Blame this.
Shackleton Mold
This is some cutting-edge stuff, to be sure. Shackleton Mold get its name from where it was recently discovered: back in 1915 a set of photo negatives from the Ross Sea Party Antarctic expedition got enclosed in a block of ice, then left frozen for a century. Yes, 1915. Which is to say, several decades before the 1947 Invasion from Beyond that fundamentally altered our microbial ecosystem and made us vulnerable to the Greys‘ genetic attacks. Sure, we fended them off, eventually — but we’ve been doing repair work ever since. The Mold has thus been an absolute godsend: covert recovery specialists were able to get a sample of the stuff before it got contaminated, which gives us a baseline for what our DNA looked like before we all got infected.