What might the earliest human languages have sounded like?

Answer 1

Fascinating question!

A full answer is far beyond the scope of this forum, however I can cast light on:

1. a few general ideas
2. a few resources to check out

How (spoken) language is made

To approach an answer to this question, we understand how it is humans make and control their bodies to make linguistic communication possible.

1. Breath Control- Humans are able to control their breath very well in order to produce long vowels and short consonants. The position of the epiglottis and of the larynx are critical.
2. Humans have very fine control of their tongue (and tongue root) allowing for a wide number of vowel sounds and semi-vowels.
3. Humans can independently control the tongue body, tongue blade, tongue root, larynx and lips to produce many combinations of sounds.
4. Overall compared to modern great apes (non-human) we have more nerve control of our breathing systems.

Analysis of close relatives, the Neanderthals shows that indeed their vocal apparatus would not impede communication. However, their vocal tract does seem to have been "less developed, but similar" to ours. One prevailing theory holds that Neanderthals had a higher pitched voice, much like babies. There have been several attempts to "reconstruct" their speech capabilities, resulting in comical interpretations. Just google "Neanderthal speech."

Defining impossible

Something to keep in mind here is the history of 19th and 20th century Phonology. At several points in the study of the human sound/speech system, scientists have declared the boundaries of potential sounds. This means they have said this is the [x] set of sounds we can make, and this is the [y] set of sounds we can distinguish from one-another. And every time that that was declared, some sound, never before thought possible in the human mouth, was found.

Just for fun, YouTube "Bantu click sounds." Humans have an amazing sound making system. What drives it is not only complex neural control of the muscles involved, but also the perceptive side. It is possible that pre-humans had as complex a linguistic system as modern humans did (depending on far back we go...)

Not Just Sound

One interesting theory laid out in the 80s is that even if early man and pre-man had limited vocal abilities, it was manually dexterous. Therefore, perhaps much of the communication we do today with out lungs, larynges, tongues, jaws and lips, they did with their hands. A fun "imagining" of this theory can be found in the somewhat poorly written, but throughly fascinating book "Clan of the Cave Bear" my M. Jean Auel.

In fact, it is very very likely that pre-man has some sign systems. After all, modern humans speak sign languages. Some, in fact, have evolved spontaneously.

Perception drives production

Because of the complexity of the system it is hard to put limits on things. Maybe early man had sounds it could produce, but the brain side was unable to distinguish them, so, they were not used in language.

To show you that you can't detect some sounds, do this experiment.

Hold a tissue up to your nose so it covers you nose and mouth. Then say these two words:

1. Spit
2. Pit

If you are an American English speaker, the tissue will "puff" at the [p] of "pitt" but not at the [p] of spit. People who speak Hindustani can hear the difference!

Yes, the above is an example of conditioning. But can you easily control the "breathy" p and the "puff-less" p? The point is, you can't do what you can't perceive (and vice-versa) From the biological side, the systems that control the muscles of the vocal tract can evolve out of step with the systems that control sound perception.

Primordial sound

One thing that is true, is that some of the sounds we respond to, we respond to in "primordial ways." Screams, laughter, crying, and maybe some onomatopoeic things are pre-pre-pre human. Rats even laugh. We we would share those.

An underlying assumption

Pre-humans had language. That's only logical. Otherwise you are postulating the sudden appearance of a very complex system that depends on and is built up from many other interrelated systems. Or, you could re-define language as something unique to modern humans. Such definitions are prevalent in science (and politics). But are bad for research.

In conclusion

So, pre-humans probably had fewer vowels sounds than we do, and maybe fewer affricates (ch, and the /j/ in juice). Check out the links to learn more.

General debate

http://blogs.discovermagazine.com/crux/2018/11/05/the-neanderthal-language-debate/#.XPmaedNKiL8

Australopithecus sign language

https://anthrosource.onlinelibrary.wiley.com/doi/pdf/10.1525/aa.1980.82.1.02a00040

Squeaky Neanderthals—or not?

http://summer10.isc.uqam.ca/page/docs/readings/BOE_Louis-Jean/Boe_et_al_JPhonetics_20007%5B1%5D.pdf

Why the epiglottis is super duper important!

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2595319/pdf/yjbm00136-0047.pdf

Idiotic, but funny

https://www.youtube.com/watch?v=tTkF8tomobA