1 Why Cities? As an archaeologist, my favorite place in Rome is not the Colosseum or the Forum. It''s the ancient trash dump of Monte Testaccio. Right in the middle of the city, it is a giant mound of broken pottery where the ancient Romans threw away the containers used to ship wine and olive oil all around the Mediterranean. Each of those vessels was about half the height of a person and made of coarse clay that would have roughed up a stevedore''s hands. Their odd shape of two handles and a pointy base made them good for packing into a ship''s hold or standing upright on a sandy shoreline but very inconvenient for much else. After a cargo of them arrived at its destination on the bustling shores of the Tiber at the very heart of the Roman world, a few were reused and a few were recycled. Mostly, people poured out the contents and threw the containers away. Over the centuries, the pile of discards grew, with the result that one of the famous hills of Rome is actually not a hill at all but a human construction-a landfill, essentially.

Today Monte Testaccio is topped by trendy nightclubs and has been endlessly mined for construction, but there are still the remains of twenty-five million ancient containers poking up from the vegetation of the hillside. Now consider a very different metropolis. My favorite part of Tokyo? The backside of the Tsukiji fish market, the part that tourists don''t visit. Tsukiji is enormous, and the passageways are crowded with plastic buckets and barrels teeming with every kind of creature that you can imagine from the briny deep. Crabs attempt to crawl their way out of baskets, little fish are piled up in ice buckets, and great slabs of tuna glisten under the klieg lights. The market is open to everyone, with chefs and restaurant owners jostling with homemakers for a clearer view of the day''s catch. It''s a world without friendly chitchat, punctuated by the dangerous darting movements of souped-up forklifts that dodge their way in and out of the building and heap up their discards out back. Behind the market is an enormous dump of plastic-foam shipping boxes used to transport the globally sourced tuna, squid, and shrimp from each morning''s auctions.

The pile of containers is taller than a two-story building and so large that it is continually cleared by bulldozer. Some of the cartons are trampled and broken in the process, with bits and pieces that spill farther into the passageway. In between the endless runs of machinery, merchants and their helpers come to pick through the heaps of box fragments. Sorting through the pile to find ones that aren''t too broken, they carry them off to repack with fish or whatever else they''re selling. Ancient Rome and modern Tokyo are literally a world apart, but if we stand back and look at them as cities, they have identical characteristics. In addition to markets and trash, there are multistory buildings, long streets, sewer pipes, water mains, public squares, and a "downtown" zone of financial institutions and government offices. There are a thousand varieties of sounds and smells, competing with the weather and daylight that frame the skyline of the built environment. There are crowds of people-rich, poor, young, old, female, male, gay, straight, trans, abled, disabled, employed, students, jobless, residents, and visitors.

Production and consumption opportunities are scaled up in cities to provide not only more things but also more things per person, a completely ironic abundance given that urban residences tend to be much smaller than their rural counterparts. In the midst of so much abundance, the only solution is to cycle through possessions faster, turning everything into trash. It''s the act of discard that provides the most telling evidence of urban activity, whether it''s a broken potsherd from two thousand years ago or a fragment of a plastic crate that was shattered this morning. Once you start to look for the concentrated detritus of your own urban life, it''s everywhere: in the trash cans that bear the proud logo of the downtown business improvement district; in the Dumpster parked outside a building that signals a renovation taking place inside; in the garbage truck that obstructs your commute; in the legions of sanitation workers employed to sweep the streets and subways and haul away the accumulations of discards. Trash has a familiar rhythm and concentration. Holidays bring a hangover of extra-full trash bins; parades and festivals and summer weekends in the park are witnessed through their aftermath of overflowing containers. Whether directly or by proxy, an urban obsession with trash is everywhere, and once you start to look, you won''t be able to stop seeing it. Congratulations! You''re an archaeologist.

Moving your gaze upward, or to the side, you might notice that it''s not just trash that silently tells a story of urban life. Your own metropolis, even if it''s new, has many traces that reveal its history before you moved through its streets. Maybe it''s a bolt hole in the sidewalk where a telephone booth used to stand, or an out-of-use railroad track now embedded in the asphalt of a city street. Maybe it''s a building that has been updated once or twice, resulting in the pastiche of a Victorian facade with mirrored glass windows, or a modernist concrete structure fronted by flowers and cheerful painted windowsills. And maybe it''s a newly cut ditch in the street where you can see the layered pavements of prior years right up to the present. Buildings and streets and parks serve as a living map of variable time, a collection of structures that all exist simultaneously whether they were constructed a millennium ago, in your grandparents'' time, or last week. Your growing archaeological insights serve you well when looking not only at modern cities but also at the ancient cities that are found by the hundreds on nearly every continent, from famous ones such as Rome to not-so-famous ones with romantic names like Tikal, Tell Brak, and Xi''an. When we look beyond the rubble and ruins, what we unearth in our excavations of them rings true to the experiences that we have in our own cities: neighborhoods and streets, open plazas and grand buildings, lines of sight to the residences of the powerful, and marketplaces where people from all walks of life met their daily needs for food and fuel.

When we walk through the streets of an ancient city like Pompeii, we encounter an environment where everything makes sense, from the sewer grates and the narrow passageways between apartment buildings to the food stalls and the cocky ancient graffiti scribbled on the walls. Although there''s a popular impression that ancient cities were prone to collapse, the vast majority of the world''s first cities are still right underfoot in the biggest metropolitan areas today: not only Rome and Xi''an, but also London, Paris, Guangzhou, Mexico City, Tokyo, Baghdad, Cuzco, Cairo, Athens, Delhi, Istanbul . the list goes on. And those cities became interconnected with other cities that sprang up alongside them, growing into a global phenomenon that dominates the planet. Today, more than 50 percent of the world''s people live in cities, and that percentage will soon be larger. It''s predicted that by 2030, more than 50 percent of Africans, 60 percent of Chinese, 87 percent of Americans, and 92 percent of the residents of the United Kingdom will live in cities. In their layouts and constructions, ancient cities look so much like the ones we build for ourselves that it seems they should always have existed. And the growth and success of modern cities also suggest that humans thrive in urban locales.

But cities are not actually the natural condition of our species, nor did we humans need to develop cities in order to survive or to successfully colonize the world. For a million years, our ancestors had lived scattered across the landscape, housed in humble huts in everyone-knows-everyone villages. By the time cities were invented six thousand years ago, our ancestors had already done a good job of filling up all of the easy places to live and many of the difficult ones, too. They had a system of pathways to get across the land, and they had developed rafts and boats to get from place to place across the trackless water. They had moved out of caves and other natural shelters into huts that they built themselves out of stone, bamboo, or brick. People had a sophisticated repertoire of language, art, music, and dance to pass the time, and they had many ways of displaying their individual identities through ornaments and tattoos and hairstyles. They already had reverence for the dead, encoded in the placement of burial goods laid to rest with the deceased. There were plenty of objects for the living, too, because people had already invented all the essentials of life.

There were clothes to keep warm, plows to till the land, pottery and baskets to keep the harvest safe, and stone knives and bronze weapons to carve up food (and to keep enemies in check). There were domesticated plants for a steady supply of beer and bread, and domesticated animals as a ready source of milk, wool, transportation, and companionship. In sum, we had everything we needed for a successful life of small-scale farming that would still have allowed for population growth to cover the planet, one little village at a time. Clearly, that simple and straightforward village life wasn''t enough for our urban ancestors. Despite having everything that members of our species needed to survive, people wanted plenty of intangible things that they couldn''t get out there in the countryside: the thrill of a crowd, the excitement of new inventions and novel foods, and the tantalizing allure of meeting a romantic partner from beyond the confines of the village. Before there were cities, such experiences could be found only in ritual spaces that p.