Humanity lived thousands of years in the environment without any source of electric power, but in the environment with radiation they will be dead in a few days.
Use it for humanity:
“Humanity lived thousands of years in an environment without any source of electric power, but in an environment with radiation it will be dead in a few days.
However, to my ears, “Humanity” sounds more like “the state of being human” . She showed her humanity when she cared for the wounded.
For the sentence in the question, I would suggest:
- "Humans" - they, their (Humans are highly social beings, and they have their own...)
- "Mankind" - it, its (Mankind has been around for over a million years.)
- "Humankind" - it, its (for those who dislike the term "mankind") (How long has humankind been around ? It has….)
- "People" - they, their (People will be dead in a few days if they are exposed to…)
The word refers to us. We, each, are part of humanity, so we is the best fit. In the particular sentence, the problem is not with the pronoun, but with the noun humanity itself. What's wrong with just people?