus, Mamavirus,
Pandoraviruses, and Molliviruses among others), which replicate in the cytoplasm
of an infected host, are considered to be ancestors of modern eukaryotes, explaining
the formation of the nucleus which have striking analogies to viral factories in size
and structure (Abergel et al. 2015; Forterre and Gaïa 2016). The authors also stated
that the comparable large genome of giruses might have provided new genes to
proto-eukaryotes and archaea via transduction or when virus became dormant in the
host. Interestingly, DNA of giruses encodes for proteins previously only known
from living cells, particularly those supporting the translation process which opens
the door of introducing a fourth domain of life (Hurst 2000; Schulz et al. 2017).
Researchers also considered the “reduction theory,” investigating the possibility
that RNA viruses originated from RNA containing cells by regressive evolution
(a new version of the reduction theory [Forterre 2006]). This concept is not new. We
know that some members of the domain bacteria have reduced their genome and
became obligate parasites, such as Rickettsia or Mycoplasma spp. (Meseguer et al.
2003; Blanc et al. 2007). Chloroplasts and mitochondria were once independent
prokaryotic cells which led to the endosymbiotic theory (Zimorski et al. 2014). But
what about ancestors of viruses? No fossil record exists, but we know that the most
primitive self-replicating structures relate to ribozymes, quasi-species RNA, that can
act as catalysts. They don’t encode proteins, lacking the genetic code, but exhibit
structural information in form of hairpin loops, able to interact with other molecules
forming hydrogen bonds with amino acids and other complex molecules (Marek
et al. 2011; Hayden et al. 2018). Reverse transcription as we know from certain RNA
viruses can be performed by ribozymes performing the same function that we know
from the enzyme reverse transcriptase, which led to the idea that RNA viruses
derived from complex ribozymes in the RNA world, being the oldest form of viruses
on Earth. Newest virological studies based on the gene coding for the viral
RNA-dependent RNA polymerase (RdRP), the only gene that all known RNA
viruses share, revealed that dsRNA viruses evolved from +ssRNA viruses on at
least two independent occasions, whereas -ssRNA viruses evolved from dsRNA
618
A. Lauer
indeed. One only needs to be reminded that smallpox killed more than three million
people in a single epidemic when Hernando Cortéz and his conquistadores introduced smallpox to the New World (Patterson and Runge 2002).
28.3 Evolution of Viruses
Researchers generally agree that viruses evolved long before eukaryotes arose on our
planet as outlined by the “virus-first theory” (Forterre 2006). However, evidence
exists that viruses have multiple evolutionary origins (Durzyńska and GoździckaJózefiak 2015; Koonin and Yutin 2018). Were viruses able to live an independent
life in the past, meaning, were they once alive? This debate received new attention
with the recent discovery of giant viruses or giruses (Colson et al. 2017). Some of
these recently detected giant DNA viruses (e.g., Mimivirus, Mamavirus,
Pandoraviruses, and Molliviruses among others), which replicate in the cytoplasm
of an infected host, are considered to be ancestors of modern eukaryotes, explaining
the formation of the nucleus which have striking analogies to viral factories in size
and structure (Abergel et al. 2015; Forterre and Gaïa 2016). The authors also stated
that the comparable large genome of giruses might have provided new genes to
proto-eukaryotes and archaea via transduction or when virus became dormant in the
host. Interestingly, DNA of giruses encodes for proteins previously only known
from living cells, particularly those supporting the translation process which opens
the door of introducing a fourth domain of life (Hurst 2000; Schulz et al. 2017).
Researchers also considered the “reduction theory,” investigating the possibility
that RNA viruses originated from RNA containing cells by regressive evolution
(a new version of the reduction theory [Forterre 2006]). This concept is not new. We
know that some members of the domain bacteria have reduced their genome and
became obligate parasites, such as Rickettsia or Mycoplasma spp. (Meseguer et al.
2003; Blanc et al. 2007). Chloroplasts and mitochondria were once independent
prokaryotic cells which led to the endosymbiotic theory (Zimorski et al. 2014). But
what about ancestors of viruses? No fossil record exists, but we know that the most
primitive self-replicating structures relate to ribozymes, quasi-species RNA, that can
act as catalysts. They don’t encode proteins, lacking the genetic code, but exhibit
structural information in form of hairpin loops, able to interact with other molecules
forming hydrogen bonds with amino acids and other complex molecules (Marek
et al. 2011; Hayden et al. 2018). Reverse transcription as we know from certain RNA
viruses can be performed by ribozymes performing the same function that we know
from the enzyme reverse transcriptase, which led to the idea that RNA viruses
derived from complex ribozymes in the RNA world, being the oldest form of viruses
on Earth. Newest virological studies based on the gene coding for the viral
RNA-dependent RNA polymerase (RdRP), the only gene that all known RNA
viruses share, revealed that dsRNA viruses evolved from +ssRNA viruses on at
least two independent occasions, whereas -ssRNA viruses evolved from dsRNA
618
A. Lauer