Six hundred million years ago, fever appeared in animals as a response to infections: the higher body temperatures optimized their immune systems. At the time, virtually all animal species were cold-blooded. They had to sit in warm patches of habitat for extended periods of time to achieve fever-range body temperatures. For Michael Logan, a Tupper Fellow at the Smithsonian Tropical Research Institute in Panama (STRI), pathogens may be the reason why warm-blooded creatures first emerged.
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Source: Phys.org