(Classified - SPOILER ALERT)
Earth was home to a sentient human species long before common science claims humans evolved. On prehistoric Earth, a human race evolved and rose to an advanced Civilization. Details of their culture and how and why they evolved is lost to time, but it is the source of the Human Mystery as Colonists of that society became the Saresii, the Sarans and all Human species of the M-O Galaxy evolved from there. Even the Ult, who adapted to their water world by changing into Amphibians.
The Chrom are a group of Human colonists that settled the World now known as Saresii Prime. 
Who or why they called themselves Chrom is not known.
 This happened long before the First Age of Knowledge