Hairless skin among humans first evolved as a way to keep cool.
Prior to the evolution of mostly naked skin, humans were furry creatures, not unlike chimpanzees now, said Nina Jablonski, professor of anthropology at Penn State University.
About 1.5 to 2 million years ago, early humans, who were regularly on the move as hunters and scavengers, evolved into nearly hairless creatures to more efficiently sweat away excess body heat, said Jablonski.
Later, humans began to decorate skin to increase attractiveness to the opposite sex and to express, among other things, group identity.
"We can make a visual impact and present a completely different impression than we can with regular, undecorated skin," Penn University quoted Jablonski as saying.
Jablonski said both males and females used skin decoration to become more attractive to the opposite sex.
Women used make-up to increase the size of their eyes, a cue considered attractive in most cultures.
Males in some cultures used skin decoration as a way to bring out facial features to appeal to women or to look more menacing and warrior-like.
"Usually it is something with deep meaning," Jablonski said.
Jablonski said that she and other researchers based their estimate on when humans evolved hairless skin on the study of the fossil record and an examination of the molecular history of genes that code proteins that help produce skin pigmentation.
"We find a lot of evidence of when humans began to lose hair based on molecular genetics," said Jablonski.
These findings were presented at the annual meeting of the American Association for the Advancement of Science in Boston.