Does anyone know if there's evidence that what hit the Earth was a meteor and not something else?
Yes.
There's:
1) A layer of Iridium in the rock layer of the time in question, all throughout the Earth, indicating a large meteorite impact of an ELE (Extinction
Level Event) magnitude.
2) There's a crater of the appropriate size for such an impact that dates to the time in question (extinction of the dinosaurs).
3) Other geological evidence supporting a blackout period, consistent with a large impact event.
This was just the catalyst though. With the mass extinction of large prey animals, the larger carnivores starved. With the massive deforestation that
would have resulted, even smaller prey animals lost their food supply, and along with it, the carnivores that preyed on them, etc., etc.
Basically, just the small, scavenging creatures survived until millions of years later, when we see large birds on the scene (dino descendants), and
some other large creatures (mammoths, etc.). I'm sure there were others in the interim, but you get the idea.