Japan was never conquered by the Mongols. And again, I have to bring up the relative isolation of Japan from the rest of Asia. The Turkic, Mongolic, Tungusic, & Korean speakers were all very close but none of them, apart from perhaps ancient Korean peoples who came to Japan, were constantly in contact with Japan.
Didn't that isolation begin in the Edo period? Japanese is a lot older than that.
During ancient & classical times, the only people Japan had consistent relations with were Chinese & Korean civilizations. They had little or no contact with Turkic, Mongolic or Tungusic speaking peoples.
Actually, there are theories of which state the ancient chinese language lacked tones and, were polysyllabic. Thus, the change to monosyllabic tonal language can, be a self-devolopement as well as an outside influence from somewhere else.
I was under the impression that Chinese developed tone under influence from the Hmong-Mien languages.
OK, so I'll provide a summary of all the "circumstantial evidence" for Altaic that I can think of.Phonology
-vowel harmony found in every branch except, perhaps, for Japonic
-rhotic phonemes do not begin native words in any branch
-a system of complementary distribution for velar & uvular plosives & fricatives is found in Mongolic, Tungusic, & in some Turkic languagesGrammar
-SOV syntax found in every language
-all languages are agglutinative & strictly suffixing
-a case system with nominative unmarked in Turkic, Mongolic & Tungusic; a system of "particles" found in Korean & Japonic
-no grammatical gender
-no distinction of singular & plural, except for animate nouns & pronouns; the suffixes used to show plural have similarities: compare Japanese -ra
with Uzbek -lar
& Tuvan -ler
; Japanese -tachi
, Korean -deul
, Kazakh -dar