Colours represent the first human arrival in a given area.
This map shows the distribution homo erectus as well as homo neanderthalensis across the earth. These species, and their habitation of the lands that their remains are found in, far predate homo sapiens. Despite not being taxonomically human, these homonins shared many traits with us. They used fire, they very likely had language, and they almost definitely had thoughts and feelings of their own. As far as we know, they were people. It only makes sense to include them on a map which attempts to show the times at which each part of the earth first became inhabited by people.