Mining Bitcoin with pencil and paper: 0.67 hashes per day
I'm probably going to do one step wrongly and get laughed at, but here goes:<p>7 000 000 000 humans * 0.67 hashes per day = 4 690 000 000 hashes per day (or 54282.4074074 per second)<p>54282H/s would give us a daily profit of: 0.00078760 BTC ($0.30)<p>So when the evil space aliens conquers the human race, they can put us to good use to generate 30 cents worth of Bitcoins for them every day (not counting human electricity cost aka food).