Ancient Thai Script: Sukhothai period - year 1293

     It is believed that this script is the direct ancestor of modern Thai script. It was created by King Raam Kham Haeng around year 1293. Comparing to other Tai scripts(including Lao and Northern Thai) which are in circular shapes, the Sukhothai letters are more squared-like. Moreover, the script has 39 consonants as opposed to the modern Thai script which has 44. And there are only 2 tonal markers as opposed to 4 in the contemporary Thai script. The 39 consonants were probably invented to represent the ancient pronunciation which many Tai linguists argue that they are different from the contemporary pronunciation. Voiced stop and fricative sounds may have gotten devoiced: คg>kh, ฅɣ>x>kh, ชɟ>tɕh, ซz>s, ทd>th, พb>ph, ฟv>f etc. And other changes includes implosives ดd and บb>plosive d and b respectively.
     These changes may have affected the tones to shift from having at least 3 tones to 5(or 6 in other dialects). I mean before the shift you could have "va" and "fa" as sensible words, but after the shift, va > fa, so now we have "fa" and "fa." To distinguish the difference, one(or both) of them has to undergo a tone shift: fa(mid tone) and fa(rising tone) etc. This could also explain how modern Thai has come to have 3 consonant classes(eg. high f vs. low f---where the low f may have come from the ancient v).

Oh and have you ever wondered why ห is written before น to create a new sound: a high n? Well in the ancient time, these two sounds were actually a cluster which means it was pronounced hn. Linguists would go look at other languages that borrowed a Thai word that begins with hn in the ancient time, and see if in these languages still retain the hn sound. How did linguists come to conclude these changes? Well thankfully the Thai script was created based on Indic scriptures, so we can compare it with let's say the devanagari script to see how the sounds differ since there has been many Pali and Sanskrit loanwords in Thai.
For more info on the ancient pronunciation, read From Ancient Thai To Modern Dialects by J. Marvin Brown, and there are also a lot of great researches done by the late linguist William Gedney too which maybe in your local university's library. For those who are curious how the Thai r became h in many Tai dialects(and not the other way around because the creator used the same symbol to write a Pali word with an r and a Thai word with an r) such as Lao, Shan, and Northern Thai, these books might give you the answer.