More than any other species we're probably capable of meeting most natural threats in some way or other. So our extinction, when it happens, will probably come either at our own hands or some combination of environmental factors all occurring at once. That, or we just evolve away from being human.
It'll happen when silicon lifeforms take over since they will be built with more intelligence and strength and would see us as a pest. If we are lucky we may stick around for a while as "pets": assuming they has an interest in such things.