Wikipedia
The proto-Mongols emerged from an area which had been inhabited by humans and predecessor hominin species as far back as the Stone Age over 800,000 years ago. The people there went through the Bronze and Iron Ages, forming tribal alliances, peopling, and coming into conflict with early China.
The Proto-Mongols formed various tribal kingdoms that fought against each other for supremacy, such as the Rouran Khaganate from 333 to 555 AD until it was defeated by the Göktürks, who founded the Turkic Khaganate (552-744), which in turn was subdued by the growing strength of the Chinese Tang dynasty. The destruction of the Uyghur Khaganate (744 — 848) by the Yenisei Kirghiz resulted in the end of Turkic dominance in Mongolia.
The para-Mongol Khitan people founded a state known as the Liao dynasty (907-1125) in Central Asia and ruled Mongolia and portions of the eastern coast of Siberia now known as the Russian Far East, northern Korea, and North China. Over the next few hundred years, the Chinese subtly encouraged warfare among the Mongols as a way of keeping them distracted from invading China.
In the 12th century, Genghis Khan was able to unite or conquer the warring tribes, forging them into a unified fighting force which went on to create the largest contiguous empire in world history, the Mongol Empire; which was finally able to conquer the whole of China beginning with his invasion of the Jurchen Jin Dynasty, and ending with his grandson Kublai Khan's conquest of the Southern Song Dynasty; establishing the Mongol-led Yuan dynasty.