Well of course there was something special about Newton and Euler, but I have no doubt that there are thousands of mathematicians alive who could outperform both in "Intelligence" tests and Mathematical Olympiads.
They had the right skills, at the right time. Newton didn't invent gravity, his work was the culmination of nearly 1000 years of thought in the natural sciences. He didn't even "invent" calculus, i mean the concept of infintismals was considered by archimedes. Many still believe that it is to Leibniz we owe for the calculus. Even so, neither mathematician put the work on a rigorous grounding, it worked but Newton did not understand why it did so. It took the development of the limit and other great mathematicians to do so.
My point is, none of us live in a vacuum. In the future we should put the smartest child on Earth determined by genetics, on an isolated island in the pacific and come back in 20/30years and see if he has formulated a new string theory.