"This is the earliest clear example of oil paintings in the world, although drying oils were already used by ancient Romans and Egyptians, but only as medicines and cosmetics", said Yoko Taniguchi of Japan's National Research Institute for Cultural Properties, leader of the research team.

The researchers went to the Bamiyan region of Afghanistan -- notorious for the demolition of Buddha statues by the Taliban.

Although the caves were also targeted for destruction by the Taliban, a number of paintings on their walls survived from the 5th to 9th century AD.

An international team of scientists deployed a variety of complex techniques to analyse the paintings, including infrared micro-spectroscopy, micro X-ray fluorescence, micro X-ray absorption spectroscopy and micro X-ray diffraction.

"On one hand, the paintings are arranged as superposition of multiple layers, which can be very thin," said Marine Cotte, a scientist at the Centre of Research and Restoration of the French Museums.

"The micrometric beam provided by synchrotron sources was hence essential to analyze separately each of these layers."

"On the other hand, these paintings are made with inorganic pigments mixed in organic binders, so we needed different techniques to get the full picture."

It emerged that 12 out of the 50 caves were painted with oil paints, probably derived from walnut or poppy seed drying oils.

Apart from the oil-based paint, some of the layers were made of natural resins, proteins, gums, and occasionally a resinous, varnish-like layer.

The presence of proteins could indicate the use of hide glue or even egg.

As well as rewriting the history of art, the findings are a welcome addition to the otherwise scant knowledge of the Silk Road's artists.

"Due to political reasons, research on paintings in Central Asia is scarce," Taniguchi said.

"We were fortunate to get the opportunity from Unesco, as a part of conservation project for the World Heritage Site at Bamiyan, to study these samples and we hope that future research may provide deeper understanding of the painting techniques along the Silk Road and the Eurasian area."