Tons of studies and experiments had to be conducted before the human race could come up with the knowledge that we have today. For instance, imagine the scientists studying all the different kinds of mushrooms to find out that one tastes like meat, the other makes you high, and another one could take out a bull in a few seconds. The growing pains of discovery included many test subjects that would often include animals and, other times, humans. Just like when the US was in the process of understanding radioactive materials a decade after plutonium had been discovered. The government had insufficient knowledge yet on how to handle such toxic materials properly. This led them to conduct experiments on humans by injecting them with plutonium without their knowledge to study its side effects.
The Manhattan Project
The Manhattan Project was a major research and development program of the US government during World War II. It officially began in 1942, although the agencies that led to the program were first formed in 1939 after Albert Einstein wrote a letter reporting that the Nazi scientists were already in the process of working on a nuclear weapon at that time, urging them to do the same. Only then did the United States take the atomic weapons business seriously.
The most famous development of the Manhattan Project was when they produced atomic bombs, two of which were the Little Boy Bomb and the Fat Man Bomb, that were dropped on the two cities of Japan, Hiroshima, and Nagasaki. There was also this not-so-famous bomb that was supposed to be the third bomb to be dropped in Japan had they not surrendered, known as the Demon Core (know why it was called as such here.)
Although a huge chunk of the Manhattan Project was dedicated to the development and production of the weapons, a small portion of it was dedicated to studying the health effects of the radioactive materials involved in the project, which was Plutonium.