we could have had already some encounters, maybe some current life-forms in Earth were actually alien-forms initially (somehow octopuses comes to mind).

Some people on the net says that the current coronavirus situation could be an alien-species. I know it is FAR FAR from anything related to what we already know about the virus.

But it gives an idea about what could have looked an encounter with a new alien-patogen which just landed on Earth at some point at history and got to survive and thrive in the local ecosystem.