If someone fails to complete the course of antibiotics for an infection, some of infection causing bacteria may survive (having a greater resistance for those antibiotics). It will lead to bacterial population in afflicted patient, with a higher resistance than normal ones.
When these resistant bacteria reproduce, infection caused by their progeny would not be treatable with same old antibiotics.
The very primitive organism is the oldest because, over time, organisms gradually evolved into more complex organisms. First came the primitive organism, then it evolved into the more complex one, or the younger fossil.
