One of your friends is originally from Ecuador. When he was a kid, he would sneak into the forest and watch howler monkeys. On one of these trips, though, he insists that he and his friends found a human skull. What would be a valid distinction why the skull was not actually human?

The foramen magnum was not located at the bottom of the skull

Explanation:

The location of the foramen magnum in the skull is a major distinctive feature we can use in validating why we can claim the skull was not actually a human skull.  

The foramen magnum, which is a large hole that serves as a passage of the CNS as well as the spinal cord, is located at the base of the human skull, which facilitates bipedal movement in humans, i.e. humans can walk mainly on on 2 legs as the skull is positioned on top of body. In howler monkeys, this is not so. Due to the fact that they generally move quadrupedally on trees, their foramen magnum opens far more backward.

Therefore, we can conclude that the skull was not actually a human skull as the foramen magnum was not located at the bottom  the skull. The skull must have been that  of howler monkeys.

your answer is "the 1950's"