The work was created by Neanderthals, close relatives of modern humans, who until now had been considered incapable of abstract thought and expression.
"Creating paintings or carvings in caves is seen as a cognitive step in human development," said Joaquín Rodríguez-Vidal of the University of Huelva – one of the researchers whose study of the cave was published in the Proceedings of the National Academy of Sciences on Monday.
The discovery is "a major contribution to the redefinition of our perception of Neanderthal culture", prehistorian William Rendu of the French National Centre for Scientific Research told the Wall Street Journal. "It is new and even stronger evidence of the Neanderthal capacity for developing complex symbolic thought."
The work, uncovered in 2012 and measuring about one square metre, consists of eight lines cut deep into the rock that is arranged in two groups of three long cuts and two shorter ones.
Rock engravings in Gibraltar could be proof that Neanderthals were more intelligent than previously thought. Photograph: Stuart Finlayson/AP
What the engraving signifies is open to conjecture. "At this point we can only guess at its meaning," said Rodríguez-Vidal. "However, the engraving in the cave is the first directly demonstrable example of an abstract work, carried out consistently and with care and requiring prolonged and concentrated work, that has been produced in a cave."
Found alongside the engravings were 294 stone tools in undisturbed sediment dating back 39,000 years – about the time when Neanderthals became extinct – meaning the art below it must be older.
The tools are made in a signature Neanderthal style of a type that has never been found at a modern human site, the researchers say.
The Neanderthals reached Europe from Africa some 300,000 years ago.
(from http://www.theguardian.com/world/2014/sep/02/neanderthal-abstract-art-found-gibraltar-cave )