70,000 BC

Analysis of the human genome has suggested that there was a choke-point in human history where the number of humans was drastically reduced.

Some believe that this was due to a single volcanic eruption occurring 70,000 years ago give or take.

According to this theory human population may have been reduced to just a few thousand breeding pairs.

Not sure if this meets your definition of 'non-natural' or of 'history' but no other event in will have made the same ratio.