You said they probably exterminate everything.... and they obviously dont, and that's based off both the Prothean base & Neanderthal Humans on Earth, and stuff like Yahgs being too undeveloped too even bother hitting that undeveloped planet.

The Reapers land and exclusively target threat species... sure there will be collateral damage, but the Reapers wouldn't exterminate everything, they only attack lifeforms deemed "sufficiently evolved". Anything else that dies, was simply "wrong place, wrong time", but it's not harvested for the purpose of "creating another Reaper".

So in our case, the Reapers would wipe humanity out, and possibly apes & gorillas too, simply because they may be hard to differentiate between us, and then either clean up the planet immediately, or leave to harvest another planet with the cleanup immediately prior to going back to Dark Space to sleep.