From Tracy Vence at The Scientist:
Foamy-like endogenous retroviruses (FLERVs), which are found in fish and amphibian genomes, may have originated with their vertebrate hosts more than 450 million years ago, according to a study published in Nature Communications this week (January 10). … “They date back to the origins of vertebrates, and this gives us the context in which we should consider their present-day activity and interactions with their hosts.” More.
Maybe they were all part of a package with the fish and amphibians who hosted them?
See also: Endogenous retroviruses made us human? Note the role of supposed junk DNA.
and
Stasis: When life goes on but evolution does not happen very much
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