Imagine an IQ test could be taken back through time. We know the average score will be 100 now. How long would we have to go back in time for this average to drop significantly.(see note)

What I'm asking is when humans reached the current level of intelligence. Was it a century ago? A generation ago? Or were the men who painted in Lascaux just as intelligent as us?

Note: I realise that every IQ test returns an average of 100 when performed on a population of people. I'm just using this as an illustration.