Evolution Of Speech
How have humans evolved to have the ability to talk while our close cousins, the great apes, have not? No definite answer can be given to that question though theories have been put forth.
One widely accepted theory has to do with the human's assumption of an erect (standing) position and the change that this brought to the anatomy of the skull. Following the evolution of human skulls from their earliest ancestors, one major change that occured is the movement of the foramen magnum (the large hole in the skull through which the spinal cord passes) to connect with the base of the brain. In early skulls, the foramen magnum is at the back of the skull because early man walked bent over with his head held to look straight ahead. The spinal cord entered the skull from behind as it does in apes and other animals. In modern humans, the opening is on the bottom of the skull, reflecting humans' erect walk and his or her's skull placement atop the spine.
As human's position changed and the manner in which his or her skull balanced on the spinal column pivoted, the brain expanded, altering the shape of the cranium. The most important change wrought by humans' upright stance is the position of the larynx in relation to the back of the oral cavity. As man became erect his larynx moved deeper into the throat and farther away from the soft palate at the back of the mouth. This opened a longer resonating cavity that is responsible for the low vocal tones that man is capable of sounding.
The expanded brain allowed the development of the speech center where words could be stored and recalled. A more sophisticated auditory center provided the means by which speech by others of the same species could be recognized. Over time, and with greater control of the articulating surfaces, consonant sounds were added to the vocabulary. Initial sounds by hominids probably were vowels, as evidenced by current ape communication.