Defenses throughout the years have changed drastically from the all-out war mentality to more cerebral/finesse type of systems (albeit still physical) based on matchups against high powered offenses.
The decade of the seventies could have been described by Darwinism, “Survival of the fittest”, as teams seemingly recruited players from the penal system that were let out on conjugal visits to play. A myriad of rule changes during this decade (mostly initiated by the Steelers) was instituted to save the game and to a greater extent, a lot of players good health in the process.
The decade of the eighties ushered in the evolution of more technology and more developed players in terms of skillsets, height, and weight.