Once upon a time, Man was enemy of the Sea. Man would catch fish until there were no more, consuming the children of the Sea with no regard to her pleas. The monstrous ocean would swallow up entire ships in anger, devouring the sailors and crew and leaving women longing for their husbands, sons, and lovers lost at sea. When their loved ones have all perished, the women would drown themselves in the ocean, hoping to be rejoined with their loved ones.
Some say that the Sea felt sorry for the women as they drowned and created a new breed of fish that can join bodies with the humans so they may survive under the water.
Some say that the Sea could not forgive man for slaughtering all her children. The women that joined bodies with the fish were made a part of the ocean and were never allowed to return to their human forms or to rejoin with their loved ones.
Those who have seen mermaids protect sailors at sea say they are still human at heart and continue to long for life on land. Others who have seen men mysteriously drown in the presence of the siren's song say that they have become monsters and are sinking ships at the whim of the Sea.
And that is how mermaids were created.
...at least that's my take on how mermaids came to be. I'm not too proud of the background. Maybe I'll change it later if I come up with something better. Evolutionarily, I dont know what the tail.. sucker.. spike... thing would do, since it's not a swimmer, but it really creeps me out. XD She has pectoral fins, teeth, and vestigial eyes on the fish half, since their bodies are fused. People tell me she looks evil, but that is just how she was made. ^_^;
Etymology: New Latin, from German Symbiose, from Greek symbiOsis state of living together, from symbioun to live together, from symbios living together, from syn- + bios life
1 : the living together in more or less intimate association or close union of two dissimilar organisms
2 : the intimate living together of two dissimilar organisms in a mutually beneficial relationship; especially : MUTUALISM
Details here: [link]