Where did they come from? Could some of them have arrived from meteors or comets that occasionally hit the earth? I suppose some of them could have originated here, but I think some of them came from another planet or something.
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You are not alone in thinking this, and it is the subject of a number of movies, but the majority of scientists believe that viruses originated on Earth. The simplest evidence for this is that viruses are fairly specific parasites of organisms on Earth and do not jump between kingdom or even phyla or classes of organisms. This specificity suggests that the viruses evolved along with organisms here. If a virus can't move between hosts of different classes, it seems highly unlikely that they would be able to infect organisms from different planets.
Viruses are highly matched to their hosts because they use much of their host's metabolic processes. While some processes such as the genetic code are the same throughout all organisms on Earth (and among viruses as well), others like the promoters that identify when to express genes are different between different types of organisms. So when we use a virus or a plasmid to express a gene from a different organisms, such as expressing human insulin in bacteria, the control regions need to be changed from human ones on the human insulin gene to bacterial control regions that are placed in front of the insulin gene coding region. In this same line, bacterial viruses have bacterial type control regions, human viruses have human type control regions and so on. Again, this matching of virus to host suggests that viruses evolved here.
You can read about the different hypotheses of viral origins here http://en.wikipedia.org/wiki/Virus#Origi…
Viruses are highly matched to their hosts because they use much of their host's metabolic processes. While some processes such as the genetic code are the same throughout all organisms on Earth (and among viruses as well), others like the promoters that identify when to express genes are different between different types of organisms. So when we use a virus or a plasmid to express a gene from a different organisms, such as expressing human insulin in bacteria, the control regions need to be changed from human ones on the human insulin gene to bacterial control regions that are placed in front of the insulin gene coding region. In this same line, bacterial viruses have bacterial type control regions, human viruses have human type control regions and so on. Again, this matching of virus to host suggests that viruses evolved here.
You can read about the different hypotheses of viral origins here http://en.wikipedia.org/wiki/Virus#Origi…
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Some evolutionists hypothesize that viruses ‘evolved’ from bacteria by natural selection. In this process, as they become parasites, they lost all the complex protein structures that bacteria require. Others hypothesize that viruses were the first form of life, and that bacteria evolved from them (as did all other life). The fatal problem with this theory is that viruses are not living, and in order to reproduce and to make ATP, they require all of the complex cellular machinery present in bacterial cells. Other scientists speculate that a reverse symbiosis occurred, and that viruses arose out of cell parts such as bacterial plasmids and other organelles, and eventually evolved into separate forms of life.
So far evidence is lacking for each of these theories. Both bacterial plasmids and viruses contain the nucleotide sequences required to initiate replication. While these structures are necessary for the function of each, this does not prove either’s phylogeny. Further, all ‘ancient’ viruses so far discovered in ‘ancient’ amber and other places are fully developed, functional viruses.
The view now emerging of the normal relationship between viruses and genes is not so much a host/invader relationship, but a relationship more akin to bees carrying pollen from flower to flower, thus causing cross-fertilisation. Viruses carry not only their own genes, but also those of other creatures as well, especially those of bacteria. Although bacteria pass genetic information to each other using several processes such as pili transfer (see below), viral transfer is now known to be critically important.
So far evidence is lacking for each of these theories. Both bacterial plasmids and viruses contain the nucleotide sequences required to initiate replication. While these structures are necessary for the function of each, this does not prove either’s phylogeny. Further, all ‘ancient’ viruses so far discovered in ‘ancient’ amber and other places are fully developed, functional viruses.
The view now emerging of the normal relationship between viruses and genes is not so much a host/invader relationship, but a relationship more akin to bees carrying pollen from flower to flower, thus causing cross-fertilisation. Viruses carry not only their own genes, but also those of other creatures as well, especially those of bacteria. Although bacteria pass genetic information to each other using several processes such as pili transfer (see below), viral transfer is now known to be critically important.