Egyptian vowels may once have been indeterminate however, the rebus principle is a prominent feature, which suggests Hieroglyphics once wrote a non-Egyptian language with numerous homophones, such as occur in Niger-Congo. The rebus principle is based on homophony, which is not a feature of Egyptian. The same Egyptian sign is freely used for words that are phonetically diverse. But this is true also of Sumerian. It is explained by a pictographic (ideographic) source. Pictographies of various kinds (ideographic, syllabic, alphabetic) occur in West Africa, where they provide elaborate pictorial catalogues of man's (and woman's) worlds (Dalby 1967). They appear to have originally been used in initiation ceremonies, and were also employed for purposes of magic. In Africa they have a meaningful cosmic context, and are embedded in the mythology.
Africa therefore, not Egypt, is the probable source of all these pictographies. Unfortunately however documentation from Africa is relatively recent. But some of these scripts or their precedents must go back to antiquity.
That is Diop's opinion. (Campbell-Dunn, 2009a: 11-12) (emphasis mine) Besides the off-remark separating Egypt from Africa, Campbell-Dunn brings up many salient points that gives context to this discourse.
As I noted in my 2011 work Passion of the Christ or Passion of Osiris: The Kongo Origins of the Jesus Myth, the Egyptians communicated through the rebus principle. This principle is discussed by wiki as follows:
In linguistics, the rebus principle means using existing symbols, such as pictograms, purely for their sounds regardless of their meaning, to represent new words.
Many ancient writing systems used the rebus principle to represent abstract words, which otherwise would be hard to be represented by pictograms. An example that illustrates the Rebus principle is the representation of the sentence - I can - see - you, by using the pictographs of - eye - can - sea - ewe. Some linguists believe that the Chinese developed their writing system according to the rebus principle, and Egyptian hieroglyphs sometimes used a similar system. A famous rebus statue of Ramses II uses three hieroglyphs to compose his name: Horus (as Ra), for Ra; the child, mes; and the sedge plant (stalk held in left hand), su; the name Ra-mes-su is then formed
The hieroglyphic symbols are an outgrowth and expansion of old Kongo-Saharan writing symbols. These symbols can be seen among the Vai, Mende and even the ancient Cretans. A few examples can be seen below of shared iconographic symbols among the Egyptians and Kongo-Saharan speakers. The first few examples is taken from Cheikh Anta Diop‘s book Precolonial Black Africa: A Comparative Study of the Political and Social Systems of Europe and Black Africa, from Antiquity to the Formation of Modern States (1987).
The following example actually comes from Linear A in Crete and the Vai script of West Africa. The Linear A borrowed many signs from the ancient Egyptian script, but didn‘t borrow the phonetics of the sign. Practically all of the signs in Linear A match Niger-Congo words for those symbols, which brings to question whether they were borrowed necessarily from Egypt or brought with them from Africa. The first example comes from Cambpell-Dunn (2006).
The comparisons on the left are a more extensive set of Egyptian and Linear A comparisons with their Niger-Congo names, which I argue, informs us on alternate pronunciations of signs in Egyptian which are attested in the Egyptian language itself.
As we can see, the names given to the Egyptian glyphs above are actually the name of those items in Kongo-Saharan languages.
A good number of these glyphs in Linear A actually are just morphological variations of the ones in Egypt.
For instance, Linear A do ―eye‖ is a variant of Egyptian iri ―eye‖ (l <> d); Linear A se ―arm‖ is a palatalized form of Egyptian (k)a ―arm.‖
We argue that the D36 glyph a "paw, claw, hand, arm" was actually pronounced ka (PWS *ka ―arm, hand, cut‖); e.g., Egyptian a ―region, province‖ < PWS *ka ―place, home‖; PWS *gà ―place‖, PWS *gi ―to be in a place‖; PWN KI (KYI, CI) ―village, settlement.‖ Over time the k- was dropped which left us with /a/.
The Kongo-Saharan languages can tell us a lot about the Egyptian writing script.
Diop reaffirms why we should be looking at those languages from the south in regards to understanding the hieroglyphs.