This is something I've drew in 2012, though this idea has been floating around in my head since 2009. And it's something I'm still refining and working on to this day

Anyway this whole thing started with a cousin of the lobe-finned fish. Instead of using only 2 pairs of limbs for locomotion it used 3, which led to the prehistoric 6 limbed reptile that birthed the entire hexapod family tree

One of the family lines grew a colorful interdigital membrane to intimidate opponents and attract partners. Eventually that membrane gained a new function, which led to the development of wings

One particular sub-family developed a feather coat, initially to survive better in colder climates. Eventually it replaced the membrane, since it was more efficient

While one of the species retained the claws and hunting habits, the other favored speed on land and a more varied diet

The other main family line is the mammalian, which focused more on using all 6 limbs for locomotion. Over time those limbs led to particularly nimble and dextrous fingers

One of the sub-families managed to grow quite huge, valuing strength and size over speed. That also meant they needed large quantities of food, so the 1st set of limbs grew longer and pushed the body into a vertical position and put more pressure on the 3rd set. Eventually the 2nd vanished completely

The other sub-family valued speed, using the 1st set of limbs to grab and carry food, while the other 2 took care of the locomotion. The 2 descending species are still closely related, though one of them learned how to use fire and doesn't need such a large jaw as its cousin