I'm currently reading Karl Friday's Japan Emerging: Premodern History to 1850 and it has a ton of interesting information regarding the strong Korean influence on the history of early Japan:
Japanese scholars suggest that at least a million Koreans poured into the Kinki region during the fifth and sixth centuries along with these items, to the degree that, according to Shoku Nihongi, eight or nine out of ten residents in the Nara basin claimed to have descended from Paekche. Korean scholars, priests, scribes, and skilled bureaucratic functionaries were assimilated into the upper levels of Yamato society.
(...)
Still, there is no disputing the influx of all kinds of craftspeople and court nobles to the archipelago during the extended wars on the peninsula. The fact that the ninth-century Shinsen Shōjiroku lists one-third of aristocratic families as having non-Japanese origins attests to the great capacity of Yamato to absorb and use the skills and knowledge of the refugees. Yamato court scribes of the late fifth century were Paekche immigrants, facilitating the beginning of the aristocratic household records that would eventually be collected into the Nihon shoki.
My question is: shouldn't there be significant linguistic evidence for this massive movement of people? As far as I understand it Korean and Japanese share very few cognates, and their language families are completely unrelated. But if scribes and scholars and aristocrats are coming from the Korean peninsula and being rapidly integrated into Japanese elite society, shouldn't there be traces of this in Old Japanese? Especially if Koreanic speakers are the ones writing literature and compiling court records?
It's enough to make me speculate that these refugees actually spoke a Japonic language, since Japonic speakers obviously came from the Korean peninsula earlier...but linguists seem fairly certain that Baekje spoke a Koreanic language. So what's going on here?
Old Japanese and Old Korean are believed to share a number of cognates—in fact, these cognates are what provoke the perennial hypotheses about Korean and Japanese being genetically related. Vovin has done a good overview of them in his 2010 Koreo-Japonica: A Re-evaluation of a Common Genetic Origin, but possible or likely loans include some key cultural terms:
Middle and Modern Korean aspirate consonants arise from Old Korean consonant clusters including a velar or /h/, so the Old Korean form would have been something like ٭patk or ٭patx. Given Japanese syllable structure it's easy to see how this would have been loaned as patakë.
The Japanese kanji for patakë (modern hata), 畑, does not exist in China. And the earliest instance of this character is from a Korean mokkan, which further strengthens the notion that the word was loaned into Japanese.
See above. MK pwùthyè [putʰjə] was likely Old Korean ٭pwutkye [putke], if we agree with John Whitman's hypothesis that Middle Korean /jə/ comes from Old Korean *e.
OJ pë was phonetically ٭pəj. The MK word is póy [pʌj], but Ito Hideto's research suggests that MK [ʌ] derives from OK ٭ə.
Old Japanese wata is not attested in other Japonic languages or in eastern dialects of OJ, and even in western OJ itself there's another word for "sea," umi, which is a pan-Japonic word. This semantic distribution strongly suggests that wata is a loan.
Japanese phonotactics would have required the addition of a vowel after -k. In any case, the MK low tone is sometimes evidence for the loss of a final vowel in OK, so the OK form might actually have been twoki.
If Whitman is right, tyél [tjəl] is from either Old Korean ٭tel or ٭ter. Old Korean distinguished between ٭l and an unknown rhotic phoneme, but the two have merged to initial [ɾ] and final [l] in Middle Korean so that it's impossible to tell from a noun like tyél what the original pronunciation was. In any case it doesn't matter because OJ lacked /l/, and the loan relationship is obvious.
OJ kama is unattested in eastern OJ dialects, and though it's used in the northern Ryukyuan languages it seems to be a loan from Japanese. This distribution makes it likely a Korean loan.
An important totem animal in ancient East Asia. The rising tone in native MK words (marked by the colon, as in :kwom) always derives from an OK bisyllabic word, so in OK the word must have been ٭kwomV, with V being an unknown vowel.
However, we know that Proto-Korean ٭wo, which was preserved in Old Korean (by which linguists mean the Silla dialect), was raised to ٭wu in the Paekje dialect. So Proto- and Old Korean ٭kwoma was ٭kwuma in Paekje, which is a clear source for OJ kuma. (In linguistic transcriptions of Korean, <wu> stands for /u/.)
٭xatun is attested in a twelfth-century wordlist by a Chinese ambassador, and also in mokkan data and one of the rare surviving OK poems. These two Korean loans are incidentally the source of the famous Japanese katana (lit. "single blade"), which is a sword with a single-edged blade.
Another OJ word for arrow, ya, is attested in other Japonic languages, while sa is not. This distribution suggests that sa is loaned from Korean.
It's much more difficult to talk of Old Korean loans in Old Japanese because Old Korean is so poorly attested, and in a largely non-phonetic script to boot. But what evidence we can gather seems suggestive.
This analysis overlooks the simple fact that the main mode of writing and to some extent spoken communication in elite Japanese society was Classical Chinese, the same as in Baekje. Neither Korean nor Japanese had it's own writing system at the time, and different dialects of Japanese and Korean were not even mutually intelligible. In this context, Chinese served as the only writing system available and the only mutually comprehensible language, in addition to the considerable prestige and access to a magnificent corpus of material. In fact, writing was introduced on a large-scale by Korean immigrant elites, who were formally educated in the language and spread it into Japanese court. It is no coincidence that 50% of all Japanese vocabulary has Chinese origins.