Short answers, pulled from the story.
A group of settlers arrived on the Japanese archipelago in the early to mid-4th century BC. This event brought the language that would eventually become the sole national tongue of the islands.
The Chinese writing system arrived in Japan from Baekje around the start of the 5th century. This introduction of a script eventually birthed a unique written language used to represent Old Japanese sounds.
The Heian period spans from 794 to 1185. During this era, Japanese underwent a radical phonological transformation driven by an influx of Chinese loanwords.
Yamato words make up 33.8% of the total vocabulary in modern Japanese. These words represent the original language of Japan or the ancestral language of a significant portion of the historical and present Japanese nation.
Modern Japanese is considered to begin with the Edo period, which spanned from 1603 to 1867. This era marks the start of a language that incorporates vocabulary from Yamato words, kango, and gairaigo.