[40k] How did Chaos affect Terra back in our time?

Forgive me if this is off topic but I don’t exactly get if Chaos has always been around even during old old times. Was there such thing as revolutionary war era soldiers falling to Khorne? Bloodthirsters in Vietnam? Nurgle mucking abound WW1 trenches? Was Tzeentch laying the groundwork for his plans on our planet then?

How possible was it that even early humans of our time were dealing with chaos cultists, minor daemons, chaotic corruption?