What an absurd statement the QI Elves recently tweeted: https://mobile.twitter.com/qikipedia/status/1222859993394962432?s=20. Appears the original source was https://elifesciences.org/articles/49555, which says
“Our investigation indicates that humans in high-income countries have changed physiologically over the last 200 birth years with a mean body temperature 1.6% lower than in the pre-industrial era.” - a percentage calculated relative to the freezing point of water… Steve Mold has a great piece on the absurdity of relativistic temperature statements: https://youtu.be/C91gKuxutTU