Population distribution is affected by environmental factors, natural resources, proximity to water and the ability to grow or obtain food. Areas in which populations are small usually lack one of these four qualities. The availability of jobs also affects population. In order to build a thriving civilization, there are certain qualities that a geographic area must have. First the climate must be something to which people can adapt. Extremely hot or cold places are difficult in which to live as well as places with extreme weather. This is why large land masses such as Greenland and Antarctica are sparsely populated. Natural resources are also necessary in order for people to survive. People must be able to obtain the raw materials necessary to build places in which to live and work. They must also be able to maintain crops or grow livestock. Water is essential to human survival. This is why the world's earliest civilizations grew up around bodies of water. Water is a critical resource that facilitates life and can serve as a transportation medium. Water also irrigates crops and allows animals to thrive, which creates a food source for those who settle there. Lack of sufficient job opportunities may be the explanation for areas in which small populations have formed but have not significantly increased.