Why is Europe and Asia often considered two seperate continents? Isn't a continent a large land mass surrounded by water?

When I asked my high school geography teacher this in front of the calss I got the reply that they are divided by a mountain range.

To this I querried- Then why isn't there an East and West America, as right down the middle is a mountain range called- The Continental Divide?

Also, if history starts with man, then how do you get prehistoric man? Isn't this a little contradictory?