Many researchers think that the capacity for symbolic behaviors - such as art and language - is the hallmark of our species. A team working in South Africa has now discovered what it thinks is some of the best early evidence for such symbolism: a cache of ostrich eggshells dated to about 60,000 years ago and etched with intricate geometric patterns.
     Last year researchers reported pieces of ochre etched with what may be abstract designs and dated to 100,000 years ago at Blombos Cave on the Southern Cape; similar etchings dated to about 77,000 years ago were previously reported from Blombos. The Blombos team argued that this represented a continuous, long-standing symbolic tradition, but some archaeologists question whether such etchings qualify as true symbolic behavior.
     Since 1999, a team led by prehistorian Pierre-Jean Texier of the University of Bordeaux in France has been working at another site, the Diepkloof rock shelter, on the Western Cape about 180 kilometers north of Cape Town. This shelter contains evidence of several cultures that used stone tools typical of modern humans. Over the past few years, the team has uncovered fragments from an estimated 25 ostrich eggs in 18 archaeological layers dated by two separate techniques to between 55,000 and 65,000 years ago. The fragments are etched with several kinds of motifs, including parallel lines with cross-hatches and repetitive non-parallel lines, the team reported in the Proceedings of the National Academy of Sciences.
     Moreover, the team found that some of the patterns seem to have changed over time. The hatched-band motif is found only in the earlier 12 layers at Diepkloof and then disappears. The team also found a few eggshell fragments that appeared to have been pierced with a tool to make a hole in the top part of the egg. The researchers suggest that the large eggs, which had a volume of about 1 liter, might have been used as water containers, as hunter-gatherers in South Africa's Kalahari Desert have used ostrich eggshells during historical times. The Kalahari people decorated the eggshells with engravings to indicate either who owned them or what they contained.
     The team tried themselves to recreate the markings using pieces of flint.
"Ostrich egg shells are quite hard. Doing such engravings is not so easy. You have to pass through the outer layer to get through to the middle layer," Dr Texier explained. The team's experiments also suggest that the colouration of the fragments is natural and not the result of the application of pigments. The team concludes that the discovery "represents the earliest evidence of the existence of a graphic tradition among prehistoric hunter-gatherer populations."
     But is this really symbolism? Yes, says Stanley Ambrose, an archaeologist at the University of Illinois, Urbana-Champaign. "The diversity of design motifs is impressive. It is an important new addition to the corpus of evidence for the development of modern human symbolic and artistic expression in Africa." Others aren't so sure. The engravings could have been done for aesthetic purposes unrelated to symbolism, says Thomas Wynn, an archaeologist at the University of Colorado, Colorado Springs. Researchers need to demonstrate that such engravings "require symbolic thinking," rather than simply assuming that all such etchings are symbolic, says Wynn. Arguably the earliest examples of conceptual thought are the pieces of shell jewellery discovered at Skhul Cave in Israel and from Oued Djebbana in Algeria. These artefacts are 90,000-100,000 years old.

Sources: Science Magazine (1 March 2010), AlphaGalileo, BBC News (2 March 2010)