YMNK that E=mc^2 was once m=(4/3)E/c^2
Einstein was not the first person to relate energy with mass. Between 1881 and 1905, several scientists - most notably phycisist J.J. Thomson and Friedrich Hasenohrl - derived numerous equations relating the apparent mass of radiation with its energy, concluding, for example, that m=(4/3)E/c^2. What Einstein did was recognize the equivalence of mass and energy, along with the importance of that relevance in light of relativity, which gave rise to the famous equation we all recognized today.