Thank you for the link, that was fascinating and entertaining, I really liked the microscopic end. Interesting that the largest virus was right next to and just smaller than the smallest thing that a surgical mask can block out. Giant earthworm at 7m, haha.
It's funny, how many conflicting, shaky arguments there are for that sort of thing. Enrico Fermi was a famous was a very famous physicist who invented a style of solving complicated problems with estimation which are now called "Fermi problems." The premise is that you break your impossible question up into the product of as many easier-to-estimate problems as possible, and let probability (assuming you overestimate some parameters and underestimate others) take over. We used them in class to estimate things like "how many piano tuners are in the city of Chicago?" -- things we could look up easily enough... and our estimates were surprisingly close. In general (depending on how many parameters the problem has, of course), if you can estimate each of the parameters to within a factor of 10 or so, then your solution will be accurate to within a factor of 4. At least, that's what we were told, and it seemed to work that way.
Anyway, Fermi made a Fermi problem out of aliens encountering earth, and by using this method (though I have no idea what his parameters were or how he did it), they would have, for him, certainly made it here by now. His colleagues were talking about whether they thought aliens existed or not, and he simply asked "if they existed, then where are they?"
I feel like there has to be some false assumption in his calculations, because we've never even visited Mars, and yet we exist... but he was a rather well-respected Physicist, sos I thought I should at least give him his due. Perhaps they were only talking about sufficiently more technologically advanced races than ours, or something.