The human genome is about 3.1 billion nucleotides long.
8-10% of your genome is retroviral-ish. We will be conservative and go with 8%– thats 248,000,000 nucleotides of retrovirus.
The average retrovirus is about 10,000 nucleotides long. So we should have ~ 24,800 retroviral elements in our genome.
But there must be more than that, because there usually are not full-length retroviruses left in your genome (they would make retroviruses and kill you). Some have deleted genes, or deleted chunks of genes, or sometimes the entire genome is deleted except for a solo-LTR.
So, some number of retroviruses, greater than 24,800, have accidentally infected our germ-line over the course of human history? I know, I know, I always say “when you are dealing with a time-frame of billions of years, accidents happen all the time”, but so many accidents? It is also implied that accidental retroviral infection happened even more often than what we see now, because we dont see all the insertions that killed the offspring… it just boggles the mind.
A group at Oxford have found one way there can be so many retroviral elements in your (and other mammals) genomes, without requiring each and every insertion to be the result of an independent infectious event:
Though Ive said this before on ERV, I will say it again now: Your genome ‘wants’ ERVs to be silent. It ‘wants’ them to be junk. You have evolved numerous defenses against ERVs to keep them from expressing proteins. Since your genome cant forcibly and purposefully cut out an ERV after it has invaded, it does try to keep the genes silent via epigenetics. Then, nature takes its course, and the ERV accumulates mutations that render the genes defective or deleted, and the genome has done the best it can do to protect itself from a selfish gene.
What this group found, is that some ERVs lost their Envelope protein, in a seemingly preferential manner. Like, it didnt just degrade by random mutations. The ERVs lost Env first. When they did that, the ERVs essentially became prisoners. Without Env, there is no way they could ever leave their host cell again.
But its like the ERVs were making the best of their captivity. Sure, they lost Env, but they still had every other retroviral gene at their disposal. This means they could not escape their host cell, but they could reinfect it from the inside as much as they bloody wanted. Just to have fun personifying viruses, its like they were saying “Fine. You wanna trap me here? Imagonnna MESS YOU UP! Ur pretty genome aint gonna be pretty no more!”
Or alternatively, they got Stockholm Syndrome “You want to trap me here?… Okay… I LOVE IT HERE! This place is AWESOME! I am TOTALLY moving in OMFG I LOVE THIS GENOME!! I want to be a part of it forever and ever and ever and EVER! I WILL NEVER LEAVE YOU!!!!”
The end result? 22% of the viral families make up 80% of the retroviral elements in your genome. Which makes more sense than each and every retroviral element in your genome coming from a completely independent infectious event, or a reinfection event.
And really, I cant honestly blame the ERVs for not wanting to leave after they have infected us. Who wouldnt want to be trapped forever in a beautiful, intelligently designed, specially created genome like ours? (and 38 other species of mammal studied in this paper.)