From prehistoric rock art to Beyonc's pregnancy photographs, symbolism and art are a key aspect of human behavior and have been since the very beginningbut how it evolved has so far mystified scientists.

To shed light on this facet of human history, researchers writing in Proceedings of the National Academy of Sciences conducted a variety of experiments on ochre and ostrich eggshell fragments collected from Blombos Cave and Diepkloof Rock Shelter, both in South Africa, aged between 52,000 and 109,000 years old.

The choice of Blombos Cave and Diepkloof Rock Shelter came down to the fact that they have artefacts displaying engraving practices throughout a 30,000 year period, enabling scientists to determine how they have changed over an extended period of time. The oldest pieces feature simple patterns with parallel lines, but become more complex, shifting to cross-hatchings displaying greater symmetry, as time wore on.

This trend towards more intricate patterns may show how the images evolved into more effective "tools of the mind," the researchers say. In contrast to instrumental tools, like stone axes, which are used to change the environment, tools of the mind serve cognitive processes, such as communication and aesthetic enjoyment. These scratchings could be the prehistoric equivalent of a bare brick feature wall or Picasso print.

In five tests, the researchers show that markings are more salient, more memorable, more reproducible and more suggestive of style and human intent the more recent they are.

The first tested saliencyhow noticeableeach of the images were. Participants were shown patterns from artefacts collected in the Blombos Cave and Diepkloof Rock Shelter in one eye and flickering colors in the other. The researchers found that the younger the engraving, the less time it took for the patterns to permeate the participants' consciousness. While older images took on average 2.27 seconds, later images took on average 1.82 seconds.

The second study involved intentionality with participants having to rate which of two images was more likely to have been created by a human. The third required participants to replicate the images they had just seen from memory. The fourth, measuring cultural traditions, instructed participants to say whether or not a target image came from the same site (the Blombos Cave or Diepkloof Rock Shelter) as competitor images.

The researchers found the younger the engraving, the more likely it was that participants believed it had been intentionally created, the more memorable (and easy to reproduce) they found it and the more likely they were to recognize it as coming from a specific site.

There was just one factor tested that did not appear to improve as time went on. The researchers tested discriminability by presenting each participant with a target and two competitor images. The target image matched one of the competitors and the participants had to work out which one as quickly as possible. There appeared to be no differences in response times based on age or location of the engravings, suggesting the "style-signifying" elements found in the fourth experiments were passive and not active.

"That is, they evolved as a side effect of transmission and reproduction more than an explicit intention to communicate group identity, which would imply an effort to actively differentiate styles between groups," the study's authors wrote.

Taken as a whole, the results suggest the engravings were created for aesthetic purposes "evolving" to become easier to remember and imitate, the study's authors propose. They add it would be interesting to delve deeper into this hypothesis by involving a more diverse set of participants to find out if the same rules apply.

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Prehistoric Engravings Offer Clues to the Evolution of Symbolism and Art - Newsweek