From a recent NYTimes article on cave art in Spain:
The handprints common at several of the Spanish caves were stencils, probably made by blowing pigment on a hand placed against the cave wall. The oldest example, at El Castillo, proved to be at least 37,300 years old, which the scientists said “considerably increases the antiquity of this motif and implies that depictions of the human hand were among the oldest art known in Europe.”
These handprints show up all over. I'm wondering if they were put there as marks of personal identity, personal presence. Without a written language it would have been impossible to write a name on the wall. But a handprint could well serve the same function.