As we got older, our tongue and face muscles got stronger and our voice box lowered. In the back of the skull of a person who can talk the bone is curved allowing the voice box to drop. Also there's a part in our brain, the neocortex, that allows us to speak. The limbic system allows us to make noises. Chimps have both but aren't able to use the neocortex for speech.
All mammals have the limbic system.
Humans are very smart and have developed the brain so they are able to speak. There is also a bone in the back of the skull called the hyoid bone which also enables us to speak. Species without this bone cannot talk. We also learned that early humans needed some form of communication for hunting and passing information, so humans developed speech. In September our Omni teacher met with a scientis named Jeffery Laitman who explained this theory.