Every drink of tap water you take has already been drunk by 10 people before you, which means it’s been 'recycled' by another creature. Paul Frijters wrote a piece at Economics.com.au showing the math used to figure out how many people your refreshing drink has passed through over the years.
It’s not a surprise; the water we have now is the same water that’s been on the planet since the beginning, and Paul says that 10 is a low estimate; his math suggests the real number is closer to 25. Of course, he says at least 10 of those times it was ‘recycled’ by a fish.
Here's the formula.
urine ratio = (total water urinated)/(total water) = (total vertebrate biomass ever lived* urine rate)/ (total water) = (average biomass vertebrates * urine rate per year * years of vertebrates) / (total water)
See how easy that was to figure out!