I want two languages to share a root but I want them to look unrecognizable written down (with the original language having a written form, so not independently created writing systems). Are there any well known patterns of how languages can change their script over time? What is some general advise on how to make this change realistic?
The surface that things are written on. Runes were carved in trees and, as such, do not have curves. The Greek alphabet was written on tablets, and curved lines were possible. The Arabic alphabet was written quickly on papyrus. Also, some civilisations might prefer cursive script over others. Arabic letters differ whether they're isolated, at the beginning of a word, at the end or in the middle. So does your handwriting with the Roman script. But Arabic kept their cursive when the digital age came, and the Roman script got rid of it and created the same character.
Os (runes), psi (Greek alphabet, but modernised on a computer, see footnote 1), šīn (Arabic script, isolated)
ᚩ, Ψ, ش
Let's call the ancestor A. The two daughter languages will be called B1 and B2. A could be a pictographic script, from which B1 devised a syllabary but B2 kept pictographs. Or A was an abjad (alphabet without vowels) and B1 became an alphabet whereas B2 stayed an abjad.
- Although language is passed on from mother language to daughter language with diachronics, script doesn't need to. In fact, you don't often see one script in the same language for three language-generations except in some cases like Latin (but Vulgar Latin wasn't really written much). Borrow a script from a neighbouring language. This is what I recommend doing.
There are several factors in a writing system that can be subject to change:
- The style of writing: In fact, style is ever-changing in human culture, and we see new styles every generation. Old styles may be revived, or the new styles become divergent enough to be considered a new writing system. Note that handwriting from a century ago (e.g., in Germany) is already very hard to read for the current generation and special training is needed to do it.
- The set of symbols used for writing: Some symbols may fall out of use, others are newly introduced (either by borrowing (Greek Y and Z into Latin Y and Z), or by modification of existing symbols (Latin V splitting up into U, V, W), or by new invention "out of nothing" (Emoji).
- The use of diacritical marks: The writing system may be borrowed from another language and is not a perfect fit for the new language. Some devices are needed to accommodate the sounds of the new language to the writing system, like digraphs (e.g, English sh or ch), diacritical marks, additional letters, or a combination of any of the former.
Whether the introduced changes create a new writing system or just a variant of an existing writing system is merely a question of definition. I remember a heated debate on the Unicode mailing list whether Hebrew and Phoenician are one or two writing systems.