A joint project with the Wildlife Conservation Society and the Center for International Earth Science Information Network (CIESIN) at Columbia University looked globally at the influence of humans on the planet and found that:

Analysis of the human footprint map indicates that 83% of the land’s surface is influenced by one or more of the following factors:  human population density greater than 1 person per square kilometer, within 15 km of a road or major river, occupied by urban or agricultural land uses, within 2 km of a settlement or a railway, and/or producing enough light to be visible regularly to a satellite at night. 

Read more about this project and access the GIS layers (in BIL and ArcInfo export .e00 formats):

• The Human Footprint and the Last of the Wild GIS Data 
• The Human Footprint Project Page