It has been argued that species survival is the most pertinent question facing humankind. The truth of such an argument is highly debatable, but it is no doubt an important question regardless. Various obstacles stand in the way of human survival, notably limited resource availability, illnesses, pollution, and conflicting ideologies between civilizations. Both pessimistic and optimistic theories have been developed regarding humanity's ultimate fate.
Pessimists such as Neo-Malthusians contend that a growing population, complemented by depleting resources, will eventually lead to humankind' s peril. Such a process began with the so-called green revolution in agriculture, which created a foundation for a large population. As competition for resources increases, people more often act in accord with their own needs, as opposed to the needs of society. This "tragedy of the commons" pollutes the environment and depletes resources to the point where humanity can no longer sustain itself.
Optimists claim that human ingenuity can solve all problems created by a large population. They contend that population growth has provided for, rather than been adverse to, sustained development. They argue that life expectancy and standard of living have steadily increased, while infant mortality has decreased. 
While certain pessimistic (and some optimistic) theories have been discredited, some still make strong arguments. Only time will tell which is correct.