By Paul Rincon BBC Science

Signs carved into 8,600-year-old tortoise shells found in China may be the earliest written words, say archaeologists.

First attempt at writing on a tortoise shell

The symbols were laid down in the late Stone Age, or Neolithic Age.

They predate the earliest recorded writings from Mesopotamia - in what is now Iraq - by more than 2,000 years.

The archaeologists say they bear similarities to written characters used thousands of years later during the Shang dynasty, which lasted from 1700-1100 BC.

But the discovery has already generated controversy, with one leading researcher in the field branding it "an anomaly".

The archaeologists have identified 11 separate symbols inscribed on the tortoise shells.

The shells were found buried with human remains in 24 Neolithic graves unearthed at Jiahu in Henan province, western China.

The site has been radiocarbon dated to between 6,600 and 6,200 BC.

The research was carried out by Dr Garman Harbottle, of the Brookhaven National Laboratory in New York, US, and a team of archaeologists at the University of Science and Technology of China, in Anhui province.

"What [the markings] appear to show are meaningful signs that have a correspondence with ancient Chinese writing," said Dr Harbottle.

The Neolithic markings include symbols that resemble the characters for "eye" and "window" and the numerals eight and 20 in the Shang script.

"If you pick up a bottle with a skull and crossbones on it, you know instantly that it's poison without the word being spelt out. We're used to signs that convey concepts and I wouldn't be surprised if that's what we're seeing here," Dr Harbottle told BBC News Online.

Writing discovery from gravesite dig

However, Professor David Keightley, of the University of California, Berkeley, US, urged caution, particularly over the proposed link to the much later Shang script.

"There is a gap of about 5,000 years [between them]. It seems astonishing that they would be connected," he said.

He added that the link had to be proved more thoroughly.

But Dr Harbottle points to the persistence of sign use at different sites along the Yellow River throughout the Neolithic and up to the Shang period, when a complex writing system appears.

He emphasised that he was not suggesting the Neolithic symbols had the same meanings as Shang characters they resembled.

Professor Keightley added: "It's a puzzle and an anomaly; [the symbols] are remarkably early. We can't call it writing until we have more evidence."

Shaman rituals

He noted that there were indications the Neolithic culture at Jiahu may not have been complex enough to require a writing system.

But Professor Keightley did say the signs appeared to be highly "schematised" or stylised. This is a feature of Chinese written characters.

The character for "eye", similar to inscriptions in the latest find

Aggregations of small pebbles were found close to several of the tortoise shells.

The Jiahu researchers propose that the shells once contained the pebbles and were used as musical rattles in shamanistic rituals.

In one grave, eight sets of tortoise shells were placed above the skeletal remains of a man whose head was missing.

The shells come from graves where, in 1999, the researchers unearthed ancient bone flutes. These flutes are the earliest musical instruments known to date.

The research is published in the journal Antiquity.