What contemporary viruses tell us about evolution:
a personal view
Received: 25 September 2012 / Accepted: 14 February 2013 / Published online: 9 April 2013
Ó The Author(s) 2013. This article is published with open access at Springerlink.com
Summary Recent advances in information about viruses
have revealed novel and surprising properties such as viral
sequences in the genomes of various organisms, unex-
pected amounts of viruses and phages in the biosphere, and
the existence of giant viruses mimicking bacteria. Viruses
helped in building genomes and are driving evolution.
Viruses and bacteria belong to the human body and our
environment as a well-balanced ecosystem. Only in
unbalanced situations do viruses cause infectious diseases
or cancer. In this article, I speculate about the role of
viruses during evolution based on knowledge of contem-
porary viruses. Are viruses our oldest ancestors?
New technologies have changed our understanding of
viruses throughout the last ten years. Viruses are not pri-
marily pathogens, which is a biased view based on the
history of medicine. Most viruses do not cause diseases.
Viruses cause diseases if a well-established equilibrium,
which evolved over billions of years, gets out of balance.
A glance at some numbers may support the notion that
viruses are much more than just pathogens. There are 10
viruses on our planet, about 10 times more than bacteria.
There are only about 10
human beings – a small minority,
which makes us the invaders in the viral world, not vice
versa. They are present in the oceans, 10
per ml [96, 97],
in the soil, abundant in plants, and inside the human body.
Healthy humans consist of about 10
cells and harbor 10
bacteria  and an unknown number of viruses.
Bacterial information is our second genome, with a total
genetic complexity about 100 times greater than that of our
own genome. We are 99 % bacteria – with respect to the
total genetic information of our body. Viruses may be our
third genome [40, 112]. We harbor about 1.5 kg of bacteria
in our guts – 1,500 different types. Viruses are also a major
component in our guts. Two hundred types have been
detected in human gut samples based on similarities to
known viruses . Archaea and fungi are also present in
our guts [28, 80, 84, 85]. Thus, we are a superorganism as
well as a complicated ecosystem . Phages or bacte-
riophages are viruses of bacteria. They can lyse bacteria,
which gave them their name. A gut microbial gene cata-
logue is being established by ongoing metagenomic
sequencing [85, 112]. It was a surprise to learn that, instead
of a constant battle going on between viruses and cells
ﬁghting for dominance in our guts, the two are actually in a
well-balanced equilibrium .
It is the purpose of this article to discuss what we can
learn from contemporary viruses about their potential role
during the history of life and evolution – apart from
causing diseases. Their contribution to the development of
life, genome composition, genetic diversity, our environ-
ment, and our body will be evaluated here.
RNA and viroids
The beginning, when life started, was an RNA world, as
this is widely accepted today [37, 41, 42, 44]. We do not
really know how the ﬁrst nucleotides, the building blocks
Max Planck Institute for Molecular Genetics,
Ihnestr 63-73, 14195 Berlin, Germany
K. Moelling (&)
Institute for Medical Microbiology,
Gloriastr 32A, 8006 Zu
Arch Virol (2013) 158:1833–1848