The neck had to elongate to allow the larynx to descend, creating the necessary 1:1 horizontal-to-vertical ratio. Even with a shorter jaw, without elongation of the neck, the larynx had nowhere to descend without causing a suite of other problems. Based on fossilized cervical vertebrae of early humans, the neck does not appear to have elongated to anywhere near its current length until around 50,000 years ago.
