The reason: chlorinated water and antibiotics.
That's the first hypothesis of Dr. Cecil Lewis. According to Lewis—who is leading a team of scientist hunting for bacterial DNA samples in mummy guts and cave soil across North and South America—these two factors "fundamentally changed human microbiomes."
Lewis believes that "the association between antibiotics and obesity is important to explore." Indeed, there's already research that indicates a link between the use of these medicines and obesity.
