By Paul Rincon Science reporter, BBC News

The shells were probably parts of necklaces or bracelets (Image: Marian Vanhaeren/Francesco d'Errico)

The earliest known pieces of jewellery made by modern humans have been identified by scientists.

The three shell beads are between 90,000 and 100,000 years old, according to an international research team.

Two of the ancient beads come from Skhul Cave on the slopes of Mount Carmel in Israel. The other comes from the site of Oued Djebbana in Algeria.

The finds, which pre-date other ancient examples by 25,000 years, are described in the US journal Science.

The pea-sized items all have similar holes which would have allowed them to be strung together into a necklace or bracelet, the researchers believe.

It supports my thought that there are no great revolutions in the evolution of modern human behaviour - it is a gradual process Alison Brooks, George Washington University

All three shells come from the same genus of marine mollusc known as Nassarius; they were probably selected for their size and deliberately perforated with a sharp flint tool.

They represent a remarkable early expression of modern behaviour in the archaeological record, experts say.

"The interesting thing about necklaces and this kind of behaviour is that it is symbolic. When we wear items like this, we are sending a message," said co-author Professor Chris Stringer of London's Natural History Museum.

"The message may be that we are powerful, or wealthy, or sexy, that we're part of a particular group, or to ward off evil. They're not just decorative; we think they had a social meaning."

Remote locations

Chemical and elemental analysis of sediments stuck to one of the shells from Skhul showed that it came from ground layers dated to 100,000 years ago.

The style of tools at Oued Djebbana suggests the single specimen from this open-air site might be up to 90,000 years old.

Flint tools like this one may have been used to perforate the shells

The authors' case for the shells having been used as beads is based on the remote location of the sites where they were found and the nature of the perforations in them.

"The fact they are there at all means they were transported by people to [Skhul] cave; these are seashells and the sea was never that close to the cave," Professor Stringer told BBC Radio 4's Leading Edge programme. Similarly, Oued Djebbana is located about 200km (120 miles) from the Mediterranean Sea.

"We're confident these were artificially made. The position of the holes are exactly where people drill shells like this when they are making necklaces."

The objects provide a clear example of the complex, symbolic behaviour that would appear to set our species apart from the animal world.

Modern thinking

Up until recently, examples of modern behaviour before 50,000 years ago had eluded researchers, even though humans with modern-looking anatomy are known in the fossil record from about 195,000 years ago onward.

This had led some researchers to propose that modern anatomy and modern behaviour did not evolve in tandem.

The people at Skhul transported the shells from far away

Instead, they argued, a fortuitous mutation in the human brain may have triggered an explosion in human creativity 50,000 years ago, leading to a sudden appearance of personal ornaments, skilfully-crafted art, novel tools and weapons.

The discovery of 75,000-year-old Nassarius shell beads at Blombos Cave in South Africa challenged this idea. These beads even bore traces of red ochre, used as a pigment. Now the dates for beads from Skhul and Oued Djebbana further weaken the "cultural explosion" scenario, says Stringer.

Professor Alison Brooks, an expert in African archaeology at George Washington University, US, said the study was "very well researched".

"I am not surprised because I have long thought that the wide variety of bead types that we see during the Upper Palaeolithic in Europe had to have an antecedent. And this tradition is a very logical antecedent," she told the BBC News website.

"It supports my thought that there are no great revolutions in the evolution of modern human behaviour - it is a gradual process."

Cooking up

But the apparent antiquity of symbolic behaviour raises questions about the time it took for modern humans to expand into the rest of the world.

"There was a long period where modern humans survived in the African world and into part of the Near East, but never expanded into western Europe," Professor Ofer Bar-Yosef of Harvard University, US, told the BBC News website.

"I think you have a 'cooking' or 'brewing' period. Otherwise you have to explain, for example, why the industrial revolution in England took place around 1850 and rapidly expanded across the channel to Europe and then across the Atlantic to America.

"In fact, we know from historical records that the development of scientific methods and the development of machinery took about 200 years before there was a 'breakout'."

The marine shells from Skhul are held by the Natural History Museum in London, while the shell bead from Oued Djebbana is held by the Museum of Man in Paris.