We may need to relearn the notion that a body temperature of 98.6 degrees is optimal. A German physician in 1868 first identified this number.

But something funny has been happening to us humans in the subsequent 152 years.  Some of us are getting cooler.

A recent study of an isolated Amazonian population known as the Tsimane (chee-MAH-nay) lends credence to the growing scientific consensus that human body temperatures are decreasing. It’s looking more and more likely that normal is probably in the mid- to upper-97-degree range.

The heat we generate varies because of a host of factors, including time of day, our age, overall health and weight. Previous research has shown that the average body temperature of Americans is now about 97.5 degrees. A U.K. study of 35,000 adults reported a comparable number — 97.9. That’s the same as seen in a 16-year study of more than 5,000 people in the Tsimanes community.

What makes these new findings so intriguing to researchers is that they were able to precisely document how quickly temperatures were dropping — about nine-hundredths of a degree annually.

So, just what in human physiology has changed? In previous work, it’s been suggested we’re experiencing fewer infections and better health than days gone by.

But detailed data from the Tsimanes indicate it’s more than that one explanation, as infection alone didn’t account for the decrease. Improved conditions generally, from access to better clothing to anti-inflammatory drugs like ibuprofen, are seen as possibly key.

More study is needed, but it seems like the mystery of changing body temperatures is just heating up.