Q:

Individualist Intelligence

Under what circumstances could an inherently individualist species become intelligent and advanced? 
Usually, animals require social interaction for intelligence to develope.  Or at least that's what is commonly assumed.  Yet, many largely solitary animals like alligators and monitors are still smart, in some cases enough to use tools or learn commands.  
I want to make my species inherently individualistic as stated in my question: Would my species even create a government?,  but several answers stated a species that asocial would never become advanced.  
Our species became intelligent, or so I've heard, in response to rapid changes in our environment.  What rapid changes would drive a largely solitary species to become smarter? 
Ideally, I'd like it to match the evolutionary path I've laid out for the species.  They were originally aerial predators that became terrestrial, omnivores that lived largely in mountainous, temperate rainforests and fjords.  

A:

I love cats. 
Just wanted to get the most important fact out first. 
Cats evolved from solitary predators, yet they have no issue with fitting into human families and forming long lasting social relationships with humans.  They also have decent skills in reading human behaviour relevant to themselves. 
So why does a solitary predator evolve what is clearly a decent set of social skills and social intelligence? 
This is because hunting successfully requires small kittens to learn lots skills and knowledge from their mother and practice those skills with their siblings.  A kitten that is not very good at learning from observation or from playing is going to starve as an adult. 
So cats are good at observing and interacting with humans because it is just an alternate use of skill they need to have as kittens in order to survive.  And since it is so vital and in nature has to happen in limited time, this relatively narrow and specialized intelligence has to be very efficient. 
So cats are kind of dumb and instinct driven except when they are not.  When the task happens to match that gift for learning from observation or interaction, cats suddenly appear much smarter than dogs.  Or small children for that matter. 
This prelude is here because I think this is the type of intelligence you want your people to have.  They are not smart in the same way a social animal like humans are.  Instead their focus would be rapidly learning from observation or direct practice thru play. 
Well, mostly I just wanted to talk about how awesome cats are. 
Now, to the actual question. 
What would this imply for the evolution of their intellect and society? 
Well, if you want more intelligence than cats without meddling from some alien pet owner, you need to extend the "learning period" or childhood of the species.  You need to increase the complexity of the skills they need to learn from their family and the time it takes to learn them. 
You can probably use the transition to being omnivores as step one for this. 
For the rest I'd suggest tool use.  For whatever reason the species has hands or some equivalent.  For climbing trees?  Some food sources they use require use of tools to get.  Smashing crustaceans with a stone can lead to stone axes.  Using a stick to get termites can lead to spears.  Making nests and stone axes can lead to building shelters.  Shelters with spears leads to walls and protective obstacles.  Longer childhoods can lead to permanent structures and even settlements for raising children. 
Basically if you can plausibly find a case for evolution of technology, learning that technology will then require learning more skills, leading to better intelligence and longer childhood.  Which will lead to more technology, including social technology.  And so on.