Nik Taylor wrote:

> How did the first tone [of Middle Chinese] split?

Syllables with voiceless initial consonants got tone 1; syllables
with voiced initial consonants got tone 2.

Middle Chinese had three voiced aspirated initials (like Shanghainese
today), which can be written [bh] [dh] [gh].  In old tone 1, they became
[ph] [th] [kh] (Pinyin p t k), but in the other tones, they became
[p] [t] [k] (Pinyin b d g).

The old fourth ("entering") tone, which was attached to syllables
ending in -p -t -k, got divided up among the other tones in
a random way when final stops were lost.

John Cowan              [log in to unmask]
        You tollerday donsk?  N.  You tolkatiff scowegian?  Nn.
        You spigotty anglease?  Nnn.  You phonio saxo?  Nnnn.
                Clear all so!  'Tis a Jute.... (Finnegans Wake 16.5)