Where It All Began...

Hokkien (also known as "Amoy") evolved from Old Chinese and separated from the rest of the Chinese languages when the Han Chinese migrated from the north during the Three Kingdoms war in the third century AD, bringing their language with them and displacing the non-Chinese Baiyue people in the region.
Though it evolved alongside Middle Chinese through military expeditions, sharing many grammatical features and giving Hokkien literary (more formal) pronunciations. But because of the mountainous nature of the Min region, it stayed relatively isolated from the rest of the Chinese languages, keeping some of the archaic features of Old Chinese.
As such, many linguists considered Hokkien (and other Min languages) to be separated from the rest of the Chinese language families, and the closest link to Old Chinese.