Final answer:
The forest model proposes that bipedalism evolved as an adaptation to shrinking forest habitats. The ancestors of hominins lost in competition with the ancestors of Panini and had to adapt to life in the forest.
Step-by-step explanation:
The core idea of the forest model is that bipedalism evolved as an adaptation to shrinking forest habitats. According to this hypothesis, the ancestors of hominins lost in competition with the ancestors of Panini and had to adapt to life in the forest. The expanding forest habitats did not cause a cultural change, but rather drove the evolution of bipedalism as a survival strategy.