One hundred million years ago the earth’s climate was much warmer than today and vast inland seas stretched across entire continents. The land was dominated by charismatic megafauna that would one day serve as inspiration for Sir Arthur Conan Doyle’s novel The Lost World. This period is commonly referred to as the age of reptiles as our placental ancestors were barely visible. Yet it was during this period that something significant happened to them, something that would become a major part of who we are today. One hundred million years ago retroviruses infected our ancestors’ germline and hitched a ride through evolution into the present day where their DNA still exists in all of our genomes. In fact, such retrovirus infections occurred ~31 separate times in our evolution and these endogenous retroviruses (ERV’s) expanded and now make up an astounding 8% of our entire genome. This means that we owe ~240,000,000 bp of our DNA to these retroviruses!
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