Ancient Viruses Shaped Ancestors

Much of the human genome carries DNA left behind by ancient viruses that infected our ancestors millions of years ago. Scientists estimate that about 5 to 8 percent of human DNA comes from endogenous retroviruses, which are viral sequences that became permanently embedded in the genomes of ancestral humans and were then passed down through generations. Some broader estimates say an even larger share of the genome may have a viral origin when other repetitive elements are included.

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