Today, about four billion people--more than half the world's population--live in cities. This number could rise to seven billion by 2050. Cities face big challenges, including threats from climate change, food insecurity, a lack of clean water and rapid population growth, but they are also places where innovation and sustainability can thrive.

Cities: How Humans Live Together travels through time to explore questions like When and why did cities form? How did people access food and water? Where did they go to the bathroom? Peek into the past to see how cities have changed through time and explore what could make cities more sustainable and welcoming for today and tomorrow.