As humans I believe our purpose on earth is to help keep the earth going and without humans the earth would probably not get along too well.
We are not here because of god or Jesus but we're here because our parents are here and they created us because they hopefully wanted us in their family.
So the main purpose I see of me being here on earth is because my parents wanted to have a baby and raise a kid and they want me around to make their life better.
Other than that I see no other purpose on earth for at least myself but someone else may have a better purpose.
I'm sure the earth though could get by fine without us humans but I'll stick around until I'm no longer needed and nature takes it's course.