8. The first evidence of a talking pig was discovered in 1979 shortly after a Charlotte's Web convention had closed.

Ans.

The first evidence of a talking pig was discovered in 1979 shortly after a Charlotte's Web convention had closed.

Here the use of the passive may be warranted. When one talks of evidence (or data, etc.), one may wish to preserve a semblance of objectivity by burying who the finder of such evidence is.

However, if the author wishes to eliminate the passive, he/she might write it something like this:

In 1979 shortly after a Charlotte's Web convention had closed a corrections officer from Tulsa, Oklahoma, discovered the first evidence of a talking pig.