I was thinking more on my idea that the verb "to be" in Indo-European was borrowed from Semitic. There's a common tendency, I find, for people to want to reject something like this outright because of the instinctive belief that "to be" is so basic and intrinsic to a language that it can never be borrowed. However one important rule I've learned in linguistics is: "Never say never".
First off, not all language speakers in the world find a verb for 'to be' particularly necessary or "fundamental to the language" at all. In fact, it could be claimed that most don't. However, in Europe today, since there is a predominance of Indo-European languages such as English, French, German, Italian, Spanish, etc. which all treat 'to be' as a grammatically obligatory verb, Europeans and North Americans may therefore have a distorted view of what is really seen around the world and through history (or even prehistory). Consider the Uralic languages, for example. These languages show us that we don't need the verb 'to be' to express presence or equality. We can just as well omit the verb altogether, particularly in the present tense. In fact, Proto-Indo-European itself is believed to have omitted *h₁es- too in certain situations. So it's hardly a stretch that Pre-IE should have showed even more of this zero-copula predication (as this type of omission is called), much like many of these Uralic languages today.
In my frenetic search through cyberspace, I uncovered some other neat stuff on this topic. Slovenian Romani apparently has borrowed negative copulas nije '(s)he is not' and niso 'they are not' directly from Slovenian (Matras, Romani: A Linguistic Introduction (2002), p.209). Yet Romani is not a creole because of its preservation of Indo-Aryan inflections (although it's sometimes a problem determining what constitutes a creole). We might also note the Altaic language named Santa. Yes, that's right, Santa. Sounds Christmasy doesn't it? According to S. Robert Ramsey (The Languages of China (1987), page 199): "One particularly striking adaption to Chinese is the hybrid construction made in Santa using the Chinese copula shi 'is.'" It gives an example of this phenomenon, Bi kieliesen kun shi ene we 'The person I was talking about is this one.' where both shi (from Mandarin Chinese 是 shì ) and we (a native copula) are used together at the same time to convey 'to be'. The author then goes on to explain that this language my have borrowed up to 30% of its vocabulary from Chinese due to extensive, long-term contacts with that language. Wow! Then, finally, let's ponder on the following revelation about a universal hierarchy regarding nominal and verbal borrowings in languages around the world (Elsik/Matras, Markedness and Language Change: The Romani Sample (2006), p.320): content verbs -> modal verbs -> existential verbs.
The final link basically tells us that if a language goes so far as to borrow existential verbs like 'to be', it must also have borrowed other kinds of verbs first. So in other words, it suggests that the language in question must have already underwent extensive borrowing from a neighbouring language for this to eventually happen, probably involving a high degree of bilingualism and bilingual interference as has apparently happened in the language of Santa vis-a-vis the dominant language in the area, Mandarin.
 Stassen, Intransitive Predication (1997), p.212 (link here).