This article appears in the Witness section of the spring 2020 issue of the New Humanist. Subscribe today.

It is not often we read about temperatures declining these days, but a group at Stanford University published a paper at the start of this year documenting just this phenomenon. It wasn’t about the temperature of our planet, but of the modern human body.

The study, published in the journal ELife, claims to document a decline in the average human body temperature in the US of nearly 1°C over the past 200 years. If true, this calls into question the widely held 37°C norm that was first defined by a German doctor, Carl Wunderlich, in 1851. The analysis used nearly 700,000 human thermometer readings across three large studies. The earliest data was for US Civil War soldiers, the most recent study was conducted between 2007-17.

These observations are more than a curiosity. They could aid our understanding of the astonishing changes in lifespan and health that have occured across much of the globe since the 19th century.

The authors note a number of possible explanations. These include the prevalence of temperature controls in modern buildings, meaning our metabolisms work less hard than when they had to compensate for extreme hot or cold environments. The most compelling explanation is the huge reduction in chronic infection that has occurred across wealthier nations. In the 19th century, two to three per cent of the US population were living with tuberculosis – a thousand times the prevalence of today, with the mass availability of antibiotics in many parts of the world. (The authors weren’t able to look at data from regions where infectious disease is still a significant threat and air conditioning is less common.)

The validity of these findings has been questioned by some. It is impossible to truly control for all possibilities when comparing data across such varied time periods, such as differences in the mode of temperature taking (armpit vs oral) and the characteristics of different study participants. Whatever the case, these datasets represent an exciting opportunity to consider the potential for human evolutionary change over modern time periods.