How have rules changed from when football first began up until how football currently is?

Between 1974 and 2011, the NFL moved the kickoff line three times. Each adjustment was carefully considered and made solely in response to circumstances and trends at the time.

In 2009, the rules were modified to prevent three or more defensive players from forming a wedge to block for the return man on kickoff returns. This change, aimed at reducing injuries, came about only after the Competition Committee members had meticulously examined hours of film of kickoffs.

Explanation:

obviously, the latter.