There are several reasons, including luck, for why mammals survived when most other species went extinct 66 million years ago.
First, other species besides mammals survived as well. Insects, avian dinosaurs (future birds) and some fish that lived in deeper waters survived.
Even species that lived far from the Chicxulub asteroid impact site in the Gulf of Mexico, such as those near the polar regions which were used to long periods of annual darkness were still decimated.
As stated in my previous article most of those affected were species that lived on land and thrived near coastal areas.
I discussed in that article how some coastal and land species adapted to eat carrion that fed off of the remains of those who were not so lucky still survived.
Never-the-less It’s still thought that 90% of mammals died off.
Flowering plants evolved toward the end of the Cretaceous. They didn’t exist before this time. They served as a viable food source for many animals and evolving birds.
Other factors relate to location. There were still some areas less affected.
Species that were broadly located in many places around the globe may have died off in most locations only to survive and repopulate in a few others.
The American Geophysical Institute publishes a magazine, EARTH. There is a fine write-up on the topic in the September/ October edition on page 38.
If you look at a cladogram – a chart showing what characteristics we share with our distant ancestors, and how biological changes occurred moving forward (such as forward looking eyes) you’ll find we are related to a shrew like animal and lemurs (and many before).
Our presence is a matter of luck and circumstance. But were we not here, Earth would still thrive with life.
Perhaps some other evolutionary path would have led to an advanced species that would live here instead.
I’m glad it’s us!