Ok, there's loads of theories as to why humans have a lowered larynx. I went to a talk about this in York probably about a year ago now and when I started reading about all this again it all came back and I thought I'd share Mark Jones' hypothesis with you all.
First, here's a brief history of lowered larynx theories:
Whilst other primates have the ability to lower their larynx to make vocalisations it is only humans who have it permanently lowered. This means that humans when swallowing have to raise the larynx. This is thought to have not evolved in other species as it creates a significant risk of choking on food if the larynx is not raised on time. So if it's so maladaptive how come it evolved in Humans?
Here are some existing theories:
Lieberman (PHILIP): Point Vowels (1972 ish?)
Lieberman hypothesises that the whole thing is as a result of the advantage of linguistic efficiency. A lowered larynx is crucial for point vowels i, a, u. Lieberman states that "The lack of these vowels in the phonetic repertories of other creatures, who lack a supralaryngeal pharyngeal region like that of adult Homo sapiens, may be concomitant with the absence of speech encoding and a consequently linguistic ability inferior to modern man."
Some languages such as Kabardian have a 2 vowel system, some languages don't have a point vowel (English!) and an explanation based on larynx alone does not seem to be enough when lips are required in the production of [u].
Fitch argues that a lower larynx reduces frequency of resonances within the vocal tract and this gives the speaker the ability to sound bigger than they are. This illusion of size is what Fitch claims was the original selective advantage for the larynx lowering. Additional lowering of male larynges at puberty supports this hypothesis because it is presumably the males who need to appear big and this can (and has) occur(ed) through sexual dimorphism.
(so, nothing to do with the emergence of language despite the permanent lowering of the larynx making vocalisation much easier)
Speech requires air flow (usually egressive). Speech occurs due to vocal folds opening and closing due to cycles of subglottal pressure forcing vocal folds apart and elastic return forces closing them again. So, for speech to occur subglottal pressure needs to be higher than supraglottal pressure for the vocal folds to open (this is done by compressing the lungs).
Here's the clever bit:
When the larynx is lowered it shortens the trachea by 3 - 4 cm. This, in turn, reduces the amount of lung compression needed to achieve phonation pressure. So it makes speaking much easier as well as creating the ability to be louder and have lower resonances.
He also points out that human's having a smaller glottis is as a result of this facilitating an increased airflow volume velocity so this is obviously adaptive.
Interestingly Mark also talked about humans having the same sized vocal tract as baboons, because despite them having a much higher larynx, they have a much longer snout. Mark pointed out that if human's were being selected to have longer vocal tracts then it wouldn't make sense for humans to have a reduced snout.
Unfortunately no paper seems to have been made of this yet so I can't cite anything. It's all very interesting though.