The silent have been there for thousands of years even so, apparently.
...And there might be a couple standing behind you right now. You should kill them all on sight. You're fussed that nobody has noticed monsters you can't remember?
And surely part of the Silence is that they are based on classic stories of UFO abductions and MIB encounters which nobody generally believes. Ask David Icke what he thinks about lizards living beneath the surface of the Earth. ;)
The "lizard people" - or, as we fans like to call them
, Silurians, Eocenes, or Homo Reptillia... depending on what episode we're watching - are a classic Doctor Who monster. Don't diss the lizards!
If you want to split hairs, Earth's history has been manipulated behind the scenes by the Daemons, the Fendahl, Fenric, Osirans...the Last of the Jagaroth. And just about every major historical figure has had some sort of run-in with an extraterrestrial monster.
Not to mention the fact that during the 1970s or 80s, the World (aka "Home Counties") was affected by some sort of Weird Sh*t every week often Master-minded by the international terrorist Victor Magister...
And it's a mute point anyway since they reset the Universe last Series finale!
But yes - if this show is taken at face value, humanity is incapable of getting it's self out of bed in the morning without some kind of extraterrestrial help.