In Reading in the Brain, Stanislas Dehaene asks “What does a macaque do with the brain areas that we now devote to reading?” His answer: object recognition. Experimental evidence shows that the kinds of symbols that crop up in human writing systems are the same kinds of symbols that primates are good at recognizing.
Dehaene -- “The most likely hypothesis is that these shapes were selected, either in the course of evolution or throughout the course of a lifetime of visual learning, precisely because they constituted a generic 'alphabet' of shapes that are essential to the parsing of the visual scene. The shape T, for instance, is extremely frequent in natural scenes. Whenever one object masks another, their contours almost always form a T-junction. Thus neurons that act as 'T-detectors' could help determine which object is in front of which.”
The point is clarified by some pictures from Dehaene's book that are available online. This illustration pertains to a neuron in a monkey's inferior temporal cortex that fires only in response to the image of a chair. This illustration shows how a neuron that will fire in response to a particular type of object will also fire in response to a version of the image that has been streamlined into something like a glyph.