I am writing a story that involves the colonization of an exoplanet with very similar properties to Earth. This exoplanet would be orbiting a red dwarf, but due to certain factors (it's atmosphere and the presence of one or several moons) it is NOT tidally locked. Upon arrival from humans, to their surprise this planet already harbors complex life. It has a large ocean, continents, fauna and flora much like our own earth.
My question is: despite all the similarities, would it be plausible for this planet to be radioactively hostile towards humans? What I mean by this is, despite the similarities, could the fauna and flora have adapted to thrive in an environment which would be ultimately too harsh for our own bodies in the long run?
To provide some more context as to why i'm asking this: The plot i'm working on makes it so that when humans arrive, they can breathe without their suits, and at first it seems like the planet is just like earth, but soon after they realize that prolongued exposure to the radiation within the planet leads to early offset of cancer and thus humans can't really afford to live in this planet as it is without applying some form of alteration to the atmosphere (that's a wholenother subject i'll leave for another post haha), which would lead to harmful consequences towards the native life that has evolved to thrive within this specific environment.
Love this forum, it's been super helpful, to anyone taking time out of their day to answer: Thank you!