Dating back as far as 100 ka, the Blombos ochre and the
Diepkloof ostrich egg engravings are considered among the
earliest fossil evidence of human symbolic behavior. The
engravings found on the same sites spanOf special interest to
this study is the temporal trajectory spanning more than 50 k
years from earlier simpler parallel line patterns to later
complex cross-hatchings suggesting adaptive compositional
development. Through a series of three psychophysical
experiments we test the hypotheses that the line engravings 1)
evolved to become more salient to the human perceptual
system, 2) more discriminable from each other, and 3)
increasingly associated with symbolic intent. Our finding
suggest that just as instrumental tools have been found to
undergo cumulative refinements in adaptation to their
function, the ochre and egg shell engravings evolved
adaptively to become more fit for their cognitive function as
signs.