One needs to be reasonably bright to perform in any rigorously taught discipline. That said, the sciences mainly require precision and discipline, with the significant exception of studies that press out into the nether regions of mathematics, where most people hit the wall long before reaching the cutting edge. Most scientists, however, don't operate at that level.
In my overlong academic career, the flat-out smartest person I ever met was doing political philosophy, rigorously. When I last had contact with him, he was taking a year out -- and a year, for him, was probably enough -- to learn classical Greek so he could read Plato and Aristotle in the original. He had not only read everything, he remembered it, and could abstract and analyze at the drop of a hat. In terms of mental gymnastics, he regularly left the rest of us standing in the dust. Brilliant man. Smarter than any mere scientist I've ever met.
I absolutely agree. Most scientists do not work on that level. One doesn't have to be “Mr. Universe” to lift a ten pound bag of sugar. The same is true in most sciences, you need to be smart enough, not the next Einstein.