The timeline starts at 1970 because the original article seems to have been written by someone who doesn't recognize the existence of the Bronze Age (which runs roughly 1970-1985).  The statement that the period begins in the mid-80's was added/changed by someone who does recognize the Bronze Age.  That's why the inconsistency.
What we should do is create a Bronze Age article and just get rid of the references to 1970 in here.  But it's too easy for naysayers to kill the article by selectively demanding references that aren't demanded for Gold/Silver/Modern.