Well, I guess they were previously skinny fucks who managed to survive one way or another, but eventually selective pressure made the survivors look more and more like sticks.
It's like dog breeding. But instead of humans selectively breeding dogs to be bigger or smaller or more aggressive or less aggressive, it's blind nature driving things.
If the best way to survive is by looking like a stick, then your stick ass and your stick-ass girlfriend are going to have kids that look even more like sticks. All the fat fucks get eaten. Skinny fucks survive.
Repeat 50 million times. Then everyone looks like a stick.
For a simple example on a much shorter time frame, look up "Peppered moth evolution" on Wikipedia.
In England there were these light-coloured moths that rested on trees with light coloured trunks.
The light colour of the moths made them difficult for predators to see against the light colour of the tree trunks.
Then humans invented factories that spewed black soot everywhere and turned those light-coloured tree trunks black.
What do you think happened then?
The lighter the colour of the moth, the more easy it was for predators to see them against the black tree trunks, so all the lightest ones got eaten first.
The survivors tended to be darker, and they interbred with each other and became darker still until they were as dark as the sooty trees and became almost invisible to their predators again.
That's how evolution works. Some of these things can happen very quickly, as they did with the moths.