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. As he describes it, you realize it was not human because

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

Explanation:

The foramen magnum is a large hole or opening found in the base of the skull  through which the spinal cord passes.

The position of the foramen magnum is important since it allows the skull to balance on the body hence essential for posture.

Humans are bipedal and Howler monkeys are quadrupedal hence the positioning of the foreman will be different.

That is how the skull found was realized it was not human

the way that the mass of carbon can be conserved through cellular respiration is through a process/law called the law of conservation of mass which even tho it undergoes a process, mass is not be destroyed or changed