SuperNova on 15 February 2016
The only case you might have is some microorganims somehow surviving entry into the athmosphere (wich would have had a very diffrent composition back then), crash landing in the ocean and evolving from there.
In that case nearly all life on earth would be alien, or have alien admixture.
For everything else Darwin and genetics have you covered. Aliens wiping out dinosours to have the planet to themselves is very unlikely, to put it mildly.
