A book excerpt in the Telegraph by David Attenborough asks this question:
Animals were the first things that human beings drew. Not plants. Not landscapes. Not even themselves. But animals. Why? The earliest known drawings are some 30,000 years old. They survive in the depths of caves in western Europe. The fact that some people crawled for half a mile or more along underground passages through the blackness is evidence enough that the production of such pictures was an act of great importance to these artists.
He doesn't answer the question, but gives an interesting history of the tradition of animal art, from early Christian scribes drawing emblematic animals in saints' portraits to Catesby, Audubon, and John Gould.
I happened across it because of the cave art reference. Readers interested in art history and art in science will enjoy the excerpt, which is from a book accompanying an art exhibition in Edinburgh.