Is it true that cataclysmic events can leave a little marker on DNA? I read somewhere that a volcano caused a tweak that allowed us to date its occurrence. If this is true, how does it happen? Hypothetically speaking, could a tweak cause a change in evolution? Make us develop in a different way? Sorry - I don't mean to come across as glib. Forgive me if I'm suggesting a complete nonsense. I'm not a scientist. I'm a writer who likes to play 'what if'?

you could be talking about molecular clocks: http://en.wikipedia.org/wiki/Molecular_clock

 

This is a method that evolution scientists use to calculate how far back in history species could have diverged.

A cataclysmic event could have caused selection to favor a particular set of genes, that would show up in the genome. Or, perhaps an individual with a random mutation in it's nonconding DNA was a lucky survivor in the event, so that most of the subsequent population carried this genetic marker; this is called the founder affect.

I'm not sure that they are able to pinpoint specific DNA markers with specific events, however.