Humans began to live in urban settlements about 7 thousand years ago. As humans continued to evolve over the millennia, so too did our cities.
Now that the majority of humans live in cities, we're going to be confronting a new set of problems in urban life. For one thing, natural disasters in cities can cause much greater numbers of fatalities than in sparse, rural communities. So the cities of tomorrow will need to be robust against many kinds of disaster, from earthquakes and floods, to radiation bombardment. It's possible that many cities will built partly under ground, and partly under water. They might even be built inside a single building surrounded by farms. Not only will such structures allow us to conserve space, but layers of earth and water are excellent protection against radiation.
Many future-minded designers and architects believe that cities of the future will survive these kinds of disasters partly by changing the materials we use to build. Instead of dead trees, we'll use living ones, combined with genetically modified algae and other plants that could purify water and air, as well as provide energy. In a recent book,Rachel Armstrong has described what she calls "living architecture," where cities are built with semi-living materials that can repair their own cracks and heal themselves when damaged by a quake or just regular wear and tear. She proposes rescuing Venice from drowning by engineering a living reef underneath the city. It would be made with calcium-extruding protocells that latch onto the city's existing piles, strengthening them and attracting living creatures whose shells will eventually turn into a true ocean reef.