Question: In the question; [Hollow-boned Humanoids](https://worldbuilding.stackexchange.com/questions/13619/hollow-boned-humanoids) Nathaniels humanoids were a flying mammal species called soarfolk. But let's say that I want a sapient mammal to have hollow bones but not fly.

These furred, sapient mammals are called Lokk, they evolved in the plains after leaving jungles. They are an average of 5"4 and 50 kg. The length of their arms(3.1 ft) Legs(2.8 ft) and tail(2.7 ft). They will be preferably digitigrade(though not a requirement) and of course they have hollow bones. 

How could a creature like this evolve in an earth like world?

Answer: 
Overall, make weight a major concern in some way, while maintaining that there is an advantage in having more upright height on land (*To keep them bi-pedal*).

The easiest way to do so is if gravity were considerably stronger, which may or may not be considered "Earth-*like*". There would be more advantage to have stronger, denser, bones. But the weight might not be worth having *solid* bones, so they might become hollow.

There are probably other ways to make one's weight heavy without simply adapting to lose that weight, but I can't think of them at the moment. Hopefully others will have more ideas if you don't want to mess with gravity.
Perhaps there are poisonous gases created by underground bacteria in the forest floor which are fatal to your Lokk, and those gases dissipate in sunlight (i.e. concentrations would readily reach non-fatal levels as one approaches the top of the jungle.

If combined with plants whose branches retract upon sufficient jarring (i.e. Lokk who were heavier and disturbed the branches would fall into the gases), the Lokk would probably evolve into lighter, hollow-boned organisms, who could keep themselves out of the gases, up in the treetops.