By Richard Black in Boston Science correspondent

Archaeologists have come up with an intriguing new theory on the origins of jewellery.

Some of the shells would have been from rare species

They think that at some time in the past, our ancestors started decorating themselves with primitive jewellery and beads in order to give themselves a sense of identity. The idea has been discussed at the annual meeting of the American Association for the Advancement of Science in Boston.

The researchers have discovered that certain ancient humans started wearing beads and jewellery at the same time in three different locations in east Africa, the eastern Mediterranean coast, and south-eastern Europe.

In these three sites, the first jewellery appeared around 40,000 years ago - but not at places in between. Dr Mary Stiner, from the University of Arizona, says our ancestors were quite selective about what they wore.

Group position

"The main emphasis is on shell beads," she told the BBC.

"And these are small marine shells, very attractive ones of very uniform size, and people were very picky about what they would select. It was not just any shells, and quite often it was species that were relatively rare."

Dr Stiner believes that shell beads became popular as the human population grew. With more people around, it became more important for individuals to be easily recognised.

Beads and decorations were a way of expressing individual identity and of determining which social group a person belonged to, or their status within that group.

Big cities

"The thing about ornaments is that they say a lot about who you are - both as an individual and in terms of your group affiliation.

"It becomes that much more important to maintain and define social boundaries: who is in your group, who is not; who is a suitable marriage partner, who is not; who has passed through certain age-dependent or sex-dependent initiation rights."

Dr Stiner suggests that the same link between population size and decoration exists today: the more tightly people are packed together in modern cities, the more elaborate their adornments.

"You just have to visit any major city in the world to see that the more people are interacting, the faster the evolution of style and the more heightened the visual appearance of expressions of style become."