The first source of radioactivity discovered was uranium.
Here's a timeline of the discovery of radioactivity:
* 1896: Henri Becquerel accidentally discovered radioactivity while studying uranium salts. He observed that the salts emitted invisible rays that could expose photographic plates even when kept in the dark.
* 1898: Marie and Pierre Curie isolated two new radioactive elements from uranium ore: polonium and radium.
So, while polonium was discovered shortly after uranium, it was not the first source of radioactivity. Uranium was the original source that sparked the discovery of this new phenomenon.
