Hypothesis:  The ability to speak human language is a by-product of becoming bipedal, while understanding human language precedes the ability to speak it.

Evidence:

Kanzi, a bonobo, understands English and can communicate in English via ‘speaking’ lexagrams.  However, he cannot speak English, although he can make a variety of communicative sounds such as pant-hoots.

Human 9-month-old babies understand what is said to them.  They can discriminate the sounds of different languages at 6 months.  However, they cannot speak, although they can make a variety of communicative sounds such as da-da, ba-ba, ma-ma.

Kanzi’s form of locomotion is quadrupedal knuckle-walking.

A human baby’s form of locomotion is crawling/creeping on all fours.

A chimp/bonobo’s larynx is positioned high in the throat, allowing it to breathe and drink at the same time.

A human baby’s larynx is positioned high in the throat, allowing it to breathe and drink/nurse at the same time.

An adult human’s larynx is positioned lower in the throat: trying to drink and breathe at the same time leads to choking/coughing.

 

A chimp/bonobo and a human infant are incapable of speaking as adult humans do.

However, by the time a human infant becomes a full-time biped at about 18 months – 2 years of age, the larynx has dropped into the adult human position, and the toddler can now speak.

Based on fossil evidence, human ancestors became bipeds at least 4.5 million years ago (mya).

Deductions:

It can be deduced that, as part of the evolutionary process of becoming bipedal, the larynx dropped into its current position in adults around 4.5 mya.

Does this mean that Ardi spoke as we do?  Probably not, since her brain was still close to chimp size.

However, by 2 mya, early Homo, such as the Turkana boy, had a brain size in the low end of the normal range in modern humans.  Based on archaeological evidence, he also had a much more sophisticated material and behavioral culture.

Therefore, it can be deduced that by 2 mya, if not earlier, our ancestors had complex, rich, and sophisticated language and language skills.

Conclusion:

Our ability to speak is a by-product of becoming bipeds.  Language is not a relatively recent development.  We have been speaking sophisticated languages for millions of years.