Just like their prehistoric ancestors, modern humans face the constant challenge of extracting enough resources from the accessible environment to simultaneously support multiple generations of human life.  Until around 10,000 years ago, there were never more than a few million people living on Earth at once (Ponting, 1993).  For the next 9,700 years or so, the global human population grew to around 500 million.  Just as it began to stabilize at this new level, the Industrial Revolution allowed for another massive expansion of human population to its present day level nearing 7 billion (United Nations Department of Economic and Social Affairs/Population Division, 2009).  In order to do so, humans have devised numerous technologies that can access previously unused energy resources such that much larger crop yields can be attained from each square mile of fertile land (Gates, 2009) (see Figure 1).  

    The significant question, though, is for how long?  That is, have humans surpassed the carrying capacity of their planet in the long term?  Since most of the technological breakthroughs of the Industrial Revolution depend upon nonrenewable energy and these resources are running out, the short answer is yes (Adas, 1990).  On the other hand, Homo sapiens has adapted and survived in numerous harsh environments, and who is to say that it cannot adapt to the predicted changes in the global environment?  Since the number of humans living on Earth is still rising, the need for large-scale changes in energy use is of critical importance in maintaining a reasonable quality of life for people in the future.  By making educated guesses about what the world of the future will like look like, modern scientists can foresee potential threats and begin the long process of designing the technologies needed to sustain the future world.  

    Given the extremely high human population densities in large metropolises, the task of maintaining sanitary living conditions is extremely costly, but so too is failing to do so.  There is evidence that the number of new infectious diseases has been on the rise of recent, likely as a result of the still rising population density around the world.  Some researchers have described the megacities in the developing world as breeding grounds for new pathogens and mutant forms of old diseases (Jones et al., 2008; Tatem et al., 2008).  This, along with the projected shortages of potable water, could interact to allow for the simultaneous infection of millions of people in a small area (Brown et al., 1999).  

    It is not just humans, however, who are vulnerable to elevating global population densities.  Researchers modeling the effects of human population growth on global biodiversity generally agree that the expected population growth in the coming years has the potential to drastically reduce biodiversity on Earth (Luck, 2007), perhaps even triggering a second mass extinction of carnivorous megafauna (Cardillo et al., 2004).  Between 2020 and 2050, McKee et al. (2003) predict that the number of endangered species will double.  Since habitat destruction and pollution are anticipated to be among the largest causes of this trend, researchers can model future population distributions so as to minimize the detrimental effects of humans on the environment.  

    Although the growth of cities has long gone hand in hand with pollution, habitat destruction, and other perils of the modern world, recent projections made by UN researchers shed a new light on the potential role of cities in the future of humanity.  Given that we cannot act immediately halt population growth, research indicates that the further population of cities may be our saving grace in the years to come (Kariuki, 2007).  By packing more people onto each square mile of already urbanized land, we could contain the ecologically destructive effects of humans, and in doing so, prevent further destruction of non-urbanized land and the ecosystems it supports.  Humans are nowhere close to having all the technologies needed to sustain future megacities, but the world’s remaining biodiversity may hold the keys to a fully sustainable future.  If this is the case, then cities just may be the road to adulthood that Homo sapiens have no choice but to take.  

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