For the past two months or so players have been grinding it out to unlock space-themed cosmetics in Season 3, following some Medieval skins in Season 2 before that.
That brings me to my first guess: dinosaurs. True, the meteor that hit the Earth killed rather than created the dinosaurs, but a general proximity to a meteor impact still puts this one on the table, I'd argue.
The second guess would be prehistoric, in some way. This works a little bit better with the meteor concept, though we're truncating the geologic timeline a bit here.