Two centuries ago, German physician Carl Wunderlich established that the average human body temperature was 98.6 degrees Fahrenheit (37 degrees Celcius).
Now, scientists from the University of California in Santa Barbara have observed that since Wunderlich's prognosis healthy adults' average body temperatures have dropped to 97.9 degrees Fahrenheit (36.6 degrees Celcius) in the U.K. and 97.5 degrees Fahrenheit (36.3 degrees Celcius) in the U.S.
A similar drop in temperature was also recorded in the Tsimane indigenous people in the Bolivian Amazon, but this has happened in the last two decades alone.
The team's study was published in Science Advances at the end of October.
"In less than two decades we’re seeing about the same level of decline as that observed in the U.S. over approximately two centuries," said Michael Gurven, U.C. Santa Barbara professor of anthropology and one of the study's authors.
Why humans' body temperature has dropped
It was never understood why body temperatures dropped. Some believed it was due to fewer infections occurring thanks to cleaner water, vaccinations, and medical treatment. Some infections can lead to body temperatures rising, and with fewer of them around, the body's heat may have dropped naturally.
However, the Tsimane people don't have access to these commodities and infections are still widespread — so this can't be the main reason.
Another common belief is that with new comforts such as air conditioning and heating, our bodies don't have to work so hard to regulate themselves. However, once again, the Tsimane don't have access to these household appliances, although they do have more blankets and clothes.
Gurven noted, "We used the same type of thermometer for most of the study, so it’s not due to changes in instrumentation."
No single 'magic bullet' answer to declining body temperature
Ultimately, Gurven and his team acknowledged that it may not be down to one single "magic bullet" explanation, but rather "It’s likely a combination of factors — all pointing to improved conditions."
"One thing we’ve known for a while is that there is no universal ‘normal’ body temperature for everyone at all times, so I doubt our findings will affect how clinicians use body temperature readings in practice," explained Gurven. So there's no need to worry at this stage.
Learning about how body temperature shifts over time may provide clues to a population's overall health, and assist with calculating life expectancy, for instance.