(Fast Company) — Humans are an urban species. At least 3.5 billion of us, more than 50% of the global population, lives in cities. That will hit 70% by 2050. Yet our early evolution never prepared us for living cheek-to-jowl in vast concrete settlements. This is precisely the way most of us will spend the 21st century, so the effect this has on our brains is important. A study published in the journal Nature puts some hard data behind the question by testing how city living affects the way we cope with stress. Researchers scanned the brains of 32 healthy German volunteers from both rural areas and cities during a series of daunting math problems reinforced with negative comments. As it turns out, their brains behaved quite differently.