Therefore, viruses, as well as viroids, deserve to be considered as being part of the foundation stone of Earth’s biosphere. Ignored and despised, these representatives of the microbial world have been significantly involved in shaping the evolution and ecology of all living species on Earth, in the past, currently, and will continue so in the future. With the detection of giant viruses with large genomes including genes coding for replication, one might consider a fourth domain of life for these agents. Furthermore, evidence exists that many proviruses have evolved with their host, some being part of the so-called junk DNA that we find in prokaryotes and eukaryotes alike, whereas others shaped the evolution of multicellular species becoming even necessary for the survival of the host. Many theories of viral origin and evolution are debated and, even though the fossil record of viruses is basically non-existent in the traditional sense, modern phylogenetic sequencing and electron microscopy have revealed astonishing secrets of viral diversity regarding genome, shape, and size and elucidated many sophisticated mechanisms behind their ability to thrive and replicate as obligate parasites, enslaving their host’s replication machinery for their own purposes. Since then, approximately 15 million different species of organisms have evolved. Life evolved on planet earth about 3.5 billion years ago. At least some RNA viruses appear to have evolved from self-replicating molecules in the so-called RNA-world that researchers believe to have existed before DNA evolved billions of years ago. Diversity and Evolution of Life 1 Origin and Evolution of Life and Introduction to Classification BIOLOGY Notes The planet earth came into existence sometime between 4 and 5 billion years ago.
Viruses are the most understudied microbial agents on our planet their polyphyletic origin is still shrouded in mysteries.
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