Scientists think our recent ancestors may have run hotter -- but why?

150 years ago, average human body temperatures appear to have been higher than now. Scientists are still exploring exactly why.

In the 1860s, the German physician Carl Wunderlich, was faced with a burning question. Was fever its own disease, or a symptom of an illness underneath?

At a time when thermometers were not routinely used, Wunderlich embarked on an ambitious mission. Over 15 years, he gathered the armpit temperatures of 25,000 patients, of all ages and both sexes, resulting in over a million readings.

From this vast dataset Wunderlich calculated an average, 37C (98.6F), for healthy adults. This became the benchmark for tracking normal human body temperature. If that rose above 38C (100.4F), it signalled fever, he wrote.

Even though others had already arrived at the same conclusion, Wunderlich's dataset was so large that his figure has since enjoyed an "exalted status" as a universal diagnostic tool, says Philip Mackowiak, a medical historian and infectious disease expert at the University of Maryland.

But more recent research suggests that the average normal body temperature has also changed over the last century and a half. Was the original baseline wrong? Or is a fast-changing world changing our biology?

Wunderlich "established a lot of what we know to be true" about body temperature variation says Julie Parsonnet, an infectious disease clinician at Stanford University.

From his dataset, which measured mostly working-class patients at the Leipzig hospital where he was medical director, Wunderlich discovered that normal temperature varies. He noticed that women have higher temperatures than men, temperature declines with age, and also increases with the time of day. 

Yet the average individual today has also now been shown to be cooler than their counterpart 150 years ago, with American men running about 0.59C lower than in the early 19th-Century.

A decade ago, Parsonnet's team analysed three large datasets involving multiple temperature readings from nearly 190,000 people in the United States, taken between 1862 and 2017. "The new normal [for adults] seems closer to 97.5F (36.39C)," she says.

Numerous studies, both before and since, echo Parsonnet's findings.

Nor is the downward trend limited to people living in developed countries. The average body temperature of Tsimanes, a semi-nomadic indigenous community living in the Amazon rainforest, used to hover around 37C in the early 2000s, says biological anthropologist Adrian Jaeggi at the University of Zurich in Switzerland. But in 2018, following two decades of rapid socioeconomic change, their average fell to 36.5C (97.7F).

Why have average body temperatures fallen?

A handful of explanations could explain the lower averages we see today. Parsonnet, for instance, believes, they are the result of healthier overall populations. In Wunderlich’s day, life expectancy was 38 years. Today, it's nearly double that.

"People didn’t live the healthy lives that we live now,"........

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