Final answer:
The visual predation hypothesis explains why orangutans do not have eyebrow ridges.
Step-by-step explanation:
The method outlined in class that explains why orangutans do not have eyebrow ridges is called the visual predation hypothesis. According to this hypothesis, the presence of forward-facing eyes, grasping hands and feet, and nails instead of claws in primates, including humans, may be adaptations that helped early primates succeed as predators. By having these features, primates were able to effectively hunt insects and other small prey in the forest undergrowth and canopy. Orangutans, as arboreal primates, may not have developed eyebrow ridges because they did not require them for their hunting strategies.