Something appears to be changing the temperature of the average human body, and it might be tied to our modern lifestyles.

Over the years, it seems as though people in the United States and the United Kingdom have grown steadily colder, with body temperatures coming up short of the usual 98.6 degrees Fahrenheit, or 37 degrees Celsius a generally accepted average.

New research on the Tsimane, a relatively remote indigenous tribe in Bolivia, suggests this trend is not a fluke, nor is not simply a product of high-income living. Instead, it appears to exist even in rural and tropical areas, where healthcare is minimal and infections are widespread.

When anthropologists first began working with the Tsimane in 2002, the average body temperature among adults was an average of 37 C – exactly what was measured in Europe two centuries ago.

In just sixteen years, that measurement had dropped to 36.5 C (36.53 for women and 36.57 for men), a rapid decline of 0.03 C each year.

"In less than two decades we're seeing about the same level of decline as that observed in the US over approximately two centuries," says anthropologist Michael Gurven from the University of California Santa Barbara.

The results come with a relatively high degree of confidence. The analysis is based on a large sample of 5,500 adults and about 18,000 long-term observations, taking into account numerous other factors that might influence a person's body temperature.

No matter how researchers sliced up the results – even when they only analysed completely healthy adults – the decline was still there.

Despite coming from a low-income population, the findings largely match a recent study in the US, which found "humans in high-income countries currently have a mean body temperature 1.6 percent lower [36.4°C] than in the pre-industrial era."

But the study among Americans was only conducted in a single population, and it couldn't explain why the decline happened.

Anthropologists working with the Tsimane were able to dig into more of the details. Researchers had access to clinical diagnoses and biomarkers of infection and inflammation for each and every patient in the study, which means they could test a bunch of different factors.

One of the currently leading hypotheses is that improved hygiene, clean water, and better access to medicine have reduced the number of infections humans experience and thus lowered their temperature.

While some infections among the Tsimane were indeed linked to higher body temperatures, a reduction in infections alone wasn't enough to explain the steep decline.

"Declines might be due to the rise of modern health care and lower rates of lingering mild infections now as compared to the past," says Gurven.

"But while health has generally improved over the past two decades, infections are still widespread in rural Bolivia."

In the end, the team wasn't able to find any single explanation or "magic bullet" to explain the rapid decline in body temperatures, and they say it's likely a combination of factors.

It could be that people today are better conditioned, and our bodies don't have to fight as hard to ward off infection. It may also have something to do with greater access to antibiotics, vaccinations, or other medical treatments.

For more developed communities, it could even be tied to modern luxuries, like air conditioning or heating, which makes it easier for our bodies to maintain their internal temperatures.

"While Tsimane body temperatures do change with time of year and weather patterns, the Tsimane still do not use any advanced technology for helping to regulate their body temperature," admits Thomas Kraft, who studies ethnography at UCSB.

"They do, however, have more access to clothes and blankets."

While it's clear cultural shifts in the way we warm up and cool down could influence our average temperatures, there's no doubt a whole a lot more going on. We need far more research on various populations throughout the world to figure out exactly what is happening to our temperatures and why.

The study was published in Science Advances.