Life on Earth a "very new thing"?
"But in astronomical terms human intelligence is a very new thing - indeed, so is life on Earth."
Life ius reckoned to have been around on Earth for an estiamte 3.8bn years (versus about 4.5bn for the age or the Earth itself or about 14bn years for the Universe. So that means life has existed on Earth for a third of the lifetime of the Universe,
I don't think that makes life on Earth a "very new thing", albeit that such life forms as were present at that time will not have been well equipped to observer comets.
As for comets being old friends of humanity, I suppose we owe a debt of grattitude to the one that wiped out the dinosaurs at the end fo the Cretaceous in order to provide an opportunity for our mammalian ancestors, but for many in human history comets were seen as the harbingers of disaster and not friends.
Also, on the basis that other life in the Solar system would still be considered alien, then, surely, so would comets, whether they formed within the gravitational influence of our Sun or not. Best call such comets extra-solar system rather than alien.