The mermaid remains one of the creatures that you might be able to draw a line to explain how it could have actually evolved. Given what we know about the importance of display in terms of mating, defense and so on, one might argue that a species of primate reverted to the water but maintained its human appearance as a means of attracting prey.
Countless animals evolve with false images or shapes on their bodies for the sole purpose of fooling their prey. So why not believe in mermaids?
E.B. Hudspeth (http://www.ebhudspeth.com/)