Why is the distribution of the worlds Population uneven?

please could somebody point me in the right direction, it would be a big help. thank you!

Most people live in regions where there are abundant natural resources such as water and fertile soil. Deserts, the Sahara in northern Africa and much of Australia, for example, cannot support many people. Also note this map. Why is population so sparse in the far northern regions of the Earth?

http://soils.usda.gov/use/worldsoils/mapindex/popden.jpg