You can't yet...you can find very convincing evidence, but until you can show evolution happening, it can't be proven.
Another point of view: Just compare the average persons physique to the accepted norm of the Middle Ages. Now, our brains are on average larger and we are taller. A 6 foot person in the 14th century was a like a basketball player now. An example is the old castles in Europe, most of the passages are a little short for the modern 'man' but were just right for the 'builders'.