Answer: The correct answer is : Parasitism arises several million years ago. The evidence shows that the parasites were initially free-living organisms that had contact with their host and managed to adapt to the same biochemical and biophysical environment. There are three theories about the origin of parasitism and they are:
inoculation of the agent through arthropod vectors
ingestion of the "pre-parasite" by its host
active invasion by the same parasites
You may think that there was a nutritional dependence
They are both highly selective,most of the time what binds to them. Think of it as a lock and key, if the key fits the lock and it turns then it does what it needs.
