What effect did European colonization have on the indigenous population?

It led to their decline, in some cases even to their extinction.

Explanation:

When the Europeans first came there were millions of Natives who were scattered across the continent. But when the Europeans started colonizing it, their number started decreasing. The main reasons at the beginning were wars, but then changed when the  Europeans started spreading diseases. The deadliest among them were smallpox. Millions of Natives died because of this disease.

probably 3 since archeologists and historians don’t understand there writing.