Why do we believe that 神 is likely to be a loan from Ainu?


I recently read the document ["Old Japanese
Loanwords"](http://conf.ling.cornell.edu/japanese_historical_linguistics/3.2%20Loanwords.pdf)
by Frellesvig, where he states:

> OJ _kamwi_ ~ _kamu-_ 'spirit, deity' may well be borrowed from an ancestor
> of Ainu _kamuy_ 'bear; deity'.

Ever since I heard of the word "kamuy", I figured there was a connection to 神.
But why do we believe that it is likeliest for 神 to be borrowed from "kamuy",
rather than "kamuy" being borrowed from 神, or both of them coming from a
common substratum? (Or, if this is actually a controversial stance, what is
the evidence on which Frellesvig bases his belief?)




**Archaeological Data**  
The Ainu people are descended primarily from the Jōmon people and secondarily
from the Yayoi. Modern Japanese people are descended primarily from the Yayoi,
secondarily from the Jōmon, and tertiarily from ethnic Koreans. There is still
debate whether the Yayoi emigrated to the Japanese Archipelago from Mumun
period Korea or from the Yuyue State in Eastern Zhou period China. In either
case, neither group's religious culture was as similar to modern Shinto as
Jōmon & Ainu animism is.

Keep in mind that 氏神{うじがみ} are tied to specific locations which would mean
that the emigrants (Yayoi/Japanese) either established new sacred places in
Japan or the natives (Jōmon/Ainu) had already established certain areas as
sacred.

**Linguistic Data**  
The kanji `神` predates the Yayoi by several hundred years, showing up in
oracle bone script.

> [かみ] often appears in compounds as /kamu/ (modern /kaɴ/), indicating that
> /kami/ is a bound or fused form deriving from */kamu.i/. Note that this
> final i may be the Old Japanese emphatic nominative particle い (i), likely
> cognate with Korean nominative particle 이 (i). Such fusion has occurred in
> other Japanese terms, such as 目 (me, “eye”, from ma + i) or 酒 (sake, “saké,
> liquor”, from saka + i).

～[Wiktionary](https://en.wiktionary.org/wiki/%E7%A5%9E#Etymology_1)

Even with the combination of the above evidence we can't definitively say that
`かみ` originated as `カムイ`, but it is the most likely scenario given the
available data.

Please comment if I've missed anything above.

