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
Proteins are the nutritional class which builds and maintains cells and tissues. The type are often called "structural" since they are like the scaffolding of cells and tissues.
