It's probably the right combination of a body, intelligence, social structure and environmental pressures that lead to sophisticated tool making societies that

It's probably the right combination of a body, intelligence, social structure and environmental pressures that lead to sophisticated tool making societies that pass their knowledge on down so it accumulates. So far, we're the only species on Earth to have enough of the right combination to get this far. So maybe it only happens 1 in 1000 worlds with multicellular life. How many planets does that leave in the Milky Way? That's the sort of calculation SETI does.