Between 1968 and 1991 (23 years) the 100m world record was broken 3 times (not including Ben Johnson), dropping a total of .05 seconds. In the last 18 years it has been broken 11 times for a total of .32 seconds. In the last 4 years it's been broken 6 times, taking .21 off the record.
Is this a natural progression?
Usain Bolt has taken .16 off the record in one year. The last time the record was broken by such a large margin was when Ben Johnson broke it by .12 seconds in 1987, and later tested positive for PEDs. If you take Johnson's disqualified times out of the picture, the most the record was ever broken by before Bolt was .05 seconds (also not counting the .07 it dropped in 1968, due to the 7400ft altitude of Mexico City). Even if the Mexico City times are considered, Bolt more than double the largest margin in history.
Simply a freak of nature? Maybe.
At the same time Jamaica has risen to world dominance in sprinting. This is a country with a population less than that of Los Angeles. They won a total of 2 gold medals in track from 1960-2000, then won 8 in the last 2 Olympics (6 last year).
Is this simply the result of improved training? Maybe. You decide.