Did Proto-Sino-Tibetan and Proto-Indo-European languages have the same origin? Did human develop a common language before migrated from Africa, and were most if not all the modern languages originated from the language? Thanks.
The interesting question here is whether humans developed a common language prior to migration from Africa. This is not a linguistic question in the strict sense, it is a question of human evolution and history, nevertheless linguists do occasionally engage in the question. The really big problem is that we have to first define “human”, “language” and “migrated out of Africa”. In addition, you have to be a bit more explicit about what counts as evidence, since for example we have no recordings of language from 10,000 years ago and therefore under unreasonably strict (direct observation) rules of evidence, we don’t know if humans manifested language 10,000 years ago (not a claim that I’ve heard anyone embrace). The methodology that tells us that humans probably had language 10,000 years ago also leads to the conclusion that we had language 90,000 years ago, and maybe earlier.
Hominins migrated out of Africa more than once, including Homo erectus 2 MYA (a.k.a. Out of Africa I). This raises questions about the “human” and “out of Africa” aspects – perhaps there was no “language” when those beings migrated. We must first say what the minimum essential characteristics of “language” would be, as relevant to human evolution and this question. Language is a culturally-learned system of abstract signs communicating ideas by combining individual sounds which combine by rules, and can express propositions like “I want to eat that monkey” and “I want to eat that zebra”. A system limited to a fixed system of vocal cries of the type “Zebra!”, “Monkey!”, “Snake!” is not language. It’s possible that Homo erectus only had a non-combining system of identification-calls.
Early Modern Humans developed in Africa between 300 KYA and 200 KYA, then migrated out of Africa in more than one wave. A correlary set of questions for that migration would be “is that the migration you mean, are those being humans”, and “did they have language”. There is a theory that “all of our ancestors” who left Africa did so as recently as 90 KYA. It may depend on what you mean by “human”, for example are Neanderthals included in that set, but it is extremely likely that hominins had already developed the cognitive faculties characterizing language before leaving Africa 90 KYA. As detailed in Fitch (2010), the development of language was not an instantaneous process, it involved the accumulation of numerous evolutionary changes. Is it reasonable to think that the evolutionary changes that gave rise to the ability to use language didn’t take place until after 90 KYA: was language a pre-migration fact, or a post-migration fact?
Linguists, unfortunately, have tended to take a narrow, isolationist stance on language vs. cognition. The bones-and-stones record shows that complex characteristically-human cognitive behavior predates the 90 KYA migration, and it is hard to see what complex human cognition would even be without exactly the traits that characterize language. You might accept that the package of cognitive tools had evolved but lay dormant, unrealized in the mind, but then what would be the evolutionary advantage of having language qua unrealized potential? If you only consider the facts of language (which leaves no fossils until writing is invented), you would fail to see evidence of language as a necessary component of reasoning, which does leave some indirect traces (prehistoric human artefacts).
When two languages A and B are “related”, that means that at some point, the ancestors of A and the ancestors of B spoke the same language and they were in physical contact. The difference between saying “A and B cannot be proven to be related” and “A and B are unrelated” is huge. The former is simply an admission of the limits on what we can clearly prove. We can’t clearly prove the common ancestry of Indo-European and Sino-Tibetan. The latter claim of absolute unrelatedness implies something extremely unlikely, that there exists at least one hominid population A which was so cut off from all other hominids – and remained cut off for over 90,000 years – that the original A language predates the evolutionary emergence of modern man.
Although there is no observational evidence proving that earliest hominids had a single language, the fact is that humans have always interacted when in physical proximity, and we spread our languages and idea like the common cold, so there is sufficient conceptual reason to say that at some point in prehistory, humans did have that original manifestation of the language faculty. We just have no idea what it looked like.
The short answer is: Yes, they probably have a common origin, but we are unable to make any solid claims about that common origin.
A bit longer story: Historical linguistics has a time depth of 5–10 k years for large and well-documented languages families. However, we think that human language exists much longer and predates the split between modern men and Neanderthalers 300 k years ago. If language is never created out of nothing (it can happen even today, as in the famous case of Nicaraguan Sign Language), than all languages of the world have a most recent common ancestor. At the time this most recent common ancestor was spoken, it was not the only language of the world, but all other languages of that time died out with no direct descendants. This most recent common ancestor is called proto-world and there are some bold linguists making a few claims about this language, but their theories are definitely not linguistic mainstream.