This thread is open to those of you who have any knowledge in how a language that you speak has evolved over time. As for me I know very little about the processes of sound change that English has undergone though I've heard and seen samples of how English was like in the past. I do, however (after reading a lot of articles which may or may not be reliable), have a fair understanding of how Vietnamese has changed over the past 1-2 thousand years. I was fascinated about the theory of tonogenesis and how originally non-tonal languages can somehow develop tones.

According to this article(in Vietnamese) entitled Âm vị và các hệ thống âm vị tiếng Việt (Phonology and the phonological systems of Vietnamese) they explained how Vietnamese before the year 0 had not yet developed tones. Rather, it happened when certain initials and finals were lost: (Thanks to Haudricourt)



"In the course of history, the Viet Muong languages have had a significant change: in the beginning our language was one which didn't bear tones, later onwards it appeared as it does today..."

1 - at the beginning of the common era (starting from the year 0)
2 - at the 6th century AD (500 AD)
3 - at the 11th century AD (1000 AD)
4 - today (~1900 AD+)

1 2 3 4
pa pa pa ba [pa > ba]
sla hla hla la la [sla > la]
ba ba pà bà [ba > bà]
la la là là [la > là]
pas pah pà pả bả [pas > bả]
slas hlah hlà lả lả [slas > lả]
bas bah bà pã bã [bas > bã]
las lah là lã lã [las > lã]
pax pa? pá pá bá [pax > bá]
slax ba? hlá lá lá [slax > lá]
bax ba? bá pạ bạ [bax > bạ]
lax a? lá lạ lạ [lax > lạ]

The ? represents an unknown value. Also note that all the letters used are from the modern day Vietnamese script. For more information click here.

[ba > bà]
[bas > bã]
[bax > bạ]
[pa > ba]
[pas > bả]
[pax > bá]
[la > là]
[las > lã]
[lax > lạ]
[sla > la]
[slas > lả]
[slax > lá]

Hence Là ba bà lạ! (It's the three weird ladies!) would've been something like La pa ba lax!

Sorry that I cannot competently comment on the topic, but I've to point out, there is no year 0 in our calendrical system.

I've read a little about this. Apparently the Kradai languages (Thai, Lao, etc.) also developed tone from Chinese influence. Apparently also, the Sinitic languages may have developed tone under Hmong-Mien (a.k.a Miao-Yiao) influence, which would make the Hmong-Mien languages the original tone languages of East Asia.

I'll use my signature as example.
Classical Latin, about s.I after JC (Julius Caesar), becoming Portuguese, 2110 years after Caesar's birth.
WARNING: it's just illustrative, so, I'm simplifying the sound changes, or I'll have to post about 200 of them. And I'm still an amateur linguist, so, feel completely free to warn me about mistakes.

sölum magnum homö potest esse quandö rëses nunquam prae uïdërunt creat.
In IPA,
/so:lum magnum homo: potest es:e kʷando: re:ses nunkʷam praj wi:de:runt kreat/

Final-word /m/ is lost;
/ai/ becomes /ɛ/;
/w/ becomes /β/;
/h/ is lost in any position;

So we have Vulgar Latin, it's still understandable for a Classical speaker, but really strange:
/so:lu magnu omo: potest es:e kʷando: re:ses nunkʷa prɛ βi:de:runt kreat/

Now, let's reorganize the vowels:
/a:/ and /a/ merge;
/e/ and /o/ becomes /ɛ/ and /ɔ/, merging with the old /aj/ and /aw/ diphtongs;
/e:/ and /o:/ becomes /e/ and /o/;
/i/ and /u/ becomes /e/ and /o/, merging with /e:/ and /o:/;
/i:/ and /u:/ becomes /i/ and /u/.

/solo magno ɔmo pɔtɛst ɛs:ɛ kʷando resɛs nunkʷa prɛ βideront krɛat/

Now, Latin's case system is messed up. Speakers often stick to a case, and use it in all grammatical roles. In what become Italian and Romanian, was the nominative; Portuguese, Castellan and French, was the accusative. The word-order is now strict SVO. Let's take the accusative path:

/ɔmene solo pɔtɛst ɛs:ɛ magno kʷando krɛat resɛs nunkʷa prɛ βideront/
Dark Ages Latin. Note /ɔmene/ (hominem). More changes.

/ɛ/ and /ɔ/ merge with /e/ and /o/ in non-stressed syllabes;
Word-final /st/ (found in verbs) simplifies to /t/;
Magis specializates semantically from "great, "plus", "huge" to just "plus". With this, magnum's meaning is changed too. Grande takes its role.
Thanks to pedantic monks, it becomes commonplace ape Greek and its articles with the demonstrative pronouns illa, illum, illus: illa femina is something like "she, that very woman". Since illum and illus share the same illum accusative, merged together. Sound changes make illa e illum become /ella/ and /ello/.
Res ("thing") also specializate and is used only for livestock now. Causa ("cause, reason") /kowsa/ takes its place, but its evolution is a bit irregular (/aw/ become /ow/ and not /o/).

/ello ɔmene solo pɔtɛt ɛs:ɛ grande kʷando krɛat kowsas nunkʷa prɛ βideront/
This already sounds pretty more Romance that Latin:

Word-final oclusives are lost, aswell non-stressed final /e/;
Voiced intervocalic oclusives became fricatives. "Intervocalic" counts semivowels, too;
Voiceless intervocalic consonants became voiced;
Long/doubled consonants simplify;
The /ɛa/ hiatus becomes /ia/.

/elo ɔmen solo pɔdɛ ɛsɛ grande kʷando kria kowzas nunkʷa prɛ βiðeron/

And so goes on... coda /n/ and /m/ nasalise the precedent vowel and disappear, /ð/ disappears too, /ɸ/ (original Latin F) and /β/ dentalize to /f/ and /v/, hominem specializes from "human being" to "man" (Mediaeval sexism?)... today, 2110 year after Julius Caesar's birth, this phrase became in one Neolatin lang...

/o ɔmẽ sɔ pɔdɛ seɾ grãde kʷãdo kria kojzas nunkʷa prɛvistas/
In Portuguese orthography...
O homem só pode ser grande quando cria coisas nunca previstas
(Although I always translate "homo" as "ser humano" (human being), because include women too).
A human being can only be great when create what was never seen before.